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: 



(i) 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. 



