188 CIRCULATORY SYSTEM. 



channel, do not " take their origin immediately opposite the ter- 

 mination of the vessels into the circulus arteriosus," They are 

 destined for the supply of the sole, upon which they run in radii, 

 at pretty equal distances, whose coumion centre is the toe of the 

 frog, where they end in communications with the arteries belong- 

 ing to that body. 



The Carotid Arteries. 



The right arteria innominata (having detached seven branches 

 which differ but unimportantly in their mode of origin, and not at 

 all in their general course and distribution, from the seven arte- 

 ries into which the left division resolves itself) becomes the com- 

 mon carotid: a vessel of large caliber, about an inch in length, 

 emerging through the upper part of the anterior opening of the 

 chest, having the trachea between it and the spine, above ; the 

 vena cava anterior, below it ; and dividing, as it quits the cavity, 

 into the right and left carotids. These arteries bend upward from 

 their origin, diverging as they ascend along the neck so as to 

 leave a space between them for the windpipe, which they closely 

 embrace and cling to for the first part of their course : towards 

 the middle of the neck, however, we find them gradually inclining, 

 as they ascend, to the posterior and lateral borders of that tube, 

 a line of direction they preserve during the remainder of their 

 course*. Having reached the top of the larynx, the carotid of 

 either side splits into three divisions — the external and internal 

 carotids, and the ramus anastomoticus : at which place, though 

 the trunk itself is found deeply lodged in soft parts, yet its situ- 

 ation is well indicated by the larynx (with which it is in contact) 

 below; the transverse process of the atlas, above; the angle of 

 the jaw a little in advance of it; and the coronoid process farther 

 removed above and before it. A deep incision corresponding to 

 the posterior border of the stylo-maxillaris, would sever the vessel 

 at or very close to its division. The carotid detaches — I. Several 

 unimportant muscular branches in its progress up the neck. 

 2. The th^roideal artery, coming off opposite to the top ring of 

 the trachea : a branch of no mean size, which turns round the 

 windpipe and enters the substance of the thyroid gland. In its 

 way, the thyroideal furnishes the laryngeal, a small artery that 

 perforates the ligament uniting the cricoid and thyroid cartilages, 



* The carotids lie more deeply the higher they proceed up the neck : 

 each is covered by the steruo-maxillaris ; and the readiest way to find the 

 vessel is to make a cut along the upper border of this muscle, and depress 

 it with the finger or handle of the knife. The artery is separated, except at 

 the bottom of the neck, from the jugular vein by a thin partition of muscu- 

 lar fibres; the par vagum accompanies it outwardly, and the sympathetic 

 n^rve runs between the two. 



