VERTEBRATA. 



the body, even the most solid, were originally formed, 

 and by which they are increased and supported. This 

 fluid ceaselessly circulates through two series of ves- 

 sels, ramifying with inconceivable minuteness to every 

 part of the animal. In the one set, called veins, re- 

 ceiving the blood after it has performed its renovating 

 office, we have said that it mixes with foreign matter 

 from the digested food ; thus supplied, it is carried to 

 the heart, a large hollow muscle, which alternately 

 contracts and expands without intermission ; by which 

 motions it is received and thrown forward in regular 

 pulsations. Hence, in whole or in part, it is carried, 

 in those animals which breathe air, to the lungs, a 

 spongy mass of minute cells communicating with the 

 external air. Over the surface of these cells the 

 blood-vessels ramify, and their coats become so thin 

 as to admit the union of the oxygen with the con- 

 tained blood, the result of which is an instant change 

 of its colour from dark red to vivid scarlet. The gills 

 of fishes perform the same office as the lungs. The 

 blood thus renewed is returned by veins to another 

 chamber of the heart, whence it is ejected into the 

 other set of vessels, called arteries, which, large at 

 first, but branching with innumerable ramifications, 

 convey it to every part, and at last, in a manner not 

 thoroughly under stood ; transfer it into the extremi- 

 ties of the veins, which, as we saw, carry it back to 

 be renewed again. 



The young are produced in all cases from eggs, 

 which exist in the body of the female ; in some cases 



B 2 



