CHAPTER VII.

ORGANOLOGY.—Continued.

=The Thoracic Cavity.=—The thorax, or chest is a bony, cartilaginous cage. It contains and protects the principle organs of respiration and circulation.

The thorax is bounded in front by the sternum and costal cartilages, behind by the twelve dorsal vertebrae and the posterior parts of the ribs, on the sides by the ribs, above by the root of the neck and below by the diaphragm.

In the female the thorax differs as follows from the male: Its general capacity is less, the sternum is shorter, and the upper ribs are more movable and so allow a greater enlargement of the upper part of the thorax than the male.

The capacity of the cavity of the thorax does not correspond with its apparent size externally, because, (1) the space enclosed by the lower ribs is occupied by some of the abdominal viscera; and (2) the cavity extends above the first rib into the neck. The size of the cavity of the thorax is constantly varying during life, with the movements of the ribs and diaphragm, and with the degree of distention of the abdominal viscera.

From the collapsed state of the lungs, as seen when the thorax is opened, in the dead body, it would appear as if the viscera only partly filled the cavity of the thorax, but during life there is no vacant space, that which is seen after death being filled up during life by the expanded lungs.

=Larynx.=—The larynx is the organ of voice, placed at the upper part of the air passage. It is situated between the trachea and the base of the tongue, at the upper and forepart of the neck, where it forms a considerable projection in the middle line. It is for this reason that it is of considerable importance to embalmers, for it is just opposite this projection, on either side of the neck, that the common carotid divides into the internal and the external carotid.

On either side of it lie the great blood vessels of the neck, behind it forms a part of the boundary of the pharynx, and is covered by the mucous membrane lining that cavity.

Its vertical extent corresponds to the fourth, fifth, and sixth cervical vertebrae. It is placed somewhat higher in the female than in the male.

The movements of the head affect the position of the larynx. When the head is drawn back, the larynx is lifted, and when the chin approaches the chest the larynx is depressed. During swallowing the larynx moves distinctly; during singing it moves slightly.

Until puberty there is no marked difference between the larynx of the male and that of the female. In the male after puberty all the cartilages increase in size, and the larynx becomes prominent as the Adam's apple in the middle line of the neck. In the female after puberty the increase of size is only slight.

The larynx is broad above, where it presents a triangular appearance, flattened behind and at the sides. Below it is narrow and cylindrical.

It is composed of cartilages which are connected together by ligaments and moved by numerous muscles. It is lined by a mucous membrane which is continuous above with the lining of the pharynx and below with that of the trachea.

The arteries that supply the larynx are the laryngeal arteries, branches of the superior and inferior thyroid arteries.

The superior laryngeal vein runs into the superior thyroid vein and then into the internal jugular vein, while the inferior laryngeal vein runs into the inferior thyroid vein and then into the innominate vein.

=The Trachea.=—The trachea or windpipe is a cartilaginous elastic, cylindrical tube, flattened posteriorly. It extends from the lower part of the larynx, on a level with the sixth cervical vertebra to opposite the body of the fourth dorsal, where it divides into two bronchi, one for each lung.

The trachea is in the median line of the body. It measures about four and one-half inches in length. The diameter is from three quarters to one inch, being always greater in the male than in the female.

The trachea is composed of imperfect cartilage rings, not coming quite together in the back.

The artery that supplies the trachea is the inferior thyroid artery.

The vein that withdraws the blood is the inferior thyroid vein.

_The Right Bronchus._—The right bronchus is shorter, and wider than the left bronchus. It is about one inch in length. It enters the lung opposite the fifth dorsal vertebra.

_The Left Bronchus._—The left bronchus is smaller and longer than the right. It is two inches in length and enters the lung at a point opposite the body of the sixth dorsal vertebra.

Each bronchus divides into smaller divisions called bronchial tubes.

Each bronchial tube divides into still smaller divisions called bronchioles.

Each bronchiole ends in the air cell.

=The Pleurae.=—Each lung is invested upon its external surface by an exceedingly delicate serous membrane, the pleura. This encloses the organ as far as its root, and is then reflected upon the inner surface of the thorax.

