Part 2

=Descriptive Terms.=—In order to indicate precisely the position and direction of parts of the body, certain descriptive terms are employed, and must be understood at the outset. In the explanation of these terms it is assumed here that they apply to a quadruped such as the horse in the ordinary standing position. The surface directed toward the plane of support (the ground) is termed =inferior= or =ventral=, and the opposite surface is =superior= or =dorsal=; the relations of parts in this direction are named accordingly. The longitudinal =median plane= divides the body into similar halves. A structure or surface which is nearer than another to the median plane is =internal= or =medial= to it, and an object or surface which is further than another from the median plane is =external= or =lateral= to it. Planes parallel to the median plane are =sagittal=. =Transverse= or =segmental= planes cut the long axis of the body perpendicular to the median plane, or an organ or limb at right angles to its long axis. A =frontal= plane is perpendicular to the median and transverse planes. The head end of the body is termed =anterior=, =cephalic=, or =cranial=; and the tail end =posterior= or =caudal=; relations of structures with regard to the longitudinal axis of the body are designated accordingly. Certain terms are used in a special sense as applied to the limbs. =Proximal= and =distal= express relative distances of parts from the axis of the body. The anterior face of the thoracic limb from the elbow downward is also termed =dorsal=, and the opposite face =volar=. In the corresponding part of the pelvic limb the terms are =dorsal= and =plantar= respectively. In the same regions =radial= and =ulnar= (thoracic limb), =tibial= and =fibular= (pelvic limb), may be used to designate that side of the extremity on which the corresponding bone is situated; they are therefore equivalent respectively to internal or medial and external or lateral in the animals with which we are concerned.

It is evidently advantageous to employ terms which are as far as possible independent of the position of the body in space and capable of general application, _e. g._, dorsal, ventral, proximal, etc. It is also desirable that the terms internal and external be reserved to indicate relations of depth in cavities or organs, and medial and lateral to designate relations to the median plane. Such terms are coming into more extensive use in human and veterinary anatomy, but the older nomenclature is very firmly established and cannot well be discarded at once and entirely.

OSTEOLOGY

THE SKELETON

The term =skeleton= is applied to the framework of hard structures which supports and protects the soft tissues of animals. In the descriptive anatomy of the higher animals it is usually restricted to the bones and cartilages, although the ligaments which bind these together might well be included.

In zoölogy the term is used in a much more comprehensive sense, and includes all the harder supporting and protecting structures. When the latter are situated externally, they form an =exoskeleton=, derived from the ectoderm. Examples of this are the shells and chitinous coverings of many invertebrates, the scales of fishes, the shields of turtles, and the feathers, hair, and hoofs of the higher vertebrates. The =endoskeleton= (with which we have to deal at present) is embedded in the soft tissues. It is derived chiefly from the mesoderm, but includes the notochord or primitive axial skeleton, which is of entodermal origin.

The skeleton may be divided primarily into three parts: (1) axial; (2) appendicular; (3) splanchnic.

The =axial skeleton= comprises the vertebral column, ribs, sternum, and skull.

The =appendicular skeleton= includes the bones of the limbs.

The =splanchnic skeleton= consists of certain bones developed in the substance of some of the viscera or soft organs, _e. g._, the os penis of the dog and the os cordis of the ox.

The =number= of the bones of the skeleton of an animal varies with age, owing to the fusion during growth of skeletal elements which are separate in the fœtus or the young subject. Even in adults of the same species numerical variations occur, _e. g._, the tarsus of the horse may consist of six or seven bones, and the carpus of seven or eight; in all the domestic mammals the number of coccygeal vertebræ varies considerably.

The bones are commonly divided into four classes according to their shape and function.

(1) =Long bones= (Ossa longa) are typically of elongated cylindrical form with enlarged extremities. They occur in the limbs, where they act as supporting columns and as levers. The cylindrical part, termed the shaft or body (Corpus), is tubular, and incloses the =medullary cavity=, which contains the medulla or marrow.

(2) =Flat bones= (Ossa plana) are expanded in two directions. They furnish sufficient area for the attachment of muscles and afford protection to the organs which they cover.

(3) =Short bones= (Ossa brevia), such as those of the carpus and tarsus, present somewhat similar dimensions in length, breadth, and thickness. Their chief function appears to be that of diffusing concussion. Sesamoid bones, which are developed in the capsules of some joints or in tendons, may be included in this group. They diminish friction or change the direction of tendons.

(4) =Irregular bones.= This group would include bones of irregular shape, such as the vertebræ and the bones of the cranial base; they are median and unpaired. Their functions are various and not so clearly specialized as those of the preceding classes.

This classification is not entirely satisfactory; some bones, _e. g._, the ribs, are not clearly provided for, and others might be variously placed.

