RELATION OF RESPIRATION WITH THE MUSCLES. 237 



are the pulsations of the heart so rapid and so frequently 

 repeated. In Reptiles, we have seen that respiration is 

 quite inactive and Jeeble, and they can dispense with it 

 for a great length of time. See how perfectly in accord- 

 ance with this condition is their circulation. 



Emily. The pulsations of the heart are quite slow 

 and unfrequent, and only half of the blood sent to the 

 lungs requires its action, for the other half has just re- 

 turned thence, and needs no change. 



Dr. B. In Fishes, respiration is performed by gills 

 which separate the air from the Water, and owing to this 

 disadvantage, greater activity of both the respiration and 

 circulation, is necessary to produce a given result. 



In Insects, the air enters the body by numerous pores, 

 not by a particular set of organs ; of course neither heart 

 nor blood-vessels aie required to transmit the blood to 

 these organs. 



The relations between respiration and muscular mo- 

 tion are exceedingly obvious and well defined. The 

 quantity and activity of one are generally proportional to 

 the quantity and activity of the other, for one of the 

 principal uses of respiration, is to reanisnate the muscular 

 power. 



Emily. And is not this the reason why the respira- 

 tion is always accelerated when we are taking exercise,, 

 or performing any task which requires considerable ex- 

 ertion of muscular power. Observe too, how a dog or 

 horse pants, after running violently, or for any length of 

 time. 



Dr. JB. This is the reason, undoubtedly. In the 

 birds and mammiferous animals which have the function 

 of respiration developed to its highest degree of excel- 

 lence, the muscular power is capable of the greatest and 

 longest-continued exertions. Birds, in which we just 

 observed that the respiratory function existed in its 

 greatest perfection, are almost always in the air, and the 

 locomotive feats which they sometimes perform are al- 

 most incredible. 



