610 Comparative Morphology of Chordates 



activities may arise — e.g., as when the respiratory stream of air is 

 utilized for production of vocal sound. 



There are two visceral organs which stand in especially close rela- 

 tion to the activities of the body-wall. These are the lungs and the 

 heart. The physiologic demands upon these organs are immediate and 

 constant. If either stops working, the animal almost instantly ceases 

 to run or swim. The services of other visceral organs are equally neces- 

 sary, but not so directly. The animal may run for miles while the 

 stomach and intestine contain nothing to digest. A warm-blooded and 

 very active animal must have a heart and lungs of high efficiency. 



In view of the facts just stated, it may be expected that, in the 

 following account of the mammalian visceral systems, the respiratory 

 and circulatory organs will require much more extended attention 

 than the organs of digestion and excretion. And the peculiarities of 

 mammalian reproduction give special importance to a comparative 

 account of the genital organs. 



I. Respiratory Organs 



Pharynx 



In all vertebrates the respiratory organs are closely associated with 

 the pharyngeal region of the digestive tube. In most fishes the nasal 

 cavities have no internal openings. Therefore breathing must be 

 through the mouth (or partly through spiracles in the few fishes 

 that have them). In all lung-breathing vertebrates the nasal cavities 

 open internally into the mouth or pharynx. On occasion, or in an 

 emergency, a mammal can breathe through the mouth. However, this 

 close association of functions so unlike as eating and breathing gives 

 rise to some complications. For the poikilothermous vertebrate these 

 are apparently not serious. A "cold-blooded" animal can suspend 

 breathing for a relatively long time without suffering inconvenience. 

 Further, these animals ordinarily "bolt" the food without chewing, 

 so that eating occasions only brief interruptions of breathing. The 

 mouth-cavity and throat are wide and the thing to be swallowed is 

 usually a large object. The glottis is a small, narrow slit. Although it is 

 on the floor of the pharynx, there is little danger that food may enter it. 

 In mammals the difficulties are greater. The homothermous animal 

 must breathe regularly. (Whales possess exceptional arrangements for 

 long submergence.) The greater need of oxygen requires a wide trachea 

 and an open glottis. The nasal passages are dorsal to the oral cavity. 

 The glottis being ventral, the trachea is ventral to the esophagus. 

 Therefore the pharyngeal region becomes the intersection of two im- 

 portant traffic lanes (Fig. 469), the respiratory and the alimentary. 



