730 



AVES. 



Fig. 362. 



Muscles of the superior larynx 

 of a bird. 



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 capable. 



(2069.) In the Insessorial Birds, by 

 far the most accomplished songsters, 

 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 (fig. 360, 

 A, B, n, o, z, s, ft). In the Parrots, three 

 pairs only are met with*; some of the 

 Natatores have two ; other Natatorial 

 birds, as well as the Rasores and Gral- 

 latores, only one ; and in a few, as the 

 King of the Vultures and the Condor, 

 the vocal muscles are quite deficient. 



(2070.) Not only is the respiration 

 of these highly- gifted Vertebrata thus 

 abundantly provided for, but, as an im- 

 mediate 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 auricle and of a strong 

 ventricular chamber. The right side of the heart receives the vitiated 

 blood from all parts of the system, which is poured into the corre- 

 sponding auricle by three large veins, viz. one inferior and two superior 

 vence cavce. The contraction of this auricle drives the blood into the 

 right ventricle, the auriculo-ventricular opening being guarded by a 

 broad fleshy valve, formed by the muscular substance of the heart itself; 

 and hence the venous blood is forced through all the ramifications of 

 the pulmonary arteries. 



(2071.) The aerated blood is then returned from the lungs by two 

 veins, which pour it into the left auricle ; and the left ventricle, now 

 entirely appropriated to the systemic circulation, diffuses it through the 

 body : thus, all mixture of the venous and arterial fluids being prevented, 

 the system is supplied by the left side of the heart with pure and highly- 

 vitalized blood. 



(2072.) In the nervous system of Birds there is a very perceptible 

 improvement when compared with that of Reptiles, more especially in 

 the increased proportional development of the cerebral hemispheres : 

 still, however, there are no convolutions seen upon the surface of the 

 cerebrum, neither are those extensive communications between the 

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



