RESPIRATION IN THE MAMMALIA. 155 



others of less importance. The intercostal, or the mus- 

 cles between the ribs, as the word signifies, assist in 

 elevating the sternum and ribs, and thus of enlarging 

 the capacity of the chest in the act of inspiration, or 

 drawing in the breath. But the diaphragm is by far the 

 most important muscular agent in the process of respira- 

 tion. 



Situation of the Diaphragm. This part is situated 

 transversely and obliquely scross the body, dividing the 

 interior into two parts. Its anterior attachment is to 

 the inner surface of the breast bone, thence running 

 down in the direction of the ribs, it is attached to the 

 vertebra of the loins. The heart, large blood vessels, 

 and lungs are thus situated above and behind tbe dia- 

 phragm, while the stomach and liver are situated below 

 and before it. It is firmly attached to the pericardium, 

 a membrane surrounding the heart, and through it pass 

 the esophagus and large blood vessels. 



The centre of the diaphragm is tendinous, but around 

 its whole circumference, it is composed of muscle. The 

 muscular part only, is that which contracts and dilates 

 during respiration. 



It is chiefly by the alternate contraction and relaxa- 

 tion of this muscle, that the air is drawn into, and 

 expelled from the lungs. 



The trachea, or wind pipe, as we have before shown, 

 leads from the back part of the throat to the bronchia, 

 while the latter, is merely a division of the former into 

 two parts leading to each lung. 



The bronchi, when they reach the lungs divide into 

 numerous ramifications forming air tubes throughout 

 their whole substance, so that the structure of these 

 parts appear to consist of little more than fine air tubes, 

 made of a thin and delicate membrane. 



The lungs thus constructed, are two spongy, flatish, 

 conical bodies situated within the lateral cavities of the 

 chest, which they completely iill. 



What is the situation of the diaphragm 7 What part of the dia- 

 phragm is muscular, and what part tendinous ? What portion contract* 

 and relaxes during respiration 1 What become of the bronchia after 

 they reach the lungs ? 



