CLASS MAMMALIA 639 



The Resfiratory System. — The rabbit and aii other 

 mammals breathe air by means of lungs. The glottis opens into 

 the larynx, from which a tube called the trachea or windpipe 

 arises. The trachea is held open by incomplete rings of cartilage; 

 it divides into two bronchi, one bronchus going to each lung. 

 The larynx is supported by a number of cartilages and across its 

 cavity extend two elastic folds called the vocal cords. The lungs 

 are conical in shape, and lie freely in the thoracic cavity sus- 

 pended by the bronchi. 



Air is drawn into the lungs by the enlargement of the 

 thoracic cavity. This is accomplished both by pulling the ribs 

 forward and thus separating them, as in most reptiles, and by 

 means of the diaphragm. The diaphragm is normally arched 

 forward and when it contracts it flattens, thus enlarging 

 the thoracic cavity. The increased size of this cavity results 

 in the expansion of the lungs, because of the air pressure 

 within them, and the inspiration of air through the nostrils. 

 Air is pumped out of the lungs (expiration) by the contraction 

 of the elastic pulmonary vesicles, and of the thoracic wall and 

 diaphragm. 



The Excretory System. — The urine excreted by the two 

 kidneys is carried by two slender tubes, the ureters, into a thin- 

 walled, muscular sac, the urinary bladder. At intervals the walls 

 of the bladder contract, forcing the urine out of the body through 

 the urinogenital aperture. 



The Nervous System. — The rabbit possesses a brain, cranial 

 nerves, spinal cord, spinal nerves, and a sympathetic nervous 

 system. 



The brain (Fig. 512), as in ether mammals, differs from that 

 of the lower vertebrates in the large size of the cerebral hemi- 

 spheres (f.b) and cerebellum (h.b). The cerebral hemispheres are 

 slightly marked by depressions, or sulci, which divide the surface 

 into lobes or convolutions not present in the pigeon. The 

 olfactory lobes (b.o) are very large and club-shaped. The optic 

 lobes are each divided by a transverse furrow into two. The 



