CLASSIFICATION OF THE ORGAN SYSTEMS. 45 
(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, 

Fic. 21. Schematic representation of the chief organ-systems of a generalized 
vertebrate as seen in a transverse section of the abdominal region: 
Integument—int. 
Skeleton—yv., vertebral body; a.v., vertebral arch; c.v., vertebral canal. 
Muscular system—s.m., skeletal muscle; v.m., visceral muscle. 
, Nervous system—m.s., spinal cord, with the central canal, and the dorsal (pos- 
terior) and ventral (anterior) roots of the spinal nerves; g.r.p., ganglion of the 
posterior root; r.c., ramus communicans to sympathetic trunk; r.m.a. and r.m.p., 
anterior and posterior rami of a spinal nerve; t.s., sympathetic trunk. 
Digestive system—i. intestine. 
Vascular system—ao., aorta. 
Urinogenital system—k., kidney; go., gonad (ovary or testis). 
Serous cavity—c.p., general coelom, pleuroperitoneal, or peritoneal cavity; 
p.v. and p.p., visceral and parietal parts of the serous tunic—visceral and parietal 
peritoneum; mes., mesentery. 

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 considered as forming with their associated lymph 
glands a separate lymphatic system. 
