88 



SITUATION OF THE ARTERIES. 



The former sort of communications must accelerate the 

 motion of the blood, for you know when a fluid passes 

 from a larger into a smaller channel, the forces by which 

 it is impelled remaining the same, its progress is quick- 

 ened. 



Dr. B. These arterial communications are called 

 anastomoses, and are very useful, in supplying organs 

 and parts from which, by any cause, the usual supplies 

 are cut off. This utility is very manifest in diseases of 

 of vessels which require them to be tied. Thus the two 

 great arteries which go up along the sides of the neck, 

 meet together in the base of the brain and form a single 

 vessel, so that if one of these vessels be tied, the brain, 

 you see, will be supplied with blood just as well as be- 

 fore. 



Emily. But it certainly cannot receive so much, be- 

 cause it is evident that one vessel cannot transmit so 

 much blood as two. 



Dr. B. This difficulty is obviated by a very admira- 

 ble law of the human economy. When an artery is tied 

 or otherwise obstructed, the neighboring vessels enlarge 

 their calibre, so that they ultimately transmit to the parts 

 as much blood as they received before. Such is this sin- 

 gular power of the vessels to accommodate their capaci- 

 ty to the wants of the system, that modern surgeons have 

 presumed upon it to an almost incredible degree, and 

 with such beneficial results as to redound greatly to the 

 honour of their art. When an artery is diseased, even 

 though it be the chief artery in the limb, they do not 

 hesitate to tie it up, to prevent it from bursting hs coats 

 and killing the patient by a terrible hemorrhage. Sir 

 Astley Cooper once tied up the aorta itself in the ab- 

 domen just before it divides into the iliacs, and though 

 his patient died a few hours afterwards, he attributed his 

 death to the disease rather than the operation. 



The position of the arteries evinces that great precau- 

 tion has been kept in view for their security and protec- 

 tion against external accidents, They generally lie deep 