STRUCTURE OF BONES[1]

Bones consist chiefly of =bone tissue=, but considered as organs they present also an enveloping membrane, termed the =periosteum=, the =medulla= or =marrow=, =vessels=, and =nerves=.

The architecture of bone can be studied best by means of longitudinal and cross-sections. These show that the bone consists of an external shell of dense =compact substance=, within which is the more loosely arranged =spongy substance=. In typical long bones the shaft is hollowed to form the =medullary cavity= (Cavum medullare).

The =compact substance= (Substantia compacta) differs greatly in thickness in various situations, in conformity with the stresses and strains to which the bone is subjected. In the long bones it is thickest in the middle part of the shaft and thins out toward the extremities. On the latter the layer is very thin, and is especially dense and smooth on joint surfaces.

The =spongy substance= (Substantia spongiosa) consists of delicate bony plates and spicules which run in various directions and intercross. These plates are definitely arranged with regard to mechanical requirements, so that systems of pressure and tension plates can be recognized, in conformity with the lines of pressure and the pull of tendons and ligaments respectively. The intervals (marrow spaces) between the plates are occupied by marrow. The spongy substance forms the bulk of short bones and of the extremities of long bones; in the latter it is not confined to the ends, but extends a variable distance along the shaft also. Some bones (Ossa pneumatica) contain =air-spaces= or =sinuses= within the compact substance instead of spongy bone and marrow. In certain situations the two compact layers of flat bones are not separated by spongy bone, but fuse with each other; in some cases of this kind the bone is so thin as to be translucent, or may even undergo absorption, producing an actual deficiency.

The flat bones of the cranial vault and sides are composed of an outer layer of ordinary compact substance, an inner layer of very dense bone, the =tabula vitrea=, and between these a variable amount of spongy bone, here termed =diploë=.

The =periosteum= is the membrane which invests the outer surface of bone, except where it is covered with cartilage. It consists of an outer protective fibrous layer, and an inner cellular osteogenic layer. During active growth the osteogenic layer is well developed, but later it becomes much reduced. The fibrous layer varies much in thickness, being in general thickest in exposed situations. The adhesion of the periosteum to the bone also differs greatly in various places; it is usually very thin and easily detached where it is thickly covered with muscular tissue which has little or no attachment. The degree of vascularity conforms to the activity of the periosteum.

The =marrow= (Medulla ossium) occupies the interstices of the spongy bone and the medullary cavity of the long bones. There are two varieties in the adult—red and yellow. In the young subject there is only =red marrow= (Medulla ossium rubra), but later this is replaced in the medullary cavity by =yellow marrow= (Medulla ossium flava). The red marrow contains several types of characteristic cells and is a blood-forming substance, while the yellow is practically ordinary adipose tissue.

Since yellow marrow is formed by regressive changes in red marrow, including fatty infiltration and degeneration of the characteristic cells, we find transitional forms or stages in the process. In aged or badly nourished subjects the marrow may undergo gelatinous degeneration, resulting in the formation of gelatinous marrow.

=Vessels and Nerves.=—It is customary to recognize two sets of =arteries=—the =periosteal= and the =medullary=. The former ramify in the periosteum and give off innumerable small branches which enter minute openings (Volkmann’s canals) on the surface and reach the Haversian canals of the compact substance. Other branches enter the extremities of the long bones and supply the spongy bone and marrow in them. In the case of the larger bones—and especially the long bones—the large =medullary= or =nutrient artery= enters at the so-called =nutrient foramen= (Foramen nutricium), passes in a canal (Canalis nutricius) through the compact substance, and ramifies in the marrow; its branches anastomose with the central branches of the periosteal set. The larger =veins= of the spongy bone do not, as a rule, accompany the arteries, but emerge chiefly near the articular surfaces. Within the bone they are destitute of valves.

The =lymph vessels= form perivascular channels in the periosteum and the Haversian canals of the compact substance. Lymph-spaces exist at the periphery of the marrow.

The =nerves= appear to be distributed chiefly to the blood-vessels. Special nerve-endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as sensory, and probably are concerned in mediating the muscle sense (Kopsch).

DEVELOPMENT AND GROWTH OF BONE[2]

The primitive embryonal skeleton consists of cartilage and fibrous tissue, in which the bones develop. The process is termed =ossification= or =osteogenesis=, and is effected essentially by bone-producing cells, called =osteoblasts=. It is customary, therefore, to designate as =membrane bones= those which are developed in fibrous tissue, and as =cartilage bones= those which are preformed in cartilage. The principal membrane bones are those of the roof and sides of the cranium and most of the bones of the face. The cartilage bones comprise, therefore, most of the skeleton. Correspondingly we distinguish =intramembranous= and =endochondral ossification=.

