THE COMMON CAROTID 479 



border of the sterno-mastoid there is a communicating vein l)et\veen the facial and 

 anterior jugular veins, which, as it crosses the line of the carotid artery, is in danger 

 of being wounded in the operation of tying the carotid. The descendens hypoglossi 

 nerve generally descends in front of the carotid sheath, being there joined by the 

 communicantes hypoglossi, one or two small l)ranches of the second and third 

 cervical nerves. At times this nerve runs within the sheath. There are usually 

 two lymphatic glands about the bifurcation of the artery. These are often found 

 enlarged and infiltrated in cancer of the lip and tongue. 



Behind, the common carotid lies on the longus colli and scalenus anticus below, 

 and rectus capitis anticus major above. Posterior to the artery, l;)ut in the same 

 sheath, is the pneumogastric nerve; and posterior to the sheath, the chain of the 

 sympathetic and the cervical cardiac branches of the sympathetic and pneumo- 

 gastric nerves. At the lower part of the neck the inferior thyroid arter}' courses 

 olili([Uely lieliind the carotid, as does likewise the recurrent laryngeal nerve. 



Internally, from below upwards, are the trachea and oesophagus, with the 

 recurrent laryngeal nerve in the groove between them, and the terminal branches 

 of the inferior thyroid artery, the lateral lobe of the thyroid l)ody, the cricoid car- 

 tilage, the thyroid cartilage, and the lower part of the pliarynx. At tlie angle of 

 bifurcation is a vascular structure known as the ganglion intercaroticum or the 

 carotid gland. 



Externally are the internal jugular vein and the pneumogastric nerve. On the 

 right side, at the root of the neck, the vein diverges somewhat from the artery, 

 leaving a space in which the pneumogastric nerve and vertebral artery are exposed. 

 On the left side the vein approaches and somewhat overlaps the artery, thus leaving 

 no interval corresponding to that on the right side. 



The cricoid cartilage is as a rule taken as the centre of the incision in the opera- 

 tion f<jr ligature of the common carotid artery. The incision is made in the line of 

 the vessel jnirallel to the anterior margin of the sterno-mastoid muscle. The omo- 

 hyoid forms one of the chief ralhdng points in the course of the operation for 

 ligature of the artery above that muscle, the usual situation. The artery is found 

 beating at the angle formed by the omo-hyoid with the sterno-mastoid. 



Branches. — (1) External and (2) internal carotid arteries. The common carotid 

 gives off no lateral branch, and consequently does not diminish in size as it runs up 

 the neck. It is often a little swollen just below its bifurcation, a condition that 

 should not be mistaken for an aneurismal dilatation. 



Variations of the Common Carotid Arteries 



The variations in the origin of the common carotid have been already mentioned under 

 Y.A.RrATio\s OF THE Chief Braxches OF THE AoRTic Arch (page 472). 

 The fulluwing vuriatiun.s are of surgical interest : — 



(A) The artery may cross obliquely the lower part of the trachea above the level of the 

 sternum. This may occur on the right side : (a) when the innominate is situatt-d abnormally to 

 the left of the middle line ; (i) when the right common carotid ari.ses as the second branch of the 

 aortic arch ; and (c) when the right and lel't common carotids aiise as a common stem from 

 the aorta. On the left side : when the left common carotid arises from tlie innominate. 



(B) The ri^ht common carotid maj', when arising I'rom the aorta, run behind the trachea and 

 oesophagus to the right side of the neck. 



(C) The commencement of the right common carotid may be above or below the usual sjiot, 

 according as the innominate bifurcates higher or lower than usual. A low bifurcation of the 

 innominate is .somewhat the more conunon abnormality. 



(D) The common carotid maj' run in a very tortuous manner, forming one or more distinct 

 loojts in its course uij the neck. 



(E) The artery may bifurcate higher or lower than normal. A high bifurcation is the more 

 common. The bifurcation may occur as high as the hj'oid bone, or even stj'loid process ; or as 

 low as the cricoid cartilage, or within an inch ami a half of its on'frin. 



(F) The artery may not bifurcate, but give off the branches usually derived from the external 

 division as it ascends in the neck. 



(G) The CMjmmon carotid may be absent, the external and internal carotids arising directly 

 from the aorta. 



(H) It may give off one or more of the branches usually derived from the external carotid. 



(I) It may give off a thyroidea ima. 



(J) The pneumogastric nerve may run in front of the artery instead of behind it. 



The collateral circulation (fig. 330), after ligature of the common caroti<l, is carried on chiefly 



