CHAPTER IV.

HISTOLOGY.

=Definition.=—Histology is that part of descriptive anatomy which treats of the intimate structure of the tissues as seen under the microscope.

Histology as taught in most professional schools constitutes a one year's course, but for the embalmer this is not entirely necessary and with the short term of schooling now existing it is quite impossible, but certain of the fundamental principles of histology are important. For this reason a few of the more important tissues have been discussed, not, however, in great detail, but only superficially, merely to have the embalmer acquainted with them.

=A Cell.=—A cell is defined as a nucleated mass of protoplasm endowed with the attributes of life.

_Protoplasm_ is the name applied to the semi-fluid, granular substance contained within the cell.

The simplest forms of animal life are organisms consisting of only one cell which are called _protozoa_.

Cells having similar shape and similar functions are grouped to form tissues.

Tissues are grouped together to form organs.

Every cell consists of a cell body and a nucleus. The cell body consists of a substance known as protoplasm. The nucleus is the essential part of a typical cell and is the controlling center of its activity.

Cells divide or reproduce themselves by means of direct or indirect division. In direct division the nucleus and the cell wall simply divide into two equal divisions and results in the formation of two new cells. In indirect division the process is much more complicated, and several stages must be passed through before there is a complete division.

The process of fertilization consists in the conjugation of two sexual cells. The male sexual cell is called the spermatazoon, and the female sexual cell is called the ovum.

The nucleus of the ovum in its earlier development stages is known as the germinal vessicle.

In the living organism many cells are destroyed during the various physiologic processes and are replaced by new ones. When a cell dies, changes take place in the nucleus which result in its gradual disappearance. This process is known as chromatolysis.

=Tissues.=—A _tissue_ is an aggregate of cells all having a common function.

Those important tissues with which the embalmer should be more or less acquainted are the following:

Skin, nails, hair, superficial fascia, deep fascia, lymphatics, glands, cartilage, bone, teeth, nerves, muscles, tendons, aponeuroses, ligaments, fat, mucous membranes, serous membranes, synovial membranes, arteries, veins and blood.

=The Skin.=—The skin or integument (intego, to cover) is the outside covering of the human body. It is the first tissue that is cut when operating upon the body.

The skin is the seat of the organs of touch. The multitudes of sensory nerve endings convey the sensations of temperature, pressure and pain to the brain, thus informing the brain at all times, to keep the body from harm, and in a strong and healthful condition.

The skin is also the regulator of the body temperature, for connected with the skin are sweat glands, and sebaceous glands, each having important excretory functions.

The skin is also a protective coat, very elastic, and varies greatly in thickness. It is thinnest in the eyelids and thickest over the back of the neck, back of the shoulders, palms of the hands and the soles of the feet.

The color of the skin depends upon two things, first, on the pigment, which is found, one of the discriminating points between the races, named by the color of the skin as white, black, yellow, etc.; second, the color depends upon the amount of blood in circulation, the deepest hue being in the parts exposed to the air, light and the varied temperatures. Besides these the color of the skin varies with age, pinkest in the infant and becoming yellow with old age. It varies with exposure and with climate, the people living in the north having a much different complexion than those living in the south under the tropical sun. The color of the skin also varies with certain diseases, being extremely pale in anaemia, brown in Addison's disease, and yellow in jaundice.

The skin can be said to be moveable, although in places it is attached firmly to the underlying structures, especially on the scalp, the soles of the feet, and the palms of the hands.

Upon close examination the skin discloses a multitude of openings, creases, furrows, depressions, folds and hairs.

A dimple is a permanent pit or depression due to the adhesion of the surface to parts beneath.

_Structure._—The skin consists of two intimately connected structures, the one is the _true skin_, _corium_, or _dermis_ and is the deepest layer of the skin; and the other is the _false skin_, _cuticle_, or _epidermis_, and is the outermost layer of the skin.

_The true skin_, is composed mostly of connective tissues and elastic fibers. It is the real seat of the sense of touch, for it is here that the sensory nerves have their termination. In this layer we also have the termination of the minute capillaries of the skin.

_The false skin_, contains no blood vessels or nerves, and being without these it is practically dead tissue, and to illustrate this fact one can take a needle and run it through this outside layer without the least pain or the drawing of blood.

The false skin is the part which slips off in case of skin slip. In as much as the minute capillaries end at the termination of the true skin, when putrefaction and fermentation begin there is an oozing of water from the capillaries and the surrounding tissues, between the two layers of skin, causing a blister to form, and known as skin slip.

At the lowest part of the false skin is a layer of germinal cells, from which all the other cells are derived, and becoming more flattened and horny as they are pushed farther away from the blood supply; and also a layer of pigment cells, which give the discriminating color to the skin.

In the skin are seen numerous sebaceous and sweat glands.

