THE GENERAL CIRCULATION. 91 



base of the brain, where the two vertebrals unite to form 

 the basilar artery. 



The utility of this arrangement, by anastomosis, between 

 the different divisions of the vascular system and the differ- 

 ent parts of each division, is strikingly seen in the safety 

 it confers, in the numerous accidents and operations to 

 which the human frame is daily subject. As, for instance, 

 when the main vessel leading to either extremity, as the 

 femoral or axillary artery, shall either by accident, disease, 

 or an operation, be divided or become obliterated, the limb 

 would necessarily die, were it not for this wise provision of 

 nature, in making the branches which go off from above 

 and below the point of the injured vessel, come together, 

 and thus carry on the circulation. 



The structure of the arteries, as of the vena portae, con- 

 sists of three membranes or coats, an external, middle, and 

 internal. The external is condensed cellular tissue, very 

 strong and resisting, composed of filaments closely bound 

 together, never containing any fat, and connected with the 

 surrounding parts. The middle coat, called also the mus- 

 cular, is regarded as the proper arterial tunic. Its fibres 

 are yellowish, brittle, elastic, and contractile. They sur- 

 round the artery in a circular manner, though not forming 

 complete rings, and are considered by many as essentially 

 muscular in their character. The microscope divides this 

 middle coat into three laminas, an outer, yellow, thin, elas- 

 tic a middle of circular muscular fibres, and an inner of 

 muscular, but longitudinal fibres; hence this peculiar com- 

 bination of structure and properties in this middle coat 

 explains how it is that when an artery is dilated, it returns 

 by its elasticity to its former natural state and how, from 

 its muscularity, it can be so contracted as to entirely de- 

 stroy its diameter and arrest the circulation. The experi- 

 ments of Mr. Hunter, as well as the daily operations of the 

 surgeon, conclusively establish the power of contraction in 

 the arteries. 



A circular section made in the aorta of a horse bled to 

 death, measured at first five inches and a half, on being 



