760 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



be inflated with air, and whose walls, being thin and membranous, are 

 thrown into vibration, and, together with the column of air contained in 

 the oral chamber, aid in producing the characteristic resonance of the 

 croaking of the frog (Fig. 315). 



In birds the vocal organs are double, a larynx situated at the upper 

 termination of the trachea and a syrinx at its bifurcation constituting 

 a true vocal organ. Two folds of mucous' membrane, or three in the case 

 of song-birds, project into each bronchus, and are so acted on by muscles 

 as to vary their tension and adapt them to the production of voice. The 

 superior larynx acts only as an accessory in the production of sound. 

 The number and complexity of the muscular fibres acting on these 

 membranes varies in proportion to the range of voice. In the gallinacea 

 simply a trace of muscular fibre can be recognized ; but one pair of 

 muscles is found in the eagle, three pairs in the paroquet, while five 

 pairs are found in song-birds. These muscles have a common origin in 

 the trachea, and their other extremities are inserted into the first ring of 

 the bronchus. In addition to these intrinsic muscles there are others 

 concerned in varying the length of the trachea, so as to alter the length 

 of the vocal tube and, therefore, the pitch of the note produced. The 

 superior organ, or the larynx, is entirely negative in the production of 

 sound, and the performance of tracheotomy below the larynx produces 

 but slight modification in the character of the sound produced. 



In mammals, as is well known, the greatest variation exists in the 

 character of the sounds produced. The organs for the production of 

 sound are here the larynx and the upper resonating chambers, varying in 

 shape and general character among each other, although in all built on 

 the same general plan as in man. The variations in the voice are 

 dependent upon modifications in the larynx, in the depth of the nasal 

 chambers, the shape of the pharynx, of the various sinuses, and the 

 formation of the mouth and of the laryngeal ventricles. Sound is, how- 

 ever, primarily due in all cases to the vibrations transmitted to the 

 column of air by the swaying to and fro of the vocal cords. The superior 

 or false vocal cords of man are absent in many species of mammals. 

 The glottis of the horse is distinguished by the formation of a semi-lunar 

 fold of mucous membrane below the epiglottis, which serves to form a 

 funnel-shaped cavity. The laryngeal ventricles are also well developed. 

 The voice in the horse is produced by a succession of interrupted expira- 

 tory movements, the tension of the .vocal cords gradually diminishing 

 during each complete expiration, so that, therefore, the first sounds 

 produced have a higher pitch than the last ; the superior ventricles are 

 wanting (Fig. 316). 



The larynx of the ass differs but slightly from that of the horse. 

 Here, also, there are two vocal cords, the ventricles are well developed, 