_The sweat glands_ are the organs by which a large portion of the aqueous and gaseous materials are excreted by the skin. Sweat glands are found in almost every portion of the skin, and are situated in small pits below the surface of the skin, surrounded by a quantity of adipose tissue or fat. They are small, round, reddish bodies, consisting of a single tubule, convoluted in form, which extends up through the skin and opens on the surface. The size of these glands, of course, vary, being especially large in those regions where the flow of perspiration is copious as in the axilla.

_The sebaceous glands_ are small, sacculated, glandular organs, lodged in the substance of the skin. They are found in most parts of the skin and are usually connected with the hair follicles. Each gland consists of a single duct, more or less capacious, which terminates in a cluster of small secreting pouches or saccules. These glands secrete an oily fluid, which keeps the skin soft and also oils the shaft of the hair.

=The Nails.=—The nails are a peculiar modification of the epidermis and have the same cellular structure as that of the epidermis. The nails are found on the dorsal surface of the fingers and toes and act as a protection, and enable one to pick up small objects, or to grasp more firmly any object. Were it not for the nails it would be impossible for one to pick up a needle from off the floor.

Each nail is convex on its outer surface, and its chief mass which is called _the body_ lies upon the nail bed, or true skin; the free end projects out over the surface of the finger, and is that part which is not attached below, and since it is the continuation of the epidermis, it likewise will have no nerve or blood supply and therefore can be trimmed without pain to the individual.

_The root_ is implanted in a groove in the skin and is composed of cells which have not become horny. The root is white in color and is the little half moon which you can see next to the skin.

_The matrix_ is that part of the true skin beneath the body and the root of the nail, and is so called, because, it is that part from which the nail is produced and so long as the matrix at the root of the nail is uninjured, the nail will be reproduced after an accident.

After death the nail turns black, due to the infiltration of blood into the matrix.

_Treatment by the Embalmer._—The blackened condition of the nail due to the infiltration of blood into the matrix can in many cases be overcome by carefully rubbing the nail at the time the body is being injected. After the discoloration is removed the fingers should be kept elevated so that the blood will not settle there again.

=The Hair.=—The hair, like the nails, is a peculiar modification of the epidermis and consists of practically the same cellular structure as the epidermis. Hair is found on nearly every part of the body excepting the palms of the hands and the soles of the feet, the borders of the lips, etc. It varies much in length, thickness and in the different races of mankind. In the eyelids it is short, on the scalp it is of considerable length. In other parts as the eye-lashes, the hair of the pubis region, the whiskers and beard the thickness is remarkable.

A hair consists of the root and the shaft. The root of the hair or that part implanted in the skin presents at its extremity a bulbous enlargement, called the hair bulb. Into this bulb we find the small arterial capillary circulating and at its termination the beginning of the venous capillary. In this way the hair is nourished in life. We also find a small nerve going to the hair bulb. The shaft is the remaining part or that part coming out from the skin.

The hair grows from its roots and as it grows it pushes itself out from the skin and owes its growth to the small capillary circulation, carrying pure arterial blood to each and every hair, and for this reason you can understand for yourself the erroneous idea of what is termed the “post-mortem growth of hair.” Only a few weeks ago one of the students declared that he had actually seen a subject shaved and the body at the time of the funeral was placed in a vault to await the arrival of a close relative who had to come from Europe.

Three weeks later the student, together with the undertaker and relatives, went to the vault to view the remains. The body was in a perfect state of preservation, only for a large growth of beard as the student supposed. This student had observed rightly, but he did not go far enough. He did not think of how the hair actually got its nourishment. The hair owes its life to the circulation of the blood, just as much as the heart or any other organ does, and will die and cease to grow just as soon as the body dies and the circulation is cut off. What this student saw was only an apparent growth, for after the body dies the tissues begin to shrink, squeezing the blood and fluid substances out of them, thus giving the hair cylinder a more projected appearance.

The student was very much surprised at his mistake, but after the explanation he saw that the hair owed its life to the circulation and that when this circulation was cut off, the hair must cease to grow.

The chief function of hair is that of protection from heat or cold and to help shield the brain from the effect of a blow upon the head.

The hair, next to the teeth and bones, is the least destructible part of the body.

=The Fascia.=—The fascia (fascia, a bandage) is areolar or aponeurotic tissue of variable thickness and strength found in all regions of the body and invests or surrounds the softer and more delicate organs. From its situation in the body the fascia is divided into two groups, superficial and deep.

_Superficial fascia_ is found immediately beneath the skin over almost the entire surface of the body. It connects the skin with the deep fascia and consists of areolar tissue.

