96 EVERS'S COMPARATIVE ANATOMY. 



the abdomen, but in others they extend along the neck, and even 

 into the extremities: they also penetrate the cavities and diploe of 

 the bones, a discovery for which we are indebted to Mr. Hunter. 

 This great physiologist injected the medullary cavities of the bones 

 from the trachea: he also tied this tube, and having broken the 

 humerus of a fowl, and the femur of a hawk, he found that the 

 birds respired for a short time through the artificial openings. The 

 proportion in which the osseous system of birds is permeated by 

 air has reference to their respective modes of progression, thus 

 almost every bone in the body admits air in the kite, the hawk, the 

 eagle, and other birds of high flight; and in the hornbill even the 

 phalanges of the toes contain air. Four uses have been ascribed 

 to this extension of the respiratory system in birds — first, to sub- 

 serve the function of respiration ; secondly, to aid by mechanical 

 pressure the action of the lungs; thirdly, to render the body spe- 

 cifically lighter for the purposes of flight : and fourthly, by the dis- 

 tension of the cells in the extremities to assist in maintaining the 

 wings in a state of extension, during long-continued flight. Mr. 

 Hunter supposed it contributed to sustain the song of birds and to 

 give it strength and tone. 



The air passages in birds consist of a superior larynx, a trachea, 

 and inferior larynx, and two bronchi with their ramifications. The 

 superior larynx is situated behind the root of the tongue, resting 

 on the uro-hyal element of the os hyoides ; it is composed of from 

 four to ten bony or cartilaginous pieces, and two pairs of muscles ; 

 thyro-arytenoidea and constrictores glottidis. The trachea is com- 

 posed of a series of bony or cartilaginous rings, which form com- 

 plete circles, with the exception of the two first ; they are closely 

 approximated, and sometimes overlap. Many birds, as the rasores, 

 have no inferior larynx ; in others it presents different degrees of 

 development, thus, in the genus falco there is but one pair of 

 muscles; in the parrot tribe three, and in the insessores, where this 

 organ attains its greatest decree of perfection, five. The rings of 

 the bronchi do not form complete circles, but gradually become 

 smaller, and finally disappear. 



MAMMALIA. 



In the entire of this class there is great similarity in the respira- 

 tory organs, not only to each other but to the human type of for- 

 mation. An epiglottis exists in all, and is divided at its superior 

 extremity in the seal, the hare, and the ant-eater. The larynx in 

 all the mammalia consists of the same parts generally as in man, 

 but occasionally modified in obedience to particular circumstances. 

 Thus, in the cetacea this organ ascends as far as the posterior 

 nares, and communicates with the spouting hole, which opens at 

 the top of the head by a single or double orifice, closed by a fleshy 

 valve in the form of two semicircles. The great size of the larynx 

 in the lion accounts for the powerful and terrific roar of that ani- 



