The common carotid of the right side arises from the arteria innominata at the superior outlet of the thorax, behind, and on a level with the upper portion of the right sterno-clavicular articulation, and between the sternal and clavicular origins of the right sterno-cleido-mastoid muscle. On the left side it arises within the thorax, from the arch of the aorta. As the two common carotid arteries ascend in the neck they separate from each other, and terminate one on either side opposite the superior margin of the thyroid cartilage, below the great cornu of the os hyoides, and at a point corresponding to about the third cervical vertebra, about an inch below the angle of the lower jaw. In this course they are separated inferiorly by the trachea and oesophagus, and superiorly, at a greater distance, by the larynx and pharynx. Each of the carotid arteries is contained within a fibrous sheath, formed by a process of the deep cervical fascia; the internal jugular vein and pneumogastric nerve are also contained within it. The tendinous centre of the omo-hyoid muscle may be seen crossing in front of the sheath, and attaching itself intimately to it, nearly opposite the cricoid cartilage. The common carotid of each side may be thus considered as divided into two stages,—one below the omo-hyoid muscle, the other above it. We shall first describe the relations of the right common carotid artery in these two stages, then the course and relations of the left, and afterwards point out the differences between them.