The superficial fascia varies in thickness in different parts of the body and some places, especially in the groin is capable of being subdivided into several different layers. The first layer of the superficial fascia, which is just beneath the skin, usually contains a great amount of fat or adipose tissue. This, in most text books, has been termed the subcutaneous tissue. The second layer is comparatively devoid of adipose or fatty tissue and in this we find the trunks of the subcutaneous vessels and nerves, as for example, the radial and ulnar veins in the arms and the saphenous vein in the leg.

The superficial fascia facilitates the movement of the skin, serves as a soft medium for the passage of the vessels and nerves to the skin and retains the warmth of the body, since the fat contained in its meshes is a had conductor of heat.

_Deep fascia_ or aponeurotic fascia is a dense inelastic, unyielding fibrous membrane, forming a sheath for the muscles and affording them broad surfaces for attachment. On removal of the superficial fascia, the deep fascia is usually exposed and can be seen as a dense, tough membrane, which not only binds down the muscles to each region, but gives to each a separate sheath as well as to the blood vessels and nerves.

Thus, on going down into the arm between the biceps and triceps muscles to raise the brachial artery, you would first cut through the skin, then the subcutaneous tissue, the superficial fascia and then you would come to a membrane investing the artery, vein and nerve. This membrane is the part of the deep fascia which covers the vessels, making a distinct sheath for them and you must go through this sheath before you can hope to raise the artery.

=The Lymphatics.=—The lymphatics occur in all parts of the body, and in many respects resemble the veins, one of the most striking similarities being that the lymphatics contain valves just the same as the venous system. The lymphatic capillaries are arranged in the form of a net work and resemble closely in structure the blood capillaries. These capillaries then unite to form the lymph vessels and these then convey the lymph to the subclavian veins. The lymph is a colorless fluid and contains numerous blood corpuscles known as lymphocytes. But in those lymphatic vessels, which have their origin in the walls of the small intestines, the lymph, especially during digestion, contains a great amount of fat, so that it has a milky appearance, and for this reason the lymphatics of this region, have been termed lacteals. There are two main lymphatic trunks, the one on the left side is called the thoracic duct. This duct extends from the lower border of the second lumbar vertebra, through the entire length of the thorax, and opens into the left subclavian vein, close to the point where it is joined to the left internal jugular. It receives the lymph from the lower limbs, the pelvic walls and viscera, the abdominal walls and viscera; the lower part of the right half and the whole of the left half of the thoracic viscera, the left side of the neck and head and the left arm.

The other duct is called the right lymphatic duct and receives lymph from the upper part of the right side of the thoracic wall, part of the right side of the diaphragm and the right lobe of the liver, the whole of the right arm and neck and right side of the head. This trunk is very short and empties its supply of lymph into the right subclavian vein.

Receptaculum chyli is the expanded portion of the thoracic duct just at its beginning. Its function is to receive the lacteals which come from the villi of the intestines.

Lymph glands are the enlargements of the lymph vessels. They occur frequently in the lymphatic system, being most numerous in the axillary space, the cervical region (in the neck) and in Scarpa's triangle.

The lymphatic system aids greatly in warding off such diseases as blood poisoning, anthrax, etc.

_The lacteals_ are the lymphatics which carry the chyme from the villi of the intestines and deposit it in the receptaculum chyli.

=Glands.=—The glands of the human body are divided into three classes called tubular, alveolar and tubulo-alveolar glands.

_Tubular Glands._—In these, the secreting portion consists of a long or short tubule, which may be relatively straight or variously twisted, one end of which ends blindly, while the other end opens on the free surface or into a duct.

Tubular glands may be simple, or having only a single tubule; they may be simple branched, having more than one tubule; or they may be compound branched, thus resembling the branching of a tree.

Some tubular glands would be the liver, kidneys, testes, lachrymal glands, serous glands of the mucous membranes, fundus glands of the stomach, uterine glands, the majority of the pyloric glands and the majority of the sweat glands.

_Alveolar Glands._—In these, the secreting compartments have the form of variously shaped vesicles or saccules, known as alveoli which open on the free surface or into a duct.

Alveolar glands may be either simple, simple branched, or compound branched.

Some alveolar glands would be the sebaceous glands, pancreas, mammary gland, ovary and thyroid.

_Tubulo-alveolar Glands._—In these, there is a combination of the tubular and the alveolar type. They may also be simple, simple branched or compound branched.

Some of this type would be certain of the pyloric glands, certain of the sweat glands, some mucous glands, the prostate and the lungs.

The most important glands will be discussed under the tissue or the organ in which they are situated.

=Cartilage.=—Cartilage is a transition stage between connective tissue and bone; when it is boiled it yields condrin. It is found in various parts of the body, in the adults being found chiefly in the joints, in the sides of the thorax, and in various tubes which are not kept permanently open, such as the air passages, nostrils, ears, etc. In the foetus, the greater part of the framework is cartilaginous and as the foetus matures this cartilage is finally replaced by bone. Cartilage is divided into hyaline cartilage, elastic cartilage, and fibro cartilage.

