MECHANISM OF ABSORPTION. 61 



ment, some provision for increasing the absorbing 

 power of the blood-vessels was required, and is 

 accordingly there found. Their peristaltic action 

 keeps up a steady continued pressure of the chyme 

 against the vessels, thereby regulating, to a certain 

 extent, the quantity of fluid effused from the latter, 

 at the same time that their calibre is diminished and 

 the velocity of the blood traversing them conse- 

 quently increased. It has been an interesting pro- 

 blem in physiology to explain how the foetus is 

 nourished by the maternal blood without any direct 

 vascular communication existing. On viewing the 

 maternal vessels, it is evident, from their increased 

 diameter, great length, and tortuous arrangement, 

 that the circulation through them must be much 

 retarded; while, on the other hand, the shorter 

 circuit of the foetal blood and the more rapid con- 

 tractions of the foetal heart, tend to prove that the 

 velocity of the foetal blood during its passage through 

 the placental vessels is much greater than that of the 

 maternal circulation in the same part. 



Since, therefore, it has been shown that the ab- 

 sorbing power of a stream is proportioned to its 

 velocity, it should follow that when two currents of 

 blood moving with different degrees of velocity are 

 in juxtaposition with each other, being separated 

 merely by two thin membranes, the more rapid 

 stream will not only draw in any fluid that may have 

 been already effused by the other, but may even 

 directly increase the rapidity of that exudation. 



In whatever part of the body the course of the 

 vessels is tortuous, or their coats are relaxed, a 