In intramembranous development the process begins at a definite =center of ossification= where the cells (osteoblasts) surround themselves with a deposit of bone. The process extends from this center to the periphery of the future bone, thus producing a network of bony trabeculæ. The trabeculæ rapidly thicken and coalesce, forming a bony plate which is separated from the adjacent bones by persistent fibrous tissue. The superficial part of the original tissue becomes periosteum, and on the deep face of this successive layers of periosteal bone are formed by osteoblasts until the bone attains its definitive thickness.

In endochondral ossification the process is fundamentally the same, but not quite so simple. Osteoblasts emigrate from the deep face of the perichondrium or primitive periosteum into the cartilage and cause calcification of the matrix or ground-substance of the latter. Vessels extend into the calcifying area, the cartilage cells shrink and disappear, forming primary marrow cavities which are occupied by processes of the osteogenic tissue. There is thus formed a sort of scaffolding of calcareous trabeculæ on which the bone is constructed by the osteoblasts. At the same time perichondral bone is formed by the osteoblasts of the primitive periosteum. The calcified cartilage is broken down and absorbed through the agency of large cells called =osteoclasts=, and is replaced by bone deposited by the osteoblasts. The osteoclasts also cause absorption of the primitive bone, producing the marrow cavities; thus in the case of the long bones the primitive central spongy bone is largely absorbed to form the medullary cavity of the shaft, and persists chiefly in the extremities. Destruction of the central part and formation of subperiosteal bone continue until the shaft of the bone has completed its growth.

A typical long bone is developed from three primary centers of ossification, one for the diaphysis or shaft and one for each epiphysis or extremity. Many bones have secondary centers from which processes or apophyses develop.

The foregoing outline accounts for the growth of bones except in regard to length. Increase in length may be explained briefly as follows: Provision for continued ossification at either end of the diaphysis is made by a layer of actively growing cartilage—the =epiphyseal cartilage=—which intervenes between the diaphysis and the epiphysis. It is evident that so long as this cartilage persists and grows, new bone may continue to be formed at its expense, and increase of length is possible. When the epiphyseal cartilage ceases to grow, it undergoes ossification, the bone is consolidated, and no further increase in length is possible. This fusion takes place at fairly definite periods in the various bones, and it is of value to know the usual times at which it occurs in the larger bones of the limbs at least.

After the bones have reached their full size, the periosteum becomes relatively reduced and inactive so far as its osteogenic layer is concerned; the bone-forming function may be stimulated by various causes, as is well seen in the healing of fractures and the occurrence of bony enlargements.

CHEMICAL COMPOSITION OF BONE

Dried bone consists of =organic= and =inorganic= matter in the ratio of 1 ∶ 2 approximately. The animal matter gives toughness and elasticity, the mineral matter hardness, to the bone tissue. Removal of the organic matter by heat does not change the general form of a bone, but reduces the weight by about one-third, and makes it very fragile. Conversely, decalcification, while not affecting the form and size of the bone, renders it soft and pliable. The animal matter when boiled yields gelatin. The following table represents the composition in 100 parts of ox bone of average quality:

Gelatin 33.30 Phosphate of lime 57.35 Carbonate of lime 3.85 Phosphate of magnesia 2.05 Carbonate and chlorid of sodium 3.45 —————— 100.00

PHYSICAL PROPERTIES OF BONE

Fresh dead bone has a yellowish-white color; when macerated or boiled and bleached, it is white. The specific gravity of fresh compact bone is a little over 1.93. It is very hard and resistant to pressure; a 5-millimeter cube of compact bone of the ox will resist pressure up to 852 pounds, if the pressure be applied in the line of the lamellæ (Rauber). Its tensile strength is estimated to be nearly twice that of oak.

DESCRIPTIVE TERMS

The surfaces of the bones present a great variety of eminences and depressions, as well as perforations. The prominences and cavities may be articular, or non-articular, furnishing attachment to muscles, tendons, ligaments or fascia. A number of descriptive terms are used to designate these features, and the following are some of those in general use:

=Process= (Processus) is a general term for a prominence.

A =tuberosity= (Tuber, Tuberositas) is a large, rounded projection; a =tubercle= (Tuberculum) is a smaller one.

The term =trochanter= is applied to a few prominences, _e. g._, the trochanters of the femur.

A =spine= (Spina) or =spinous process= (Processus spinosus) is a pointed projection.

A =crest= (Crista) is a sharp ridge.

A =line= (Linea) is a very small ridge.

A =head= (Caput) is a rounded articular enlargement at the end of a bone; it may be joined to the shaft by a constricted part, the =neck= (Collum).

A =condyle= (Condylus) is an articular eminence which is somewhat cylindrical; a non-articular projection in connection with a condyle may be termed an =epicondyle=.