_Hyaline cartilage_ is found in the nose, larynx, trachea, and bronchi.

_Elastic cartilage_ is found in the epiglottis and the cartilages of the larynx.

_Fibro cartilage_ is found at the point of insertion of the ligaments, into the body of the bone, such as the cartilage which helps to hold the femur or long bone of the thigh into the hip.

=Bones.=—Bone results from the calcification of cartilage or fibrous tissue. It is a highly specialized form of connective tissue. There are two varieties of bone; dense or compact bone and cancellous, loose, or spongy bone. Compact bone is dense, like ivory, and is always found on the exterior of bones.

Cancellous bone is found in the interior of bones, and has a lattice-work appearance.

Bone consists of one-third animal or organic matter and two-thirds earthy or inorganic matter. These proportions, however, vary with age. In youth it is nearly half and half, while in the adult the earthy is greatly in excess. It also varies with disease. With some defect of nutrition, the bone is deprived of its normal proportion of earthy matter, while the animal matter is of unhealthy quality, and we have as a result, a disease called rickets, so common in the children of the poor. The earthy or inorganic matter consists of phosphate, carbonate, fluoride of calcium, sodium chloride, and phosphate of magnesium. The animal matter consists of fat collagen, which when boiled with water is resolved into gelatin.

To illustrate the two substances, take a bone and place it in dilute hydrochloric acid. The acid will eat out all the mineral matter and we have left only the animal matter. After this operation one can take the bone and can bend it into any position whatever, which experiment shows that the animal matter gives elasticity to the bone.

The second experiment would be to put the bone on a bed of hot coals and burn it. Only the animal matter will burn and we will have the mineral matter remaining. After this operation one will find that the bone is very brittle and will easily break, which experiment shows that the mineral matter gives stability and support to the bone.

If a cross section is made of any long bone, such as the humerus, and this section placed under the low power of the microscope, the Haversian canal system can be discerned. The Haversian canal system consists of the numerous small openings or canals through which the blood vessels ramify in distributing the nourishment to the bone. Around each individual canal are seen smaller spaces arranged in a circle. These are known as the lacunae (small lakes). Going from the lacunae are smaller canals which take on the name canaliculae, and joining all the lacunae together, making the appearance of concentric circles, we have the lamellae. The outside covering of the bone is called the periosteum and the inside covering is called the endosteum. Most of the long bones and many of the smaller bones are supplied by a nutrient artery, which enters the bone near its center, enters the bone marrow, and divides into two branches, one going up and the other down in the marrow. The blood is then distributed through the Haversian canal system. Veins emerge from the long bones in three places: 1. One or two large veins accompany the nutrient artery. 2. Numerous veins emerge from the articular extremities. 3. Many small veins arise in and emerge from the compact substance.

Bones are divided, according to shape, into four classes: long, short, flat and irregular.

_Long Bones._—These bones are usually used as a system of levers to confer the power of locomotion. A long bone consists of a shaft and two extremities. The shaft is a hollow cylinder within which is the medullary canal. The extremities are somewhat expanded for the purpose of articulation, and to afford a broad surface for the attachment of muscles. The long bones are as a rule curved in two directions to give greater strength to the bone. Some examples of this class of bone are the clavicle, radius, ulna, humerus, femur, tibia, fibula, metacarpal, metatarsal, and the phalanges.

_Short Bones._—These bones are placed in that part of the skeleton where there is need for strength and compactness, and where the motion of the part is slight and limited. Some examples of this class of bone are the bones of the carpus and tarsus (in the hand and the foot).

_Flat Bones._—Flat bones are found where the principle requirement is either extensive protection, or the need of a broad surface for the attachment of muscles. Some of the bones of this class are the occipital, parietal, frontal, nasal, lachrymal, vomer, scapula, sternum, and the ribs.

_Irregular Bones._—These bones are such as from their peculiar shape and form can not be grouped under any of the preceding heads. Some of the bones of this class are the vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, etc.

If the surface of a bone is examined, certain articular and non-articular eminences and depressions will be seen.

_Articular Eminences._—Examples of this class are found in the heads of the humerus and the femur.

_Articular Depressions._—Examples of this class are found in the glenoid cavity of the scapula and the acetabulum.

_Non-articular Eminences._—These are designated according to their form.

A tuberosity is a broad, rough, and uneven elevation.

A tubercle is a small, rough prominence.

A spine is a sharp, slender, pointed eminence.

A ridge, line, or crest is a narrow, rough elevation, running some way along the surface.

_Non-articular Depressions._—These are of variable form, and are described as notches, sulci, fossae, grooves, furrows, fissures, etc. These non-articular eminences and depressions may serve to increase the extent of surface for the attachment of ligaments and muscles or may receive blood vessels, nerves, tendons, ligaments, or portions of organs.

