40 ANATOMY OF THE RABBIT. 
teria; (c) platelets, minute nucleate amoeboid cells found in the 
blood stream. 
In all multicellular animals the tissues are removed more or less 
extensively from the surfaces of absorption and excretion. Blood 
and lymph are the media through which tissue metabolism is 
maintained, and the vascular system is the mechanical means by 
which the continuous circulation of fluid is brought about. The 
composition of the blood varies from time to time according to the 
individual functions performed. Oxygen and food materials are 
carried to the tissues. Carbon dioxide and waste materials of 
other kinds are carried to organs from which they can be excreted. 
Blood, however, has been shown to vary in composition in different 
species of animals, and to be chemically homologous in related ones; 
while it possesses immunizing properties towards bacterial diseases, 
also_differing in different species and individuals. 
TERMINOLOGY. 
In special or descriptive anatomy it is necessary to employ an 
extensive system of terminology in order that the various structures 
of the body may be individually designated, classified, and referred 
to their respective positions. The terms used for this purpose may 
be classified into four groups, as follows: (1) general terms— 
those included in the names of parts, but applicable in themselves 
to similar structures (arteries, nerves, etc.) in various parts of the 
body; (2) specific terms or names of parts; (3) regional 
terms—those defining areas (topographic); and (4) terms of 
orientation. 
Although few in number, the terms of orientation may be 
regarded as the most generally useful terms of descriptive anatomy. — 
This is because they are based on very general relations of the body 
and are therefore of wide application. For this reason they are 
here selected for definition to the exclusion of others of a more 
restricting or individualizing kind. 
In all vertebrates we may recognize a longitudinal axis, 
corresponding, in general, to the line formed by the vertebral 
column. In the usual or prone position of the body this axis is 
horizontal. The uppermost surface is then described as dorsal, 
