XII PHYLUM CHORDATA 349 



orly, — 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 respiration 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 heemoglobin. 



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 

 arterial blood, i.e., blood which contains abundance of 

 oxygen. The veins have thin, non- elastic walls which col- 

 lapse when the vessel is empty and contain valves ; the con- 

 tained venous blood is darker in colour than the arterial, and 

 has been deprived of oxygen in the tissues. Both arteries 

 and veins ramify extensively, the ultimate branches being of 

 very small size. Connecting together the ultimate branches 

 of the arteries and the ultimate branches of the veins is a 

 system of microscopic vessels — the capillaries. 



The heart is ventral and anterior in position. In the dog- 

 fish it will be found to lie in a space, the pericardial cavity, 

 between the two rows of gills, and separated behind from the 