Canals or foramina are channels or openings in bone through which pass the nerves and blood vessels.

=Teeth.=—In the human body we find two sets of teeth. One appearing in childhood, and are known as milk teeth, twenty in number, the permanent teeth replacing these about the sixth year.

There are thirty-two permanent teeth, divided into four incisors, two canines, four bicuspids and six molars.

Teeth are made up of three different substances, which are known as enamel, dentine and cement.

The enamel is a very hard substance, the hardest in the body, and may be compared to quartz. The enamel covers the entire tooth down as far as the gums.

The cement is a continuation of the enamel below the gums, and is closely adherent to the dentine. The cement consists of bone tissue, but the lamellae as a rule do not contain Haversian canals.

The dentine is, next to the enamel, the hardest tissue of the tooth, and composes the main body of the tooth. The pulp cavity is found within the center of the tooth, with the opening toward the jaw bone. The tooth is nourished by a nutrient artery and vein and nerve which pass into the pulp of the tooth.

=Nerves.=—Nerves are divided into two general classes, called medullary and non-medullary nerves. The non-medullated type arise mostly from the sympathetic system, while the medullated type arise from the brain and cord. As a rule, the nerves of the body follow the course of the arteries, and are generally found in the same sheath with the artery and vein.

They are easily distinguished from the arteries and veins by touch and by their color, being very inelastic and fibrous, hard to the touch, and unlike the artery or vein, since they have no central opening.

=Muscles.=—Myology is that branch of anatomy which treats of the muscles. The muscles are formed of bundles of reddish fibres, endowed with the property of contractility. In the body we find two kinds of muscular tissue, called voluntary and involuntary muscle. The voluntary type is characterized by the striped appearance which it displays when seen under the microscope, and for this reason it is called striped or striated muscle. It is so named “voluntary” because it is capable of being put into action and controlled by the will. The involuntary muscles do not present any striped appearance, and consequently are called unstriped or non-striated, and are not under the control of the will. An example of voluntary muscle would be any muscle of the bony framework as for example, the biceps or triceps.

An example of involuntary muscle would be those of the intestines and stomach, the muscles of the bladder and uterus and the walls of the arteries and veins, etc.

When viewed under the microscope, the muscle is seen to be composed of many fibrils. The sheath covering each fibril is called the sarcolemma, and contains within its boundaries the muscle plasma, or protoplasm, and a nucleus. Many of the fibrils when grouped together constitute the entire muscle.

The muscles get their blood supply from the nutrient artery, which ramifies the tissues, the smallest capillaries coming in contact with each muscle cell.

=Tendons.=—Tendons are white, glistening, fibrous cords, varying in length and thickness, sometimes round, sometimes flattened, of considerable strength, and devoid of elasticity. It consists principally of a substance which yields gelatin.

Tendons do not have a direct blood supply.

=Aponeuroses.=—Aponeuroses are flattened or ribbon-like tendons, of a pearly-white color, irridescent, glistening, and similar in structure to the tendons.

=Ligaments.=—Ligaments consist of bands of various forms, serving to connect the articular extremities of bones. They are strong bands of smooth, silverwhite fibrous tissue.

A ligament is pliable and flexible, so as to allow the most perfect freedom of movement, but at the same time it is tough and strong, so as not to yield readily under the severe applied force, and for this reason they serve as good connecting links for the binding of bones together.

_Poupart's Ligament._—Poupart's ligament extends from the crest of the ilium to the top of the pubic bone. This ligament is of utmost importance to the embalmer, as it serves as a guide to locate the femoral artery. By placing the thumb on the crest of the ilium and the second finger on the top of the public bone, then letting the first finger drop midway between the two, which would be the center of Poupart's ligament, we have a point which marks the exit of the artery from the body and the beginning of the femoral artery.

Poupart's ligament also forms the base of Scarpa's triangle. The structure of this triangle will be taken up later.

=Fat.=—Fat is a deposit of an oil in the cells of the tissues, just beneath the skin, giving roundness and plumpness to the body, and acting as an excellent non-conductor for the retention of heat.

So tiny are these cells, that there are over sixty-five million in a cubic inch of fat. As they are kept moist, the liquid does not ooze out; but, on drying, it comes to the surface, and thus a piece of fat feels oily when exposed to the air. The quantity of fat varies with the state of nutrition. In corpulent persons, the masses of fat beneath the skin, in the mesentery, on the surface of the heart and the great vessels, between the muscles, and in the neighborhood of the nerves, are considerably increased. Conversely, in the emaciated we sometimes find beneath the skin cells which contain only one oil drop. Many masses of fat which have an important relation to muscular actions—such as the fat of the orbit or the cheek, do not disappear in the most emaciated persons. Even in starvation, the fatty substance of the brain and spinal cord are retained.