The _pulmonary pleura_ is the portion investing the surface of the lung, and dipping into the fissures between its lobes.

The _parietal pleura_ is that which lines the inner surface of the chest.

The space between these two layers is called the cavity of the pleurae, (the pleural cavity); and contains nothing but a very little clear fluid.

In the healthy condition the two layers are in contact and there is no real cavity, but after death the lungs become collapsed and separate from the walls of the chest. Each pleura is therefore a shut sac, one occupying the right, and the other the left half of the thorax, and they are perfectly separated from one another. The two pleurae do not meet in the middle line of the chest, excepting for a short distance between the second and third pieces of the sternum—a space being left between them, which contains all the viscera of the thorax excepting the lungs; this is called the mediastinum.

The mediastinum then, is the space between the right and left pleural sacs.

The arteries of the pleura are derived from the intercostal, internal mammary, musculo-phrenic, thymic, pericardiac, bronchial.

The veins correspond to the arteries.

=The Lungs.=—The lungs are the essential organs of respiration. They are two in number, placed one on each side of the chest, separated from each other by the heart and the contents of the mediastinum. A healthy lung hangs free within the pulmonary space. The lung is suspended by the root. The root of the lung is formed by the bronchial tubes, pulmonary artery, pulmonary veins, bronchial arteries, bronchial veins, etc., all of which are enclosed by the reflections of the pleurae.

The _root of the lung_ may be described as being that part where all the great blood vessels and the bronchial tubes, enter the lungs.

In many cases the lung does not hang free, but as a result of former pleurisy, the area of the pulmonary pleura is adherent to the parietal pleura.

Each lung is conical in shape, and presents for examination, an apex, a base, and two surfaces.

_The Apex_ forms a tapering cone which extends into the root of the neck about an inch and a half to two inches above the level of the top of the first rib.

_The Base_ is broad and concave and rests upon the convex surface of the diaphragm, which separates the right lung from the upper surface of the right lobe of the liver and the left lung from the upper surface of the left lobe of the liver, the stomach, and spleen.

_Surfaces._—There are two in number. The external, costal or thoracic surface is smooth, convex and corresponds to the form of the cavity of the chest. The inner or mediastinal surface is concave, and the middle portion, where all the vessels enter and leave the lung is called the root.

_Lobes._—Each lung is divided up into lobes. The right lung has three lobes, and the left lung has two lobes.

_Weight._—The weight of both lungs together is about 42 ounces, the right lung being a little heavier than the left. The lungs are heavier in the male than in the female. The male lungs weigh from 42 to 45 ounces, and the female lungs weigh from 32 to 35 ounces.

_Color._—The color of the lungs at birth is a pinkish white, in adult life a dark slate color, mottled in patches and as age advances this mottling assumes a black color.

_Substance._—The substance of the lung is of a light porous, spongy texture. It floats in water, if it has once been filled with air. It is elastic and for this reason we always find the lung collapsed after death.

The structure of the lung is such that the blood brought by the pulmonary artery comes into close relation with the air in the air-cells which enters from the bronchioles. The blood gives off carbon dioxide to the air-cells and the air in the cells furnishes oxygen for the blood. The process of respiration causes the dark blood brought from the heart by the pulmonary arteries to return to the heart as red blood in the pulmonary veins.

_Arteries._—The bronchial arteries supply the lungs with nutrition.

The pulmonary arteries convey venous blood from the heart to the lungs to be purified.

_Veins._—The bronchial veins carry off the impure blood from the lungs.

The pulmonary veins convey the blood which has been purified by the lungs, back to the heart.

=The Mediastinum=.—The mediastinum is the space left in the middle portion of the chest by the non-approximation of the two pleurae. It extends from the sternum in front to the spine behind.

Within it are the contents of the thorax, excepting the lungs. The mediastinum may be divided into two parts.

