OP THE VESSELS IN GENERAL. 245 



volume of the vessel, increases from the trunk to its ramifica- 

 tions. The cavity presents exactly, as we have already said 

 [354,] of the vessels themselves, the cylindric form in each 

 division; that of a cone diminishing in size from the trunk to 

 one of the last divisions; and that of a cone increasing from 

 the trunk to all its ramifications. 



360. The texture of vessels is formed of several layers 

 placed one over the other, and more or less distinct. 



The internal membrane is thin, whitish, more or less dia- 

 phanous, uniform, without any visible fibres, every way con- 

 tinuous, but different in the arteries and veins. It very much 

 resembles serous membranes, and is moistened by a liquid, 

 the origin of which is not well known. It forms, according 

 to the different kinds of vessels, a greater or smaller number" 

 of valves or duplicatures, arranged in such a manner as to per- 

 mit the passage of the fluids in the direction of the circulation, 

 and preventing their retrograde course. 



The external coat, which must not be confounded with the 

 cellular sheath which closely surrounds the vessels, is thicker 

 than the internal, is fibro-cellular, and is generally formed 

 with filaments, which are oblique with respect to the direction 

 of the vessel, and which decussate each other. 



Between these two membranes, a third which is fibrous, is 

 observed, it is distinct in all the arteries that can be dissected, 

 as well as in the larger veins. 



361. The external membrane of the vascular system, and 

 especially the middle coat or membrane of the vessels, which 

 are provided with it, are formed with a particular fibre. This 

 fibre has been named elastic fibre, elastic fibrous tissue, &c. 

 although the greater number of the organs are elastic and 

 fibrous, but because it possesses this property in the highest 

 degree. Its elasticity had already been observed by Nicholls, 

 J. Hunter, and Ed. Home;* some modern anatomists and 

 chemists have made it an object of their study.t 



It not only forms the parietes of the vessels, but of those of 



* Croonian, Lecture an Muscular Motion, in Philos. Trans, ann. 1795. 

 f H. Hauff, de systeme telse elastic^ &c. Tubing*, 1822. Chcvreul, from 

 an unpublished note. 



