GENERAL CONSIDERATIONS. 517 



of the arteries vyith the bones is important to the surgeon ; as it enables him, 

 temporarily, to interrupt the circulation in these vessels by exercising external 

 pressure ou the points of their course which correspond to the several bones, 

 and thus diminish their calibre by flattening them. 



e. By virtue of their deep situation, the arteries are, in general, distant 

 from the skin ; there are, nevertheless, some which course immediately beneath 

 the inner face of that membrane ; these are only found about the head and 

 in the extremities. 



/. Lastly, all the arteries are enveloped by a layer of connective tissue, 

 ■which forms around them a kind of sheath, generally difficult to tear with 

 the fingers alone, and which isolates from the neighbouring parts, but chiefly 

 the veins. This connective tissue, more or less abundant according to the 

 regions, is always loose enough to allow the arteries to roll and be displaced 

 with the greatest facility, and thus to glide away from incisive bodies 

 accidentally introduced into the tissues. 



Anastomoses,. — Very often the arterial branches are united to each other by 

 communications, which have received the name of anastomoses, and which 

 assure the distribution of the blood in regulating its flow. There are 

 distinguished : 



1. Anastomoses hy convergence : formed by two vessels joining at their 

 terminal extremity in an angular manner, to form a third and more 

 voluminous trunk. 



2. Anastomoses hy arches or hy inosculation ; due to the junction of two 

 principal branches, which are inflected towards each other, meet, and unite to 

 form a single and curvilinear canal. 



3. Anastomoses by transverse communication : represented by ramifications 

 thrown transversely between two parallel arteries. 



4. Mixed or composite anastomoses : in which are found a combination of 

 the different types enumerated above. 



A knowledge of the anastomoses of vessels is of the highest practical 

 interest ; as these communications permit the surgeon, in extreme cases, to 

 tie the principal artery of a region without the latter experiencing any 

 considerable nutritive disturbance ; the blood continuing to arrive by the 

 collateral vessels which, at first very small, gradually dilate from the 

 excentric pressure to which their walls are submitted. But these anasto- 

 moses, if they offer this immense advantage, have also their inconveniences : 

 we allude to the difficulties experienced in arresting haemorrhage in wounds 

 of certain organs, owing to the relations of the principal vessel with its 

 communicating collaterals. 



Mode op Distribution. — The branches an artery distributes in the 

 neighbouring organs are distinguished as terminal and collateral. The 

 arterial trunks, after finishing a certain course, divide into several branches — 

 nearly always two, which, as new arteries, continue the primary vessel and 

 take the name of terminal hranches, because they really begin at the terminal 

 extremity of that vessel. 



The collateral vessels originate at various distances along the course of 

 the arteries, and proceed in a lateral direction : these collateral branches 

 increase in number as the arteries become more superficial, the ramifications 

 being particularly abundant around the articulations, and in the organs which 

 are prominent on the surface of the body. This abundance of vessels is 

 intended to maintain a moderate temperature in those parts which, by their 

 structure or situation, are exposed to sudden chills. 



The distinction between the terminal and collateral branches of arteries 

 36 



