728 AVES. 



the lungs (c c), and by innumerable canals distribute air throughout 

 their spongoid substance ; but the main trunks of the bronchial tubes, 

 passing right through the pulmonary organs, open by wide mouths, 

 represented 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 inter- 

 communicating cells, all of which are freely permeated by the atmo- 

 spheric 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. 



(2062.) 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 ex- 

 panded 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. 



(2063.) 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 rarefied, 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 leverage and 

 firmer purchase ; so that their efforts are materially favoured. And, 

 lastly, it is owing to the capacity of the air-cells that the Singing Birds 

 are enabled to prolong their notes to that extent which renders them 

 pre-eminent among the vocalists of creation. 



(2064.) In connexion, therefore, with the respiratory system of the 

 feathered races, it will be advisable, in the next place, to consider 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. 



(2065.) The trachea is of very great proportionate length in corre- 

 spondence with the elongated neck commencing at the root of the 

 tongue, and extending into the thoracic cavity, where it divides into two 

 bronchial tubes, one appropriated to each lung (fig. 360, 1 1}. The 

 trachea of Birds is composed of cartilaginous rings, which are very 

 generally ossified, each ring, with the exception of two or three imme- 

 diately beneath the upper larynx, forming a complete circle (fig. 361, A) 

 surrounding the tracheal tube : these rings are enclosed between the soft 



