216 SPECIAL PHYSIOLOGY. 



thyroid cartilage and hyoid bone, which, communicating with the ventricles 

 of the larynx, and with other cavities above them, called the laryngo-pharyn- 

 geal sacs, cause a remarkable increase in the resonance of the laryngeal appa- 

 ratus. The bray of the ass probably depends, in great part, on the presence 

 of two large sacs situated between the vocal cords and the inner surface of the 

 thyroid cartilage. Among the Marsupials, some, as, e. (/., the kangaroo, pos- 

 sess membranous vocal cords which fold upon themselves ; the arytenoid mus- 

 cles cannot therefore stretch them. A few Mammalia are unprovided with 

 vocal cords, and are therefore mute, as for example, the giraffe, armadillo, and 

 porcupine. The vocal ligaments are also absent in the Cetacea ; some of these 

 are able to utter a lowing or bellowing sound ; this is produced during the act 

 of expiration, when, the mouth being closed, they expel the water, with which 

 that cavity has become filled in the act of feeding, through the nasal opening, 

 or openings, in the upper part of the head : the noise produced in this act can- 

 not be regarded as a vocal sound. The voice of Mammalia is always in a 

 minor key. 



The vocal apparatus in Birds differs altogether from that of Mammalia, 

 both as regards its anatomical structure, and the manner in which sounds are 

 produced by it. Biids are provided with a larynx corresponding in situation 

 to that of Mammalia, presenting, however, a marked contrast in many parts 

 of its structure ; and being, moreover, totally unconnected with the production 

 of sound. This part of the respiratory apparatus is called the superior larynx. 

 The true organ of voice, the inferior larynx, is situated at the lower end of 

 the trachea, immediately before it bifurcates to form the two bronchi. It pre- 

 sents various modifications, both in form and structure, in the different Orders 

 of birds ; in some, it forms an exceedingly delicate and complicated apparatus ; 

 it is a double organ, except in the parrot and a few other birds, and is almost 

 always symmetrical. It is composed of several of the lower rings of the 

 trachea, united together, so as to form a tube, which presents, at its lower 

 extremity, two projections, an anterior and a posterior one ; passing between 

 these, in most birds, is a slender rod of bone, called the os transversale, which 

 serves to connect them together ; this part of the trachea opens below by two 

 oval apertures, into the right and left bronchi. The upper margin of the bony 

 rod gives attachment to a fine delicate membrane, the membrana semilunaris, 

 which is directed upwards ; connected with its lower margin is another mem- 

 brane, called the membrana tympaniformis, which is, in reality, formed by that 

 part of the wall of the bronchus which is made up simply of membrane ; for 

 the bronchi, in Birds, are not formed of complete rings of bone and cartilage, 

 joined by membrane, like the windpipe, but are only partially strengthened by 

 bony or cartilaginous pieces, so that their adjacent or opposed parietes are 

 membranous ; and it is these parietes that form the tympaniform membrane. 

 In some birds, this membrane is very small and rudimentary ; it is highly de- 

 veloped in singing birds, and still more so in speaking birds ; it is continuous 

 with the membrana semilunaris, and can therefore, when thrown into vibra- 

 tions, render the latter tense. The inferior larynx is provided with special 

 muscles, in the more perfect forms, with five pairs of muscles, the office, of 

 which is to regulate the distance between the vocal membranes, and to alter 

 their tension by elevating the first cartilage of each bronchus ; in some birds, 

 however, the inferior larynx has no special muscles. The entire organ is 

 absent in vultures. 



The seat of voice in birds has been shown by experiment to reside in the 

 inferior larynx, the tympaniform and semilunar membranes being the ana- 

 logues of the vocal cords in Mammalia. Division of the trachea about its cen- 

 tre, in singing birds, does not arrest vocalization, although the notes emitted 

 are, of course, rendered feebler by the existence of such an opening. Again, 

 if the bronchi, together with the inferior larynx, be removed from the body, 

 and air blown through them, the resulting sounds closely resemble the natural 

 notes or cries of the bird. The absence of vocal ligaments, or cords, in the 

 superior larynx, at once excludes all idea of its being concerned in vocaliza- 

 tion ; it no doubt, however, exercises a considerable influence in modifying the 

 vocal sounds. It may, moreover, be seen to move simultaneously, with the 

 mouth during the action of the inferior larynx, in singing birds. It is yet 



