764 



COMPARATIVE ANATOMY 



tightened or relaxed to alter the pitch of the note caused by their vibra- 

 tion. A pocket lies anterior to the cords, the laryngeal ventricle (sinus 

 of Morgagni), one on each side, quite small in most mammals, but well 

 developed in the anthropoid apes to large vocal sacs. In the chimpanzee 

 there is a median vocal sac in addition. These act as resonators and add 

 strength to the voice. 



The larynx is prolonged in whales and marsupials so that it projects 

 into the choana behind the soft palate. This is an adaptation to the 

 manner of taking food from the water and breathing at the same time 

 in the whales, while in the young marsupials the milk is forced into the 

 mouth by the muscles of the mammae of the mother, an arrangement 

 that prevents strangulation. 



The trachea (Fig. 442) in the higher forms has a series of cartilagi- 

 nous rings forming its walls. It varies in length and size as well as the 

 quantity of cartilage strengthening its walls in the different genera. It 

 is as a rule shortest in lizards and often convoluted in turtles. The carti- 

 laginous rings may be entirely complete or the dorsal part of the ring 

 may be of membrane. It is usually longest in birds. 



It is interesting to note that the larynx never forms the voice organs 

 of birds. In this form of animal life, the sound producing parts are 

 formed from membranes which also vibrate by the passage of air, but 

 this voice organ is located at the point where the trachea divides into 

 bronchi and is known as a syrinx (Fig. 443). The most common form 

 of this organ is that in which the last rings of the trachea unite to form 



a resonating chamber, the tym- 



9 y f* **i$ panum, while folds of mem- 



brane, called internal and ex- 

 ternal tympanic membranes 

 (not to be confused with the 

 similarly named structure in 

 the ear), extend into the cavity 

 from the median and lateral 

 wall of each bronchus. 



In some instances there is 

 also an internal skeletal ele- 

 ment called a pessulus, bearing 

 a semilunar membrane on its 

 lower surface. This type of 

 syrinx may be a symmetrical 

 and even may form a bony 

 resonating vesicle. There are 

 various muscles attached to 

 trachea and bronchi which per- 

 mit an alteration of the tension 

 of the folds in all forms of 



Fig. 443. 



Columba livia. The lungs with the posterior end 

 of the trachea, ventral aspect, a.in., aperture of 

 anterior thoracic air-sac ; br., principal bronchus ; 

 br' ,br.' r br"'., secondary bronchi; p. aperture of ab- 

 dominal air-sac ; p.a., pulmonary artery entering 

 lung ; p.in., aperture of posterior thoracic air-sac ; 

 p.v., pulmonary vein leaving lung-; sb.b., aperture 

 of interclavicular air-sac; sp. b., aperture of cervical 

 air-sac; sy., syrinx; tr., trachea. (From Parker's 

 Zootomy. ) 



