GENERAL ANATOMY. 23 



this is not the case when we come to compare the entire leg of a bird with the 

 entire hind leg of a squirrel, — the homology is then incomplete. There are 

 other terms, used technically, that occur throughout the entire science of des- 

 criptive anatomy: thus, the spinal or vertebral column is considered to be the 

 longitudinal axis of the bird's body, its c<j>linUc extremity bearing the head, 

 and its caudal end, the tail. So, too, there is an anterior half of the body, ft 

 posterior half, and the two lateral halves. In the latter, as a rule, there are 

 corresponding parts throughout, while in the former some structures only, are 

 serially anologous, being comparisons confined principally to the limbs and the 

 skeletal girdles supporting them. Anterior and posterior may also be used 

 relatively in describing the same structure or any part of it, as the anterior 

 and posterior part of the skull, or any charcter of the same may be anterior 

 or posterior to any other character. In much the same manner we employ 

 the terms distal and proximal, in a relative sense as being near or remote from 

 the imaginary centre of the body. The distal extremity of the humerus articu- 

 lates with the bones of the forearm, while its proximal end is in the glenoid 

 fossa. The manus is the distal division of the pectoral limb and is distad to 

 its proxitnad division, which is composed of the humerus. The hasal part of 

 an organ or structure is generally its proximal part, as the base of the heart; 

 the base of the skull. On the other hand the ajMX, or apical end is usually 

 the tip or distal extremity, as the apex of the beak; the apex of the heart or 

 lungs. Aspects of a part or organ relatively corresponding to the back or the 

 belly sides are designated respectively as dorsal and ventral,' they find their 

 explanation in the fact that the heart, lungs, and digestive organs are upon the 

 ventral side of the spinal column, in contradistinction to the structures found 

 dorsad or upon the opposite aspect. Vent7'ad upon the sternum we find the 

 keel. In our descriptions an imaginary plane is supposed to pass through the 

 longitudinal axis; it is at right angles to the transverse axis. With reference 

 to this plane, parts of structures, characters and so forth, may, relatively 

 speaking, be inner or median, outer or lateral. The lateral aspect of the skull 

 sometimes conceals those parts situated mesiad to them. 



The topographical anatomy of a bird is, of itself, an extensive subject, 

 and demands the employment of a large number of technical terms in its de- 

 scription. In the first place birds are the only vertebrates possessing feathers, 

 and the anatomy of these integumentary structures alone offers a wide field 

 for study. In their entirety they constitute the plumage of a bird or its 

 ptilosis. Their peculiar development has been fully described by many 

 writers while the science oi pterylography, or the various arrangements or im- 

 plantations of the feathers upon the skin, has for a long time been employed 

 as an important factor in classification. A bird's body, again, is divided 

 into various parts for the purpose of description, as the head, the neck. 



