The _primary carotid_, capr. After leaving the heart, this very large vessel passes cephalad and laterad for some distance on the left side of the body and then gives off, from its anterior side, the large left subclavian artery, sc.s., to be described later. After giving off the subclavian artery, it makes a short loop, still farther to the left, and then turns sharply mediad to pass to the head in the median plane directly dorsal to the œsophagus. Its distribution in the cervical and cephalic region will be described later. The mate to the œsophageal branch, oe (near heart), of the right subclavian which was mentioned above is apparently sometimes given off from the primary carotid near its base (as shown in Fig. 62) and sometimes as a branch of the left pleural artery. The left _subclavian_ artery, sc.s., although it has a different origin, has the same branches as described in connection with the right subclavian. The exact order in which the first of these (the thyroid, th; the internal mammary, im¹; the collateralis colli, cc; the pleural, plu; and the vertebral, v¹) are given off is, as might be expected, subject to some variation. The _collateralis colli_, cc (following Bronn’s nomenclature), whose origin was noted above, will now be discussed; since the two are alike only one need be described. After leaving the subclavian, it passes cephalad, at the side of the trachea and œsophagus, in company with the internal jugular vein, so that in this part of its course it would seem to be the internal carotid artery. It gives numerous small twigs to the trachea and œsophagus, oe. In the region of the posterior part of the huge jaw muscle it is connected directly, x, with the adjacent branch, cm (called by Bronn the common carotid) of the primary carotid, and indirectly, x¹, with a complicated group of branches from the common carotid. Cephalad to the connective x¹, which extends dorsad and is hence foreshortened in the figure, the collateralis colli gives off a small vessel, y (too large in Fig. 62), to the shoulder and skin; it then sends a fairly large branch, jm, into the large jaw muscle, close to which it now lies. Next a small branch, lg, is sent to the larynx. Continuing cephalad and laterad (in Fig. 62 it is drawn farther to the side than it actually lies) for a short distance farther, it divides into three branches: (1) a short twig, mg, that goes to the musk gland on the side of the mandible and to the skin of that region; (2) a large branch, the _mandibular_, md, that enters the large foramen on the mesial side of the mandible and extends in the cavity of that bone throughout its entire length; (3) the _lingual_ artery, l¹, which in turn divides, some distance cephalad, into two branches, one extending along the lateral region, the other nearer the mid-ventral surface of the tongue. It is seen, then, that the collateralis colli arteries supply directly the lower side of the head--tongue, mandible,