RESPIRATION LN THE MAMMALIA. 151 



others gf less importance. The intercostal, or the muscles 

 between the ribs, ar the word signifies, assist in elevating 

 the sternum 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 respiration. 



455. Situation of the Diaphragm. This part is situ- 

 ated transversely and obliquely across 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 the diaphragm, 

 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 relaxation 

 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 bronchia, 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, coni- 

 cal bodies, situated within the lateral cavitie% of the chest, 

 which they completely fill. 



What is the situation of the diaphragm ? What part of the diaphragm 

 is muscular, and what part tendinous? What portion contracts and 

 relaxes during respiration? What becomes of the bronchia after they 

 reach the lungs * 



