THE ARTERIES. 



fTlHE distribution of the systematic arteries is like a highly ramified tree, the 

 -L common trunk of which, formed by the aorta, commences at the left ventricle, 

 while the smallest ramifications extend to the peripheral parts of the body and the 

 contained organs. Arteries are found in all parts of the body, except in the hairs, 

 nails, epidermis, cartilages, and cornea; the larger trunks usually occupy the most 

 protected situations, running, in the limbs, along the flexor surface, where they 

 are less exposed to injury. 



There is considerable variation in the mode of division of the arteries : occasion- 

 ally a short trunk subdivides into several branches at the same point, as may be 

 observed in the celiac artery and the thyrocervical trunk : the vessel may give off 

 several branches in succession, and still continue as the main trunk, as is seen in 

 the arteries of the limbs; or the division may be dichotomous, as, for instance, when 

 the aorta divides into the two common iliacs. 



A branch of an artery is smaller than the trunk from which it arises; but if an 

 artery divides into two branches, the combined sectional area of the two vessels 

 is, in nearly every instance, somewhat greater than that of the trunk; and the 

 combined sectional area of all the arterial branches greatly exceeds that of the 

 aorta; so that the arteries collectively may be regarded as a cone, the apex of 

 which corresponds to the aorta, and the base to the capillary system. 



The arteries, in their distribution, communicate with one another, forming 

 what are called anastomoses, and these communications are very free between 

 the large as well as between the smaller branches. The anastomosis between trunks 

 :>f equal size is found where great activity of the circulation is requisite, as in the 

 brain; here the two vertebral arteries unite to form the basilar, and the two ante- 

 rior cerebral arteries are connected by a short communicating trunk; it is also 

 round in the abdomen, where the intestinal arteries have very ample anastomoses 

 oetween their larger branches. In the limbs the anastomoses are most numerous 

 ind of largest size around the joints, the branches of an artery above uniting 

 >vith branches from the vessels below. These anastomoses are of considerable in- 

 terest to the surgeon, as it is by their enlargement that a collateral circulation is 

 established after the application of a ligature to an artery. The smaller branches 

 :>f arteries anastomose more frequently than the larger; and between the smallest 

 twigs these anastomoses become so numerous as to constitute a close .network 

 that pervades nearly every tissue of the body. 



Throughout the body generally the larger arterial branches pursue a fairly 

 straight course, but in certain situations they are tortuous. Thus the external 

 maxillary artery in its course over the face, and the arteries of the lips, are extremely 

 tortuous to accommodate themselves to the movements of the parts. The uterine 

 arteries are also tortuous, to accommodate themselves to the increase of size which 

 the uterus undergoes during pregnancy. 



The Pulmonary Artery (A. Pulmonalis) (Figs. 503, 504). 



The pulmonary artery conveys the venous blood from the right ventricle of the 

 heart to the lungs. It is a short, wide vessel, about 5 cm. in length and 3 cm. in 



(543) 



