Mammalia: Visceral Systems 615 



directly concerned in production of sound, although it has been al- 

 leged that they are responsible for the cat's purring (Fig. 471, diagrams 

 A-CA'-C). 



At the moment when food is passing across the pharynx, breathing 

 is stopped. The glottis is closed, the epiglottis is bent back over the 

 laryngeal aperture, and the soft palate is moved upward against the 

 roof of the nasopharynx, thus blocking the nasal passages. Any defect 

 in the coordination of these several events may cause choking. 



In whales and their allies, as in crocodilians, aquatic living in- 

 creases difficulties in the pharyngeal region, which accordingly exhibits 

 various modifications serving to emphasize the separation between the 

 respiratory and the alimentary tracts. In sperm-whales the laryngeal 

 wall and, especially, the epiglottis are greatly elongated so that the 

 larynx projects forward into the nasopharynx, fitting it tightly and 

 thus providing direct tubular connection between nasal and tracheal 

 passages. Under similar necessity but quite different circumstances, 

 the elongated larynx and epiglottis of the newborn kangaroo and other 

 marsupials are inserted into the rear of the nasopharynx. This is an 

 adaptation to the fact that, at time of birth, the marsupial is not yet 

 sufficiently developed to be able to suck milk. For some time after 

 birth, the young animal (actually a fetus) is continuously attached to 

 a nipple of a mammary organ, and milk is injected into the young by 

 contraction of a muscle (the cremaster) at the base of the nipple. 

 Arriving in the pharynx, the stream of milk divides and passes around 

 the tubular larynx and on into the esophagus, without interfering 

 with the breathing. 



Compared to the larynx of a mammal, the reptilian larynx (Fig. 

 60) is poorly developed, a fact consistent with the very inferior vocal 

 abilities of reptiles. Cartilages apparently corresponding to the cricoid 

 and arytenoids of mammals may be recognized. In the alligator certain 

 folds on the inner surface of the laryngeal wall are probably concerned 

 with the animal's roar, but the folds evidently do not correspond to 

 mammalian vocal folds. As a vocal organ, the larynx of frogs and toads 

 is much better developed than that of reptiles. In the laryngeal region 

 of birds, cricoid and arytenoid cartilages are present but the region is 

 only slightly differentiated, the vocal function having been shifted to 

 the syrinx at the posterior end of the trachea. 



The walls of the trachea are supported by regularly spaced trans- 

 verse bands of cartilage which are usually incomplete rings, the defi- 

 ciency being on the dorsal side where the trachea is in close proximity 

 to the esophagus (Fig. 168). The dorsal gap in each skeletal ring makes 

 the trachea more easily compressible and so facilitates passage of a 

 bolus of food along the adjacent esophagus. Just anterior to the lungs, 



