26 Anatomy of the; Rabbit. 



f 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 repre- 

 sent 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 de- 

 velopment. 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 ap- 

 pendages, 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 the 

 vertebral 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 out- 

 standing glandular appendages — the oral glands, the liver, and the 

 pancreas. 



(6) The respiratory system, comprising the lungs, and respiratory 

 passages, namely, the bronchi, the trachea and the larynx. With this 

 system may also be included the accessory respiratory passages formed 

 by the nasal fossae. 



(7) The vascular system, comprising the organs of circulation, 

 namely, the heart, arteries, capillary vessels, and veins. The lymph- 

 conducting canals are also portions of the circulatory system, but since 

 they are largely independent of the bloodvessels, they are usually con- 

 sidered as forming with their associated lymph glands a separate 

 lymphatic system. 



(8) The urinogenital system, comprising the reproductive and 

 excretory organs, together with their common ducts — the urethra of the 

 male and the vestibulum of the female — and the associated bulbourethral 

 gland. The reproductive organs comprise, in the male, the central organs 

 or testes, and the deferent ducts, both of which are paired, the unpaired 

 seminal vesicle, and the paired prostatic and paraprostatic glands. In 

 the female, the reproductive organs comprise the paired ovaries, uterine 

 tubes and uteri, together with the unpaired vagina. The excretory 