_The superior mediastinum_ is that portion of the interpleural space which lies above the level of the pericardium. This space contains the arch of the aorta, innominate, part of the left carotid artery, part of the left subclavian artery, the upper half of the superior vena cava, the upper half of the innominate vein, the left superior intercostal vein, trachea, esophagus, thoracic duct, remains of the thymus gland, etc.

The inferior mediastinum is divided into three portions:

_The anterior mediastinum_ is that portion in front of the pericardium. It contains nothing but some loose areolar tissue.

_The posterior mediastinum_ is that portion back of the pericardium. It contains the descending thoracic aorta, the greater and lesser azygos veins, the esophagus, the thoracic duct, etc.

_The middle mediastinum_ is that part within the pericardium or heart sac. It is the largest space of all the mediastinal spaces. It contains the heart, the ascending aorta, the lower half of the superior vena cava, the vena azygos, the bifurcation of the trachea, the pulmonary artery, etc.

The middle mediastinum is sometimes called the cardiac cavity, because it contains the heart.

=The Pericardium (Heart Sac).=—The pericardium is a serous sac in which is located the heart and the commencement of the great blood vessels.

Behind we find the bronchi, esophagus and descending thoracic aorta. To the sides we find the pleura, the phrenic nerve and the accompanying vessels. In front we find the sternum and the remains of the thymus gland. It is attached above to the great blood vessels and below to the diaphragm.

=The Heart.=—The heart is a hollow, muscular organ of a conical (cone shaped) form, placed between the lungs and enclosed in the pericardium.

The heart is placed obliquely in the chest. The base is directed upward, backward and to the right, and corresponds to the dorsal vertebrae from the fifth to the eighth inclusive.

The apex is directed downward, forward and to the left and corresponds to the space between the cartilages between the fifth and sixth ribs.

The exact location of the apex of the heart would be ¾-inch to the inner side, and an inch and one-half below the left nipple, or about three and one-half inches from the middle line of the sternum or breast bone.

The heart is placed behind the sternum, and extends about three inches to the left of the median line, and about one and one-half inches to the right, or in other words, about one-third of the heart lies to the right of the median line, and two-thirds lies to the left of the median line.

The heart in the adult measures five inches in length, three and one-half inches in breadth in its broadest part, and two and one-half inches in thickness. The weight of the male heart varies from ten to twelve ounces, and that of the female from eight to ten ounces.

The capacity of the ventricles of the heart averages about three and one-half ounces of blood to each ventricle, and the auricle a little less than four ounces, making the total capacity of the heart average about fifteen ounces.

The heart is divided by a muscular septum (separation wall) into two lateral halves, which are named respectively the right or venous side and the left or arterial side. The septum is called the longitudinal septum. Each side of the heart is further sub-divided into an upper and lower compartment, the upper on each side is called the auricle and the lower the ventricle. The upper and lower compartments of the heart (auricles and ventricles) are separated by the auricular-ventricular septums (meaning a separation between the auricle and ventricle).

The superior and inferior venae cavae empty into the right auricle of the heart, also the blood from the coronary sinus.

In fact, this compartment receives all the venous or impure blood from all parts of the body, and sends it through what is known as the tricuspid valve into the right ventricle or lower compartment. After getting into the right ventricle, the blood is sent forth into the lungs by first passing through the pulmonary semi-lunar valve into the pulmonary artery, which enters the lungs at the root of the same.

This would then finish the circulation through the right side of the heart, and after the purification has been accomplished by the lungs, we find the blood being returned to the left side of the heart through the four pulmonary veins. The pulmonary veins extend from the lungs (two on each side) to the left auricle (upper compartment of the heart) and deliver the purified blood to the left or arterial side. The course of the blood from the left auricle is downward into the left ventricle (or lower compartment) through what is known as the bicuspid or mitral valve.

The blood is then sent out into the body to nourish all the tissues, by being forced through the aortic semi-lunar valve into the great aorta artery. The circulation is then completed by the blood running into the branch arteries and from them into the smaller branches and into the capillaries from which the course of the blood is into the smaller veins and into the larger veins, finally terminating into the two large trunk veins, the ascending (or inferior) and descending (or superior) venae cavae. Of these two large trunk veins the ascending vena cava is the only one to have a valve at its termination (eustachian). The functions of this valve are to prevent a backward flow of blood into the vein from the auricle.

