762 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



cords are scarcely perceptible; the ventricles are ample, but their 

 openings are very narrow. The voice in dogs is capable of greater scope 

 than in other of the domestic animals, with the exception, perhaps, of 

 the cat, which is characterized by an almost equal development of the 

 upper and lower vocal cords. 



The mechanism of the production of the voice, therefore, depends 

 upon the expulsion of a blast of air between two free membranous rims, 

 which are thus thrown into vibration and whose tension and position are 

 capable of modification. The change in the position and tension of the 

 vocal cords is accomplished through the action of the laryngeal muscles. 

 The laryngeal muscles fulfill a double function. In respiration, as has 

 been already mentioned, the glottis is widened during inspiration and 

 the vocal cords tend to approach each other during expiration. In the 

 production of voice the vocal cords are almost always in close contact. 



The glottis may be dilated by the action of the crico-arytenoid 

 muscles. When they contract the arytenoid cartilages are drawn back- 

 ward, downward, and toward the middle line, so that, therefore, the 

 vocal processes in which the vocal cords are inserted must be separated. 

 A large triangular space is thus formed between the vocal cords 

 as in inspiration. 



The glottis is constricted by the contraction of the transverse 

 arytenoid muscles, which extend from both outer surfaces of the 

 arytenoid cartilages along their entire length. When these muscles, 

 together with the oblique arytenoid, contract, the arytenoid cartilages 

 are approximated and the glottis closed. During the production of 

 voice the vocal processes of the arytenoid cartilages must be closely 

 approximated, and to accomplish this it is necessary that they be rotated 

 inward and downward. This result is brought about through the 

 contraction of the thyro-arytenoid muscles, which are imbedded in the 

 substance of the vocal cords, and when they contract they so rotate the 

 arytenoid cartilages that the vocal processes turn inward. The glottis 

 is, therefore, narrowed to a mere slit in the anterior part, while a 

 triangular space through which respiration takes place remains open 

 posteriorly. 



The vocal cords vary in tension according to the degree of con- 

 traction of the crico-thyroid muscles, which pull the thyroid cartilage 

 downward and forward. At the same time the crico-arytenoid muscles 

 act upon the arytenoid cartilages, drawing them slightly backward and 

 maintaining them in that position. 



In the production of voice, not only must the vocal cords be thrown 

 into tension in the manner above described, but the triangular space of 

 the respiratory part of the glottis between the arytenoid cartilages must 

 likewise be closed. This is accomplished by the contraction of the 



