PHYSIOLOGICAL ANATOMY OF THE ARTERIES. 243 



pression. In some situations, also, as in the vessels going to 

 the brain, particularly in some inferior animals, it is neces- 

 sary to moderate the force of the blood current, on account 

 of the delicate structure of the organs in which they are dis- 

 tributed. Here Nature makes a provision in the shape of 

 anastomoses ; by which, on the one hand, compression of a 

 vessel simply diverts, and does not arrest, the current of 

 blood, and on the other hand, the current is rendered more 

 equable and the force of the heart moderated. 



The arteries are provided with membranous sheaths, of 

 greater or less strength, as the vessels are situated in parts 

 more or less exposed to disturbing influences or accidents. 



Researches into the minute anatomy of the arteries have 

 shown that they are possessed of three pretty well marked 

 coats. As these vary very considerably in arteries of different 

 sizes, in their description, it is convenient to divide the ves- 

 sels into three classes. 



1. The largest arteries / in which are included all that are 

 larger than the carotids and common iliacs. 



2. The arteries of medium size that is, between the 

 carotids and iliacs and the smallest. 



3. The smallest arteries ; or those less than -^ to -^ of an 

 inch in diameter. 1 



The largest arteries are endowed with great strength and 

 elasticity. Their external coat is composed of white or in- 

 elastic fibrous tissue. According to Kolliker, -this coat is no 

 thicker in the largest vessels than in some of the vessels of 

 medium size. In some medium-sized vessels it is actually 

 thicker than in the aorta. This is the only coat which is 

 vascular. 



The middle coat, on which the thickness of the vessel de- 



1 This is essentially the division made by Kolliker (Manual of Human Micro- 

 scopic Anatomy, London, 1860, p. 485). Some anatomists make five or even 

 more coats to the arteries. The three coats are pretty well marked, each pos- 

 sessing distinctive properties. The numerous coats which are sometimes given 

 are, many of them, simple layers of the same tissue. The division into three coats 

 is more simple and physiological. 



