xii PHYLUM CHORDATA 347 



In the wall of the sac, immediately outside the delicate 

 internal epithelium, is a rich network of blood-vessels, into 

 the blood contained in which oxygen from the air in the 

 cavity of the lung readily passes, while the carbonic acid 

 is at the same time given off. In the Rabbit the lung is 

 of much more complicated structure, but the essential 

 relations are the same. 



In the Lizard the lungs lie in the anterior part of the 

 general body cavity. In the Rabbit the anterior part of the 

 body-cavity, containing the lungs and the heart, is separated 

 off from the posterior part, containing the greater part of the 

 enteric canal and other organs, by a muscular partition 

 concave posteriorly the diaphragm, the anterior portion 

 of the cavity being known as the cavity of the thorax, and 

 the posterior as that of the abdomen. 



The air in the lungs, as it is constantly losing oxygen and 

 gaining carbon dioxide, requires to be frequently renewed ; 

 and the respiratory movements which have already been 

 referred to are the movements indicative of this renewal : in 

 the movement of inspiration air is drawn into the lungs, 

 which become fully distended ; in that of expiration, the 

 greater part of the air is driven out again, and the lung 

 collapses. In the Rabbit inspiration and expiration are 

 effected by the movements of the ribs and of the diaphragm, 

 by which the dimensions of the cavity of the thorax are 

 increased or diminished. 



The blood-vascular system is highly developed in all the 

 three examples. The blood is of a red colour owing to 

 the presence of red corpuscles containing a red colouring- 

 matter termed hemoglobin. 



The blood vessels are of three kinds arteries, veins, and 

 capillaries. The arteries have firm and elastic walls, which 

 do not collapse when the vessel is empty : they contain 



