212 BIOLOGY. [Book ii. 



This second membrane, closely joined to the first, contains fibre- 

 cells, this time transversal, which already indicate the rise of 

 the contractile tunic of the larger vessels. Finally, these 

 capillaries, with their transversal fibre-cells, are continued along 

 with vessels still larger, from 0'"'",060, to 0°™,140, which, besides 

 the two preceding tunics, have a third external tunic, a coating 

 of laminous longitudinal fibres. These last vessels, already 

 visible to the naked eye, have direct continuance in the arterioles 

 and the veinules. 



In diverse regions of the body, the arteries have, apart from 

 the capillaries, transversal channels of communication, which 

 connect them direct with the veins. These are small vessels 

 furnished with a muscular layer, comparatively thick. If these 

 vessels contract, the blood is compelled to pass through the 

 muscular network ; if, on the contrary, the last contractile rings 

 of the arterioles shrink up and cause the occlusion of the capil- 

 laries, the blood passes through the collateral vessels, and, in 

 that case, it is no longer serviceable to nutrition. This mechanism 

 is observable in certain glands, and notably in the venous 

 apparatus of the liver, where it serves to regulate the glyco- 

 genical secretion, to deprive of blood the secreting cells, or to 

 allow the adequate ration thereof to reach them.^ 



We can now form a general idea of the province of the capil- 

 laries in nutrition. The sanguineous stream, driven first of all 

 by the heart, then by the arteries, directs its course toward the 

 capillary network, for a time in an abrupt and pulsatile fashion, 

 then more and more regularly. Arriving at the capillaries, the 

 arterial blood moves at a moderate and uniform rate, at least in 

 the normal state. We have seen that in the finest canalicules 

 the globules only pass one by one, and so to speak, by friction. 

 Then they and the plasma which bathes them and carries them 

 on are only separated from the anatomical elements constituting 

 the tissues by a thi n homogeneous membrane of which we have 

 spoken. The osmotic conditions are therefore very favourable 



^ CI. Bernard, Lefons sw Us ProprUUs des Tissus vivants, pp. 415, 416. 



