CHAP. XXVIII.] ANASTOMOSES OF ARTERIES. 323 



from the heart, by the atttachment to that organ of each of the two 

 great vessels (the aorta and the pulmonary artery) which form the 

 main trunks of their respective systems. The middle coat of each 

 of these vessels is inserted into or adherent to the concavity of 

 three festoons composed of rounded cords of white fibrous tissue. To 

 the central portion of the convexity of each of these the muscular 

 fibre of the heart adheres closely in the case of the pulmonary 

 artery; but in that of the aorta, one festoon and the half of 

 another gives attachment to muscular fibre, whilst the other half of 

 this festoon, and a third are attached to a part of the fibrous zone, 

 which forms the base of the inner lip of the mitral valve. Along 

 the line of these festoons the lining membrane of the heart be- 

 coming continuous with that of the arteries forms three semilunar 

 curtains which are strengthened by processes of fibrous tissue con- 

 tinuous with the festoons. These folds constitute the valves of 

 these vessels, the only ones of the arterial system. We shall recur 

 to these by and bye. 



Arteries convey the blood to the various parts of the body, by 

 the subdivision of their trunks and the giving off of branch-vessels 

 at various points. The branches for the most part come off at an 

 acute angle with the continued trunk, so that the new stream of 

 blood does not experience any great diversion from the direction of 

 its parent stream. In a few instances, however, this arrangement 

 is not observed, as in some of the intercostal and lumbar arteries, 

 which form nearly a right angle with the aorta from which they 

 originate. 



Anastomoses of Arteries. The manner in which different arterial 

 trunks communicate with each other indirectly, is one of the most 

 interesting points in the anatomy of the arterial system. This 

 anastomosis of arteries often affords the means of supplying the 

 nutrient fluid to a limb after its principal artery has been obli- 

 terated, small collateral channels enlarging more or less for the 

 reception of a greater supply of blood than they were wont to con- 

 vey. Hence the study of the intercommunicating vessels has had 

 great influence upon the surgical treatment of aneurisms and 

 wounds of arteries. 



One of the most simple of these anastomoses is found in the 

 union of two arteries, originating from different trunks, to form one 

 as the vertebral arteries unite to form the basilar ; another kind 

 is when two vessels from the same or different trunks form, by their 

 union, an arch from the convexity of which others come off, which 

 form similar reunions and arches, giving off smaller branches which 