Fat collects as pads in the hollows of the bones, around the joints and between the muscles, causing them to glide more easily upon each other. As marrow, it nourishes the skeleton, and also distributes the shock of any jar the limb may sustain.

Fat does not gather within the cranium, the lungs or the eyelids, where its accumulation would clog the organs.

=Mucous Membranes.=—Mucous membranes line all the open cavities of the body, or all those cavities which communicate with the outside.

At the edges of the openings into the body, the skin seems to stop and give place to a tissue which is redder, more sensitive, more liable to bleed, and is moistened by a fluid or mucous, as it is called. Really, however, the skin does not cease, but passes into a more delicate covering of the same general structure, and it is to this that the name mucous membrane is applied.

The entire alimentary canal, the entire respiratory tract, and the genito-urinary tract, are lined with a mucous membrane. Mucous membrane secretes a mucous fluid.

=Serous Membranes.=—Serous membranes line the closed cavities of the body. The pleurae, the pericardium and the peritoneum are examples of serous membranes. Serous membranes secrete a serous fluid.

=Synovial Membranes.=—Synovial membranes are serous in character, and consist of loose connective tissue, containing fat, vessels and nerves, its inner surface being usually lined with secreting cells. The fluid secreted is yellowish-white or slightly reddish, resembling very much the white of an egg. It contains fats, salts, albumen, extractives from the lymph, and a fluid known as synovia. The chief function of this fluid is to act as an oil to lubricate the joints and surfaces in which there is any friction.

Synovial membranes are divided into three classes, known as articular, bursal and vaginal.

_Articular synovial membranes_ are found in every free movable joint.

_Bursal synovial membranes_ are sacs interposed between the surfaces which move upon each other, producing friction, as in the gliding of a tendon or of the integument over projecting bony surfaces.

_Vaginal synovial membranes_ serve to facilitate the gliding of a tendon in the bony canal through which it passes.

=Arteries.=—The arteries are cylindrical vessels which serve to convey the blood from both ventricles of the heart to every part of the body. They are called arteries from the Greek words which mean “to contain air,” and they were supposed, by our ancients, to have this function until the time of Galen, when he refuted this opinion and showed that these vessels, though for the most part empty after death, actually contained blood. The distribution of the arteries may be compared to a tree, the common trunk of which corresponds to the aorta, and the smallest twigs corresponding to the minute capillaries. When one artery communicates with another it is said to anastomose, and this communication is very free between the larger as between the smaller branches. Anastomosis between trunks of equal size is found where great activity of the circulation is requisite, as at the base of the brain, where the two vertebrals unite to form the basilar artery.

In the limbs and arms the anastomoses are more numerous and of larger size around the joints. The branches of the artery above, unite with branches, from the vessels below. These anastomoses are called collateral circulations. The principal ones of interest to the embalmer are those of the deep brachial uniting with the recurrent radial and ulnar arteries, forming the collateral circulation in the arm; the deep femoral uniting with the recurrent posterior and anterior tibials, forming the collateral circulation in the leg; the superficial and deep mammary arteries, branches of the subclavian artery uniting with the superficial and deep epigastric arteries, branches of the external iliac, forming the collateral circulation over the abdomen and chest, and may be considered the longest collateral circulation in the body.

A terminal artery is one which forms no anastomoses; such vessels are found in the heart, brain, spleen, kidneys, lungs and mesentery.

_Structure._—An artery consists of an internal, a middle and an external coat.

The inner coat consists of endothelial cells and elastic fibrous tissue, sometimes arranged longitudinally, but usually they form a distinct fenestrated membrane (similar to a doorscreen).

The middle coat consists mostly of elastic tissue and white fibrous tissue.

The external coat is called the fibrous coat. It contains fibrous connective tissue and elastic tissues.

_Vasa-Vasorum._—Running in the outer wall of the artery, we find small capillary vessels, and their function is that of nourishing the outer wall, for the blood which passes through the artery does not nourish the artery from within, but depends on these small capillaries, called vasa-vasorum, for their nutrition.

The individual sheath, or arterial sheath, the covering for the artery, is composed of connective tissue, and at places may adhere very tightly to the artery.

=Veins.=—The veins are the vessels which carry the blood from the capillaries back to the right auricle of the heart, and are found in nearly every tissue of the body. They commence as venous capillaries, uniting together into larger and larger veins, until we have the great ascending and descending venae cavae. In form the veins are perfectly cylindrical, like the arteries, but with this difference, that their walls collapse when empty and that they contain valves.

_Structure._—The vein has about the same structure as the artery, only that the middle coat is much thinner and less elastic than the artery, and for this reason it easily collapses.

Veins are divided into superficial, deep and sinuses. Superficial veins are found between the layers of the superficial fascia, just underneath the skin.

