484 THE BLOOD- VASCULAR SYSTEM. 



the artery and vein, on a plane posterior to both. On opening the sheath these 

 three structures are seen to be separated from one another, each being enclosed in 

 a separate fibrous investment. 



Relations. At the lower part of the neck the common carotid artery is very 

 deeply seated, being covered by the integument, superficial fascia, Platysma, and 

 deep cervical fascia, the Sterno-mastoid, Sterno-hyoid. and Sterno-thyroid muscles, 

 and by the Omo-hyoid, opposite the cricoid cartilage ; but in the upper part of its 

 course, near its termination, it is more superficial, being covered merely by the 

 integument, the superficial fascia, Platysma, deep cervical fascia, and inner margin 

 of the Sterno-mastoid, and, when the latter is drawn backward, it is seen to be 

 contained in a triangular space, bounded behind by the Sterno-mastoid, above by 

 the posterior belly of the Digastric, and below by the anterior belly of the Omo- 

 hyoid. This part of the artery is crossed obliquely, from within outward, by the 

 sterno-mastoid artery; it is crossed also by the superior and middle thyroid veins, 

 which terminate in the internal jugular; and, descending on its sheath in front, is 

 seen the descendens hypoglossi nerve, this filament being joined by one or two 

 branches from the cervical nerves, which cross the vessel from without inward. 

 Sometimes the descendens hypoglossi is contained Avithin the sheath. The middle 

 thyroid vein crosses the artery about its middle, and the anterior jugular vein below; 

 the latter, however, is separated from the artery by the Stemo-hyoid and Sterno- 

 thyroid muscles. Behind, the artery is separated from the transverse processes of 

 the vertebrae by the Longus colli and Kectus capitis anticus major, the sympathetic 

 nerve being interposed between it and the muscles. The recurrent laryngeal nerve 

 arid inferior thyroid artery cross behind the vessel at its lower part. Internally, it 

 is in relation with the trachea and thyroid gland, the later overlapping it, the inferior 

 thyroid artery and recurrent laryngeal nerve being interposed : higher up, with the 

 larynx and pharynx. On its outer side are placed the internal jugular vein and 

 pneumogastric nerve. 



At the lower part of the neck the internal jugular vein on the right side diverges 

 from the artery, but on the left side it approaches it, and often overlaps its lower 

 part. This is an important fact to bear in mind during the performance of any 

 operation on the lower part of the left common carotid artery. 



PLAN OF THE RELATIONS OF THE COMMON CAROTID ARTERY. 



In front. 



Integument and superficial fascia. Omo-hyoid. 



Deep cervical fascia. Descendens and Communicans hypoglossi 

 Platysma. nerves. 



Sterno-mastoid. Sterno-mastoid artery. 



Sterrio-hyoid. Superior and middle thyroid veins. 



Sterno-thyroid. Anterior jugular vein. 



Externally. Internally. 



Internal jugular vein. /^ Trachea. 



Pneumogastric nerve. ( Thyroid gland. 



Recurrent laryngeal nerve. 



Inferior thyroid artery. 



Larynx. 



Pharynx. 

 Behind. 



Longus colli. Sympathetic nerve. 



Rectus capitis anticus major. Inferior thyroid artery. 



Recurrent laryngeal nerve. 



Peculiarities as to Origin. The right common carotid may arise above or below the upper 

 border of the sterno-clavicular articulation. This variation occurs in one out of about eight 

 cases and a half, and the origin is more frequently below than above ; or the artery may arise 

 as a separate branch from the arch of the aorta or in conjunction with the left carotid. The 

 left common carotid varies more frequently in its origin than the right. In the majority of 

 abnormal cases it arises with the innominate artery, or, if the innominate artery is absent, the 

 two carotids arise usually by a single trunk. It rarely joins with the left subclavian, except in 

 cases of transposition of the arch. 



