THE VERTEBRATES: MAMMALS 241 



the organs of taste. The oesophagus is always a simple 

 straight tube, but the stomach varies greatly, being usually 

 simple, but sometimes, as in the ruminants and whales, 

 divided into several distinct chambers. The intestine in 

 vegetarian mammals is very long, being in a cow twenty 

 times the length of the body. In the carnivores it is com- 

 paratively short 'in a tiger, for example, but two or three 

 times the length of the body. 



The blood of mammals is warm, having a temperature of 

 from 35 C. to 40 C. (95 F. to 104 F.). It is red in color, 

 owing to the reddish-yellow, circular, non-nucleated blood- 

 corpuscles. The circulation is double, the heart being com- 

 posed of two distinct auricles and two distinct ventricles. 

 Air is taken in through the nostrils or mouth and carried 

 through the windpipe (trachea) and a pair of bronchi to the 

 lungs, where it gives up its oxygen to the blood, from which 

 it takes up carbon dioxide in turn. At the upper end of 

 the trachea is the larynx or voice-box, consisting of several 

 cartilages attaching by one end to the vocal cords and by the 

 other to muscles. By the alteration of the relative position 

 of these cartilages the cords can be tightened or relaxed, 

 brought together or moved apart, as required to modulate 

 the tone and volume of the voice. 



The kidneys of mammals are more compact and definite 

 in form than those of other vertebrates. In all mammals 

 except the Monotremes they discharge their product through 

 the paired ureters into a bladder, whence the urine passes 

 from the body by a single median urethra. Mammary 

 glands, secreting the milk by which the young are nourished 

 during the first period of their existence after birth, are 

 present in both sexes in all mammals, though usually func- 

 tional in the female only. 



The nervous system and the organs of special sense reach 

 their highest development in the mammals. In them the 

 brain is distinguished by its large size, and by the special 