Deep veins accompany the arteries, and are usually enclosed in the same common sheath with the artery.

Sinuses are venous channels, which in their structure and mode of distribution differ altogether from the veins. They are found only in the interior of the skull, and consist of channels formed by a separation of the two layers of the dura mater.

=Blood.=—The blood of the body is contained in a practically closed system of tubes, the blood vessels, within which it is kept circulating by force of the heart beat. It is usually spoken of as the nutritive liquid of the body, but the functions may be stated explicitly, although still in quite general terms, by saying that it carries to the tissues food stuffs after they have been properly prepared by the digestive organs; that it transports to the tissues oxygen, absorbed from the air by the lungs; that it carries from the tissues various waste products formed in the processes of dissimilation; that it is the medium for the transmission of the internal secretion of certain glands; that it aids in equalizing the temperature and water contents of the body.

The total quantity of blood in the body has been determined approximately for man as one-thirteenth of the body weight. The specific gravity of human blood in the adult may vary from 1.041 to 1.067, the average being about 1.055.

The blood is composed of a liquid part, the plasma, in which float a vast number of microscopical bodies, the blood corpuscles, known respectively as the red corpuscles, the white corpuscles or leucocytes, of which in turn there are a great many different kinds, and the blood plates.

_Blood plasma_, when obtained free from corpuscles, is perfectly colorless, in thin layers, for example, in microscopical preparation; when seen in large quantities it shows a slightly yellowish tint. The red color of the blood is not due, therefore, to coloration of the blood plasma, but is caused by the mass of red corpuscles held in suspension in the liquid. The proportion by bulk of plasma to corpuscles is usually given roughly as two to one. The blood plasma is composed of two substances, blood serum and blood fibrin. You have noticed that blood, after it has escaped from the vessels, usually clots or coagulates. The clot, as it forms, gradually shrinks and squeezes out a clear liquid, to which the name blood serum has been given. Serum resembles the plasma of normal blood in general appearance, but differs from it in composition. Here it is sufficient to say that blood serum is the liquid part of the blood after coagulation has taken place. You can prepare this experiment for yourself: If shed blood is whipped vigorously with a rod or some similar object while it is clotting, the essential part of the clot, namely the fibrin, forms differently from what it does when the blood is allowed to coagulate quietly. It is deposited in shreds on the whipper. Blood that has been treated in this way is known as defibrinated blood. It consists of blood serum plus the red and white corpuscles, and as far as appearances go it resembles exactly the normal blood; it has lost, however, its power of clotting.

_Red blood corpuscles_ are bi-concave, circular disks, without nuclei; their average diameter is 7.7 microns (1 micron equals 1-25,000 of an inch); their number, which is usually reckoned as so many to a cu. millimeter, varies greatly under different conditions of health and disease. The average number is given as 5,600,000 per cubic millimeter for males and 4,500,000 per cubic millimeter for females.

The number of red corpuscles also varies in individuals with the constitution, nutrition and manner of life. It varies with age, being greatest in the fetus and in the new-born child. It varies with the time of the day, showing a distinct diminution after meals. In the female it varies somewhat with menstruation and pregnancy, being slightly increased in the former and diminished in the latter condition.

The red color of the corpuscles is due to the presence in them of a pigment, known as hemoglobin. Owing to the minute size of the corpuscles, their color when seen singly under the microscope is a faint yellowish red, but when seen in mass they exhibit the well-known blood-red color, which varies from a scarlet in arterial blood to a purplish red in venous blood, this variation in color being dependent upon the amount of oxygen contained in the blood in combination with the hemoglobin. The function of the red blood corpuscles is to carry oxygen from the lungs to the tissues. This function is entirely dependent upon the presence of hemoglobins, which have the power of combining easily with the oxygen gas.

_White blood corpuscles or leucocytes_ contain no hemoglobin or coloring matter. They have a nucleus or center spot. Their size varies from 5 to 12 microns, and are less numerous than the red corpuscles, being in this proportion: one white corpuscle to 500 red corpuscles. The chief functions of the white corpuscles are: (1) That they protect the body from pathogenic or disease-producing bacteria. In explanation of this action it has been suggested that they may either ingest the bacteria and thus destroy them directly, or they may form certain substances, defensive proteids, that destroy the bacteria. White corpuscles that act by ingesting the bacteria are spoken of as phagocytes (meaning to eat the cell). (2) They aid in the absorption of fats from the intestines. (3) They aid in the absorption of peptones from the intestines. (4) They take part in the process of blood coagulation. (5) They help in maintaining the normal composition of the blood plasma in proteids.

_Blood plates_ are small circular or elliptical bodies, nearly homogeneous in structure, variable in size, always much smaller than the red blood corpuscles. Less is known of their origin, fate and functions than in the case of the other blood corpuscles, but there is some considerable evidence to show that they take part in the process of coagulation or clotting.

