14 ON THE STRUCTURE AND 



upwards, and which serve to prevent the blood from return- 

 ing into the ventricle. 



The terminations both of the arteries and veins are exceed- 

 ingly minute. It is from these that the blood deposits its 

 molecules, destined to replenish the solids, and that various 

 kinds of humours are filtrated. From the constant motion 

 of the juices, from the friction of the solids on one another, 

 and from other causes, many particles are lost. These are 

 repaired by the circulation of the vital fluid. If the nou- 

 rishing particles that are added be more than those that are 

 lost, the body is said to grow ; that is, the fibres of which 

 the solids consist become gradually more and more enlarged. 

 Growth, however, has its bounds, at which it stops : for 

 when, by continual nourishment, the fibres are rendered 

 thick and hard, the interstices at length are so filled up, and 

 the fibres become so close, that no nourishing particles can 

 pass between them, and consequently the body ceases to 

 grow. At last the vessels become so gross that the juices 

 cannot force their way through them, but remain thick and 

 unelaborated. Their motion grows languid, and when at 

 last it stops, the body ceases to live, or dies. For a little 

 while after death it retains its organic structure; but the 

 parts soon begin to separate and decompose : they fall clown, 

 and take their place in the mineral kingdom, from which 

 they were originally formed. 



The blood, in its circuit through the body, does not re- 

 turn to the heart till it has passed through the lungs. Here, 

 in its contact with the air, it undergoes a process which is 

 absolutely necessary towards rendering it capable of afford- 

 ing the proper nutriment to the system ; and it is in con- 

 sequence of this process that it attains that heat which 

 it afterwards communicates to all the different parts of the 

 body. After the blood has passed the lungs it is of a ver- 

 milion colour, and frothy consistence : but, when on ite 



return 



