486 THE BLOOD-VASCULAR SYSTEM, 



and the sheath of the vessel is exposed. This must be opened with great care on its inner 

 or tracheal side, so as to avoid the internal jugular vein. This is especially necessary on 

 the left side, where the artery is commonly overlapped by the vein. On the right side there is 

 usually an interval between the artery and the vein, and not the same risk of wounding the 

 latter. 



The common carotid artery, being a long vessel without any branches, is particularly suitable 

 for the performance of Brasdor's operation for the cure of an aneurism of the lower part of the 

 vessel. Brasdor's procedure consists in ligaturing the artery on the distal side of the aneurism, 

 and in the case of the common carotid there are no branches given off' from the vessel between 

 the aneurism and the site of the ligature ; hence the flow of blood through the sac of the 

 aneurism is diminished and cure takes place in the usual way, by the deposit of laminated fibrin. 



Collateral Circulation. After ligature of the common carotid the collateral circulation 

 can be perfectly established, by the free communication which exists between the carotid arteries 

 of opposite sides, both without and within the cranium, and by enlargement of the branches of 

 the subclavian artery on the side corresponding to that on which the vessel has been tied the 

 chief communication outside the skull taking place between the superior and inferior thyroid 

 arteries, and the profunda cervicis and arteria princeps cervicis of the occipital ; the vertebral 

 taking the place of the internal carotid within the cranium. 



Sir A. Cooper had an opportunity of dissecting, thirteen years after the operation, the case 

 in which he first successfully tied the common carotid (the second case in which he performed 

 the operation). 1 The injection, however, does not seem to have been a successful one. It 

 showed merely that the arteries at the base of the brain (circle of Willis) were much enlarged on 

 the side of the tied artery, and that the anastomosis between the branches of the external carotid 

 on the affected side and those of the same artery on the sound side was free, so that the external 

 carotid was pervious throughout. 



The External Carotid Artery. 



The external carotid artery (Fig. 283) commences opposite the upper border of 

 the thyroid cartilage, and. taking a slightly curved course, passes upward and for- 

 ward, and then inclines backward to the space between the neck of the condyle 

 of the lower jaw and the external meatus, where it divides into the temporal and 

 internal maxillary arteries. It rapidly diminishes in size in its course up the neck, 

 owing to the number and large size of the branches given off from it. In the 

 child it is somewhat smaller than the internal carotid, but in the adult the two 

 vessels are of nearly equal size. At its commencement this artery is. more super- 

 ficial, and placed nearer the middle line than the internal carotid, and is contained 

 in the triangular space bounded by the Sterno-mastoid behind, the Omo-hyoid 

 below, and the posterior belly of the Digastric and Stylo-hyoid above. 



Relations. It is covered by the skin, superficial fascia, Platysma, deep fascia, 

 and anterior margin of the Sterno-mastoid, crossed by the hypoglossal nerve, and 

 by the lingual and facial veins ; it is afterward crossed by the Digastric and 

 Stylo-hyoid muscles, and higher up passes deeply into the substance of the parotid 

 gland, where italics beneath the facial nerve and the junction of the temporal and 

 internal maxillary veins. Internally is the hyoid bone, wall of the pharynx, the 

 superior laryngeal nerve, and the ramus of the jaw, from which it is separated by 

 a portion of the parotid gland. Externally, in the lower part of its course, is the 

 internal carotid artery. Behind it, near its origin, is the superior laryngeal nerve ; 

 and higher up, it is separated from the internal carotid by the Stylo-glossus and 

 Stylo-pharyngeus muscles, the glosso-pharyngeal nerve, and part of the parotid 

 gland. 



PLAN OF THE RELATIONS OF THE EXTERNAL CAROTID. 



In front. 



Skin, superficial fascia. 

 Platysma and deep fascia. 

 Anterior border of Sterno-mastoid. 

 Hypoglossal nerve. 

 Lingual and facial veins. 

 Digastric and Stylo-hyoid muscles. 



Parotid gland with facial nerve and temporo-maxillary vein in its 

 substance. 



1 Guy's Hospital Reports, i., 56. 



