44 ANATOMY OF THE RABBIT. 
preliminary statement of how certain outstanding features of the 
rabbit may be explained, as to the nature of their origin, and what 
grades of organization they may be presumed to illustrate. It will 
be recognized that the mammals are in most respects less specialized 
than man, and must accordingly show in their grades of organization 
various stages through which man must be assumed to have passed. 
CLASSIFICATION OF THE ORGAN SYSTEMS. 
The term organ-system is employed in descriptive anatomy 
to designate a group of organs which cooperate in.a general function. 
In many respects the systems represent primitive functions, and 
it is therefore largely on account of the independent elaboration 
of these that the systems may be recognized also on a structural 
basis as groups of organs allied in origin and development. The 
exact number of systems recognized depends on certain arbitrary 
distinctions, the following being those usually distinguished: 
(1) The integumentary system, comprising the skin, and its 
appendages, namely, the hairs and the general cutaneous, 
mammary, and inguinal glands. 
(2) The skeletal system, comprising the cartilage and bone 
elements of the skeleton, with their connections. 
‘ (3) The muscular system, comprising all contractile structures 
of the body. Since, however, the involuntary muscles are arranged 
as muscle layers in connection with visceral organs, the muscular 
system is usually considered as including only the individual or 
voluntary muscles of the skeleton and skin. 
(4) The nervous system, comprising the central nervous 
system (the brain and spinal cord) and the peripheral nervous 
system, the latter consisting of the paired cranial and spinal nerves 
with their associated ganglia. A special portion of the peripheral 
nervous system is set apart as the sympathetic nervous system. 
The latter consists of a pair of ganglionated sympathetic trunks 
lying along the ventral surface of theverteb ral column, and of two 
series of ganglia, prevertebral and peripheral, connecting the 
trunks with the visceral organs. 
(5) The digestive system, comprising the digestive tube and 
its outstanding glandular appendages—the oral glands, the liver, 
and the pancreas. 
