22 MAEINE AND AQUATIC TRESPASSERS. 



the region of the chest. They are twenty in number, 

 ten on either side, and in the whale they are enor- 

 mously developed, so as to produce the remarkable 

 appendage to respiration which has been mentioned. 



This, however, is not all. Beside this enormous 

 mass of arteries, there is a corresponding network of 

 veins, which appear to form reservoirs for the blood 

 which has been passed through the system and 

 become useless. 



Even in these veins there is a difference in struc- 

 ture. In most veins there is a series of valves, which 

 allow the blood to pass forward but not backward ; so 

 that, no matter how rapidly the heart may beat, the 

 blood is propelled forward with corresponding force. 



Any of my readers, who have been accustomed to 

 running, or have subjected themselves to training, are 

 familiar with the phenomenon called " second wind." 

 When the first half mile or so has been passed, the 

 action of the heart seems to be so much in advance of 

 that of the lungs that the runner feels half-choked, 

 gasps, and is almost unable to proceed. If, however, 

 he doggedly perseveres, the feeling of oppression soon 

 goes off, and he can then run for almost any distance 

 without distress, as far as respiration goes. His legs 

 may fail him, but his lungs will not. 



The fact is, that after a little time, the blood that 

 has been violently forced through the system, has had 

 time to equalize its rate of progress with the increased 

 pressure, and breathing becomes as easy as if the 

 runner were seated at rest. 



In the reserved vein-system of the whale tribe, 

 however, there are scarcely any valves, so that the 



