RESPIRATORY SYSTEM 67 



the walls of the long tubular proboscis, this apparatus is entirely 

 wanting. 



Trachea. — The narial passages have the organ of smell situated 

 in their upper part, and communicate posteriorly with the 

 pharynx, and through the glottis Math the " trachea " or windpipe, 

 a tube by which the air is conveyed to and from the lungs. The 

 permanent patency of the trachea during the varied movements of 

 the neck is provided for by its walls being stiffened by a series of 

 cartilaginous rings or hoops, which in most mammals are incomplete 

 behind. Having entered the thorax, the trachea bifurcates into the 

 two bronchi, one of which enters, and, dividing dichotomously, 

 ramifies through each lung. In some of the Cetacea and 

 Artiodactyla a third bronchus is given off from the lower 

 part of the trachea, above its bifurcation, and enters the right 

 lung. 



Larynx. — The upper end of the trachea is modified into the 

 organ of voice or " larynx," the air passing through which to and 

 from the lungs is made use of to set the edges of the " vocal cords," 

 or fibrous bands stretched one on each side of the tube, into vibra- 

 tion. The larynx is composed of several cartilages, stich as the 

 "thyroid," the "cricoid," and the " arytenoid " which are moved 

 upon one another by muscles, and suspended from the hyoidean arch. 

 By alteration of the relative position of these cartilages the cords 

 can be tightened or relaxed, approximated or divaricated, as 

 required to modulate the tone and volume of the voice. A median 

 tongue-shaped fibro-cartilage at the top of the larynx, the "epiglottis," 

 protects the " glottis," or aperture by which the larynx communi- 

 cates with the pharynx, from the entry of particles of food during 

 deglutition. The form of the larynx and development of the vocal 

 cords present many variations in different members of the class, 

 the greatest modification from the ordinary type being met with in 

 the Cetacea, where the arytenoid cartilages and epiglottis are united 

 in a tubular manner, so as to project into the nasal passage, and, 

 being grasped by the muscular posterior margin of the palate, pro- 

 vide a direct channel of communication from the lungs to the 

 external surface. An approach to this condition is met with in the 

 Hippopotamus and some other Ungulates; it is indeed so general 

 as an abnormality, that Howes suggests that an internarial epi- 

 glottis may have been a primitive feature common throughout the 

 class. Nearly all mammals have a voice, although sometimes it is 

 only exercised at seasons of sexual excitement. Some Marsupials 

 and Edentates appear to be quite mute. In no mammal is there 

 an inferior larynx, or " syrinx," as in birds. 



Diaphragm. — The thoracic cavity of mammals differs from that 

 of the Sauropsida in being completely separated from the abdomen 

 by a muscular partition, the " diaphragm," attached to the vertebral 