The heart has three walls, the inner wall is called the endocardium, the middle wall is called the myocardium, and the outer wall is called the epicardium.

The heart is surrounded by a serous sac called the pericardium.

The heart receives its blood supply from the coronary arteries, which are branches of the ascending aorta, just after it leaves the aortic semi-lunar valve.

The coronary veins bring the venous blood back from the tissues of the heart and empty into the coronary sinus, back of the right auricle of the heart.

The veins which originate about the region of the right auricle, empty directly into the right auricle of the heart through the valves of Thebesii.

=The Alimentary Canal.=—The alimentary canal is a muscular membranous tube. It is about thirty feet in length, and extends from the mouth to the anus. It is lined throughout by a mucous membrane.

The following outline will show the parts of the alimentary canal:

Mouth Pharynx Oesophagus Stomach { Duodenum Small Intestines { Jejunum { Ileum

{ Caecum Large Intestines { Colon { Rectum

The accessory organs to the alimentary canal are the following:

Teeth, Salivary glands, Liver, Spleen, Pancreas.

=The Mouth.=—The mouth is placed at the commencement of the alimentary canal. It is a nearly oval shaped cavity.

In this cavity the mastication of the food and the insalivation of the food takes place.

=The Teeth.=—The structure of the teeth has been considered under the head of tissues.

=The Palate.=—The palate forms the roof of the mouth. It consists of two portions: The hard palate is in front and the soft palate is in the back.

=The Salivary Glands.=—By the term salivary glands is usually understood the three chief glands on each side of the face.

The parotid gland is placed near the ear. The submaxillary gland is placed below the jaw. The sublingual gland is placed below the tongue.

These glands secrete the salival juices which are brought into the mouth by three small ducts, where it aids in the digestion of the food. The digestive action of the saliva is limited to the starchy foods. Its action is to change starches into sugars.

It also fulfills other important functions. By moistening the food it enables us to reduce the material to a consistency suitable for swallowing and for manipulation by the tongue and other muscles. The saliva also serves as a kind of lubricator that insures the smooth passage along oesophageal canal.

=The Pharynx.=—The pharynx is that part of the alimentary canal, which is placed behind, and communicates with the nose, mouth and larynx. It is a muscular, membranous tube which extends from the back of the mouth and under surface of the skull to the level of the cricoid cartilage or to a point between the fifth and sixth cervical vertebrae.

The pharynx is about four and one-half inches in length.

Seven openings communicate with it, as follows:

Two posterior nares, two eustachian tubes, mouth, larynx, esophagus.

=The Esophagus.=—The esophagus or gullet is a muscular canal about nine or ten inches in length, extending from the pharynx to the stomach.

It begins at a point between the fifth and sixth cervical vertebrae and descends along in front of the spine through the posterior mediastinal space, passes through the diaphragm, and entering the abdomen, terminates in the stomach wall at a point opposite the tenth dorsal vertebra.

At its commencement it is placed in the median line and gradually inclines to the left as it passes forward to the esophageal opening to the diaphragm.

The esophagus is from one-half to an inch in diameter.

Arteries.—The arteries which supply the esophagus are the esophageal, which are branches from the aorta.

Veins.—The esophageal veins empty into the ascending vena cava.

=The Diaphragm.=—The diaphragm (a partition wall) is a dense, muscular, fibrous septum, placed obliquely across the trunk. It separates the thoracic from the abdominal cavity, forming the floor of the thoracic and the roof of the abdominal cavity.

It is attached in front to the ensiform process of the sternum, on the sides to the inner surface of the cartilages and bony portions of six or seven inferior ribs, and behind it is attached to the lumbar vertebrae.

The diaphragm has three openings, as follows: opening for the esophagus, opening for the aorta, opening for the ascending vena cava.

The diaphragm is the principal muscle of respiration.

The arteries which supply the diaphragm are the phrenic arteries.

The phrenic veins receive the blood from the diaphragm.