38 ANATOMY OF THE RABBIT 



THE GENERAL FEATURES AND GROUND PLAN 

 OF THE ORGAN-SYSTEMS 



It has already been stated as a general principle that the 

 structure of an organism is the expression of an underlying plan 

 and pattern, in the elaboration of which embryonic development 

 and ancestry play a very large part. The manner in which the 

 comparative method is applied in interpretation may be demon- 

 strated by reference to any part of the body of an animal; and in 

 the following pages will be found, under the head of the respective 

 systems, a preliminary statement of how the origin of certain out- 

 standing features of the rabbit may be explained and what grades 

 of organization they may be presumed to illustrate. It will be 

 recognized that the lower mammals are in many respects less 

 specialized than man and must accordingly show in these respects 

 various stages through which the human species must be assumed 

 to have passed. Nevertheless, this principle is not applicable to 

 all parts and cannot be assumed to be true in any particular case 

 without critical examination. 



Classification of the Organ-Systems 



The term organ-system is employed in descriptive anatomy to 

 designate a group of organs which co-operate 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 comprises the skin, and its 

 derivatives, such as the claws (or nails), hairs, and various glands 

 of epidermal origin. 



(2) The skeletal system comprises the cartilage and bone 

 elements of the skeleton, with their connections. 



(3) The muscular system comprises all contractile structures 

 of the body. Since, however, the involuntary muscles are arranged 

 as muscle layers in connection with visceral organs, the muscular 



