Part 3

(1) Sense Organs

(2) Common Integument.

The term =topographic anatomy= designates the methods by which the relative positions of the various parts of the body are accurately determined. It presupposes a fair working knowledge of systematic anatomy.

=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

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).

FIG. 1.—FRONTAL SECTION OF LARGE METATARSAL BONE OF HORSE, POSTERIOR PART. ]

FIG. 2.—SAGITTAL SECTION OF LARGE METATARSAL BONE OF HORSE.

S.C., Compact substance; S.s., spongy substance; C.m., medullary cavity; F.n., nutrient foramen. Note the greater thickness of the compact substance of the inner and anterior parts of the shaft. ]

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.