606 AVES BIRDS. 



must suppose the cricoid to be represented by three distinct ossicles, 

 two of which (c, c) are lateral, while the third or central portion 

 (e) supports the arytenoid bones (/,/), which are moveably arti- 

 culated with its anterior margin. The arytenoid bones themselves 

 are of an elongated form, and each presents a long process (g, g) 

 for the insertion of the muscles that act upon them. These 

 arytenoid bones are moved by two pairs of muscles ; the superficial 

 pair (thyro-arytenoidei, Jig. 276, B) serving to pull asunder, 

 while the more deeply seated (constrictores glottidis^Jig. 276, A) 

 bring together the lips of the glottis. 



(673.) It is the lower larynx, 

 situated at the opposite extremity of 

 the trachea, at the point where that 

 tube gives off the bronchi, that 

 the real vocal apparatus of birds is 

 situated ; and in the more perfect 

 Singing Birds a very important 

 set of muscles is appropriated to 

 perform those delicate movements 

 that regulate the condition of the 

 air-passages at this part, and thus 

 give rise to all the varieties of 

 tone of which the voice is ca- 

 pable- 



In the Insessorial Birds, by 

 far the most accomplished song- 

 sters, five pairs of muscles are connected with the inferior larynx ; 

 and so disposed as to influence both the diameter and length of the 

 bronchial tubes (Jig. 274, A, B, n, o, z 9 s, h). In the Parrots, three 

 pairs only are met with ;* some of the Natatores have two ; other 

 natatorial birds, as well as the Rasores and Grallatores, only one ; 

 and in a few, as the King of the Vultures and the Condor, the 

 vocal muscles are quite deficient. 



(674.) Not only is the respiration of these highly gifted Vertebrata 

 thus abundantly provided for, but, as an immediate consequence of the 

 necessity for supplying the system with pure and highly oxygenized 

 blood, the heart, hitherto but imperfectly divided, becomes now 

 separated into two distinct sets of cavities, each composed of an au- 

 ricle and of a strong ventricular chamber. The right side of the 

 heart receives the vitiated blood from all parts of the system, 

 * Vide Yarrell on the Organs of Voice in Birds. Linn. Trans, vol. xvi. 



