THE STRUCTURE AND ACTIVITIES OF MAMMALS 407 



cavity. This is accomplished both by pulling the ribs forward 

 and then separating them and by means of the diaphragm. The 

 diaphragm is normally arched forward (up in man, Fig. 273), 

 and when it contracts, it flattens, thus enlarging the thoracic 

 cavity. The increased size of this cavity results in the expan- 

 sion 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 lung 

 vesicles and of the thoracic wall and diaphragm. 



Excretion. Waste products are cast out of the body by the 

 kidneys and skin. The kidneys are the principal excretory 



MetL 



FIG. 277. Brain of dog. Side view. 



I-XII, cranial nerves; B.ol, olfactory lobe; HH, cerebellum; Hyp, hypo- 

 physis ; Med, spinal cord ; NH, medulla oblongata ; Po, pons Varolii ; VH, 

 cerebrum; Wu, cerebellum. (From Wiedersheim.) 



organs. The urine which they extract from the blood 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 urino- 

 genital aperture. In the skin of man are numerous sweat glands 

 and sebaceous glands which aid the kidneys in excreting waste 

 material. 



Nervous System. The nervous system is very highly de- 

 veloped in mammals. The brain (Fig. 277) differs from that 

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



