604 AVES BIRDS. 



mination of Jig. 274, the arrangement adopted will easily be under- 

 stood : the bronchi derived from the bifurcated inferior extremity 

 of the trachea plunge into the anterior face of the lungs (c, c), and 

 by innumerable canals distribute air throughout their spungoid 

 substance ; but the main trunks of the bronchial tubes, passing 

 right through the pulmonary organs, open by wide mouths, repre- 

 sented in the figure, into the cavity of the thorax, into which the 

 air likewise freely penetrates. The whole thoracico-abdominal 

 cavity is moreover divided by septa of serous membrane into 

 numerous intercommunicating cells, all of which are freely per- 

 meated by the atmospheric fluid, which in most instances is 

 admitted into the very bones themselves, and even penetrates to 

 the interspaces between the muscles of the neck and limbs ; thus, 

 in some birds of powerful flight, gaining free access to almost every 

 part of the system. 



The mechanism by which the air is drawn into, and then expelled 

 from, this extended series of respiratory cells, is sufficiently simple ; 

 the whole being accomplished by the movements of the expanded 

 sternum, assisted slightly by the abdominal muscles. The descent 

 of the sternum from the vertebral column necessarily enlarges the 

 capacity of the chest, and, acting like a great bellows, sucks in air 

 through the trachea, which not only fills all the spongy substance of 

 the lungs, but penetrates to all parts whereunto air is admitted; 

 while the ascent of the sternum, and consequent contraction of the 

 thoracico-abdominal space, alternately effects its expulsion. 



The results obtained by this unusual arrangement are of great 

 importance in the economy of the feathered races. In the first 

 place, the perfect oxygenization of the blood is abundantly secured. 

 Secondly, from the high temperature of the blood, the air drawn in 

 becomes greatly rarified, and thus materially diminishes the specific 

 gravity of the bird. Thirdly, from the inflation of the whole body, 

 the muscles, more especially those of flight, act with better lever- 

 age and firmer purchase ; so that their efforts are materially favoured. 

 And, lastly, it is owing to the capacity of the air-cells that the Sing- 

 ing Birds are enabled to prolong their notes to that extent which 

 renders them pre-eminent among the vocalists of creation. 



(67&.) In connection, therefore, with the respiratory system of 

 the feathered races, it will be advisable, in the next place, to con- 

 sider the construction of the air passages, whereby the atmospheric 

 fluid passes into and out of the body; and more especially of the 

 organs of voice connected with them. 