_Coagulation of the Blood._—One of the most striking properties of the blood is its power of clotting, or coagulating, shortly after it leaves the blood-vessels, or if any foreign elements come in contact with it. The general changes in the blood during this process are easily followed. At first perfectly fluid, in a few minutes it becomes viscous, and then sets into a soft jelly, which quickly becomes firmer, so that the vessel containing it can be inverted without spilling the blood. The clot continues to grow more impact, and gradually shrinks in volume, pressing out a greater or smaller amount of clear, faintly yellow liquid, to which the name blood serum is given. The essential part of the clot is the fibrin.

Fibrin is an insoluble proteid not found in normal blood. In shed blood, and under certain conditions while still in the blood-vessels, this fibrin is formed. In forming, it shows an exceedingly fine network of delicate threads that permeate the whole mass of the blood and gives the clot its jelly-like character. The shrinking of the threads causes the subsequent contraction of the clot. If the blood has not been disturbed during the act of clotting, the red corpuscles are caught in the fine fibrin mesh-work, and as the clot shrinks these corpuscles are held more firmly, only the clear liquid of the blood being squeezed out, so it is possible to get specimens of serum containing few or no red blood corpuscles. The white corpuscles or leucocytes, on the contrary, although they are also caught at first in the forming meshes of fibrin, in latter stages of the clotting they readily pass out into the serum, on account of their power of having movement. If the blood has been agitated during the process of clotting, the delicate net work will be broken in places, and the serum will be more or less bloody—that is, it will contain numerous red blood corpuscles. If during the time of clotting the blood is vigorously whipped with a bundle of fine rods, all the fibrin is deposited as a stringy mass on the whipper, and the remaining liquid part consists of serum plus red corpuscles. Blood that has been whipped in this way is known as defibrinated blood. It resembles normal blood in appearance, but is different in composition; it can not clot again. The way in which fibrin is normally deposited can be easily demonstrated by taking a drop of blood on a slide and covering it with a cover slip, allow it to stand several minutes until coagulation is complete, and view under a microscope. If the drop is examined, it is possible by careful focusing, to discover in the spaces between the masses of corpuscles many examples of delicate fibrin net work. The physiological value of the clotting of blood in life is that it stops hemorrhages by closing the openings of the wounded blood vessels, but the clotting of the blood after death, is to the embalmer one of the bugbears, and a real method of preventing it, or of dissolving the clot after it has once formed in the blood vessels is one of those difficult problems which remains as yet unsolved.

Since we have no real method of preventing coagulation in the blood vessels, let us search out the things which will hasten or retard this coagulation. Blood coagulates normally within a few minutes after it is liberated from the blood vessel, but this process may be hastened by increasing the amount of foreign substance with which it comes in contact. Thus the agitation of the liquid in quantity or the application of a sponge or handkerchief or the application of heat hastens the onset of clotting.

Coagulation in drawn blood may be retarded or prevented altogether by a variety of means, of which the following are the most important:

(1) By cooling.

(2) By the action of neutral salts.

(3) By the action of oxalate solutions.

(4) By the action of sodium fluoride.

Summary.—To summarize then, the following statements may be made:

(1) The immediate factor necessary to the clotting of the blood is the fibrin.

(2) That blood does not clot normally in the blood vessels before death.

(3) That after death blood remains for a long time without clotting, provided some outside agent is not introduced to cause it.

Such an agent may be the blood coming in contact with the air, or the blood drainage tube. The one point then to be emphasized is that when a vein is cut, and the blood begins to flow, you know that the blood is not in a coagulated condition. Then work rapidly, put the blood drainage tube quickly into the vein and draw off as much blood as you can before it begins to clot at the end of the tube. The great trouble has been, that the embalmer does not work with precision. He first raises the vein, and exposes it on the surface of the incision. He then raises the artery. He places the drainage tube into the vein, but shuts it off till he is ready with the artery. Now, by the time he has placed the arterial tube in the artery, injected a few bulbs full to see that all is in working order, and has perhaps attended to a few other duties, he is amazed to find that the blood will not flow, that it has clotted. What is the reason? He gave it time to clot after the drainage tube was inserted.

A better procedure would be not to touch the vein until every other procedure has been attended to. Then raise the vein, insert the drainage tube and withdraw the blood quickly, and at the same time keep injecting slowly into the arterial system to keep up the needed pressure to keep the blood flowing.

(4) That when a clot is once formed in a blood vessel, it is not dissolved by the addition of fluid or any other solution.

(5) That sometimes when the blood has become clotted at the end of the drainage tube, it can be loosened up or be slightly pushed away by attaching the pump to the drainage tube and injecting a few bulbs of fluid, which, when it runs out, will again start the flow of blood.