A =trochlea= is a pulley-like articular mass.

A =glenoid cavity= (Cavitas glenoidalis) is a shallow articular depression, and a =cotyloid cavity= or =acetabulum= is a deeper one.

The term =facet= is commonly applied to articular surfaces of small extent, especially when they are not strongly concave or convex.

The terms =fossa=, =fovea=, =groove= or =sulcus=, and =impression= are applied to various forms of depressions.

A =foramen= is a perforation for the transmission of vessels, nerves, etc.

A =sinus= or =antrum= is an air-cavity.

Other terms, such as =canal=, =fissure=, =notch=, etc., require no explanation.[3]

VERTEBRAL COLUMN

The =vertebral column= (Columna vertebralis) is the fundamental part of the skeleton. It consists of a chain of median, unpaired, irregular bones which extends from the skull to the end of the tail. In the adult certain vertebræ have become fused to form a single bony mass with which the pelvic girdle articulates. Vertebræ so fused are termed =fixed= or =“false” vertebræ= (Vertebræ immobiles), as distinguished from the =movable= or =“true” vertebræ= (Vertebræ mobiles).

The column is subdivided for description into =five regions=, which are named according to the part of the body in which they are placed. Thus the vertebræ are designated as =cervical=, =thoracic= (or dorsal), =lumbar=, =sacral=, and =coccygeal= or =caudal= (Vertebræ cervicales, thoracales, lumbales, sacrales, coccygeæ). The number of vertebræ in a given species is fairly constant in each region except the last, so that the =vertebral formula= may be expressed (for the horse, for example) as follows:

C_{7}T_{18}L_{6}S_{5}Cy_{15–21}.

The vertebræ in a given region have special characters by which they may be distinguished from those of other regions, and individual vertebræ have characters which are more or less clearly recognizable. All typical vertebræ have a common plan of structure, which must first be understood. The parts of which a vertebra consists are the =body= or =centrum=, the =arch=, and the =processes=.

The =body= (Corpus vertebræ) is the more or less cylindrical mass on which the other parts are constructed. The anterior and posterior extremities of the body are attached to the adjacent vertebræ by intervertebral fibro-cartilages, and are usually convex and concave respectively. The dorsal surface is flattened and enters into the formation of the vertebral canal, while the ventral aspect is rounded laterally, and is in relation to various muscles and viscera. In the thoracic region the body presents two pairs of demifacets (Foveæ costales) at the extremities for articulation with the heads of two pairs of ribs.

The =arch= (Arcus vertebræ) is constructed on the dorsal aspect of the body. It consists originally of two lateral halves, each of which is considered to consist of a pedicle and a lamina. The pedicles form the lateral parts of the arch, and are cut into in front and behind by the =vertebral notches= (Incisura vertebralis cranialis, caudalis). The notches of two adjacent vertebræ form =intervertebral foramina= for the passage of the spinal nerves and vessels; in some vertebræ, however, these are complete foramina instead of notches. The laminæ are plates which complete the arch dorsally, uniting with each other medially at the root of the spinous process.

The body and the arch form a bony ring which incloses the =vertebral foramen= (Foramen vertebrale); the series of vertebral rings, together with the ligaments which unite them, inclose the =vertebral canal= (Canalis vertebralis), which contains the spinal cord and its coverings and vessels.

The =articular processes=, two =anterior= and two =posterior= (Processus articulares craniales, caudales), project from the borders of the arch on either side. They present joint surfaces adapted to those of adjacent vertebræ, and the remaining surface is roughened for muscular and ligamentous attachment.

The =spinous process= (Processus spinosus) is single, and projects dorsally from the middle of the arch. It varies greatly in form, size, and direction in different vertebræ. It furnishes attachment to muscles and ligaments.

The =transverse processes= (Processus transversi) are two in number and project laterally from the side of the arch, or from the junction of the arch and body. In the thoracic region each has a facet for articulation with the tubercle of a rib (Fovea costalis transversalis). They also give attachment to muscles and ligaments.

Some vertebræ have also a =ventral= or =hæmal spine=.

=Mammillary processes= (Processus mammillares) are found in most animals on the last thoracic and anterior lumbar vertebræ between the transverse and anterior articular processes or on the latter.

=Accessory processes= (Processus accessorii), when present, are situated between the transverse and posterior articular processes.

=Development.=—The vertebræ are developed by ossification in the cartilage which surrounds the notochord and forms the sides of the neural canal. There are =three primary centers of ossification=, one for the body and one for each side of the arch. =Secondary centers= appear later for the summit of the spinous process (except in the cervical region), the extremities of the transverse processes, and the thin epiphyseal plates at the extremities of the body.