GENERAL ANATOMY OF THE SANGUIFEROUS SYSTEM. 215 



been separated while this external cellular coat has remained 

 entire. 



The arteries are supplied with their proper blood-vessels and 

 lymphatics ; (i)asa vasorum.) It is to be observed, that these 

 blood-vessels are not derived from the artery on which they run, 

 but from the contiguous vessels. 



These vessels have nerves also, which are rather small in size, 

 when compared with those which go to other parts. 



Arteries appear to have a cylindrical form, for no diminution 

 of diameter is observable in those portions of them which send 

 off no ramifications. 



When an artery ramifies, the area of the different branches 

 exceeds considerably that of the main trunk. Upon this prin- 

 ciple, the aorta and its branches have been compared to a cone, 

 the basis of which is formed by the branches, and the apex by 

 the trunk.* 



The transverse section of an artery is circular. 



There are no valves in the arteries, except those of the ori- 

 fices of the aorta and the pulmonary artery, at the heart. The 

 valves of the pulmonary artery have been described in vol. i. 

 p. 509, and those of the aorta have an exact resemblance to 

 them, but are rather larger. 



The course of the arteries throughout the body is obviously 

 calculated to prevent their exposure to pressure, or to great 

 extension from the flexure of the articulations by which they 

 pass. With this view they sometimes proceed in a winding 

 direction ; and when they pass over parts which are subject to 

 great distension or enlargement, as the cheeks, they often 

 meander ; and, therefore, their length may be increased by 

 straightening, without stretching them. 



Their course appears sometimes to have been calculated to 

 lessen the force of the blood, as is the case with the Internal 

 Carotid and the Vertebral arteries. 



In the trunk of the body the branches of arteries generally 



* According to Brussiere the relation of the branches of the aorta to its 

 trunk is as 25 lo 16 ; Helvetius reckons the orifice of the aorta in comparison 

 with its branches as 64 to 71. Lassus. — h. 



