POSTERIOR BRANCHES OF THE EXTERNAL CAROTID.-iSt. The occipital artery ( a. occi pitalis) arises from the posterior side of the external carotid, opposite the lingual or the facial ; it sometimes but rarely comes from the internal carotid ; it passes at first a little ob liquely backwards along the lower border of the posterior belly of the digastric muscle which overlaps it; it crosses over the ninth pair of nerves which winds beneath it just at its origin, the internal carotid artery, internal jugular vein, and spinal accessory nerve ; and passing backwards between the transverse pro cess of the atlas and the mastoid process of the temporal bone it is lodged in a groove in this latter bone, which is internal to the inser tion of the posterior belly of the digastric ; it crosses the outer border of the rectus capitis latemlis muscle, and continuing its course beneath the stemo-cleido-mastoid, trachelo mastoid, splenius capitis and trapezius, and over the obliquus superior and complexus, it ascends tortuously over the superior part of the occipital bone, where it becomes cutaneous and anastomoses with branches from the tem poral, posterior auris, and opposite occipital. The first branches of the occipital are small, and are distributed to the sterno-mastoid, di gastric, and stylo-hyoid. muscles, and to the lymphatic glands in the neighbourhood ; the bmnches which enter the sterno-mastoid are sometimes considerable, and anastornoso freely in the substance of that muscle with the branches which it receives from the superior thyroid.
The sterno-mastoid muscle very frequently receives a large branch at this part arising dis tinctly from the external carotid. This Professor IIarrison considers should be classed among the regular branches of the external carotid, and he has described it under the name of a. sterno-mastoidea.* While the occipital artery is covered by the sterno-mastoid, trachelo-mastoid, and splenius, it gives branches to these muscles, some of which descending anastomose with branches of the cervicalis profunda and the vertebral ; those which ascend are distributed to the supe rior attachments of these muscles ; amongst them there is one branch occasionally found which penetrates into the cranium by the mas toid hole, and is distributed to the dura mater, under the name of posterior meningeal of the occipital.
When the occipital artery comes out from beneath the splenius muscle it divides into those branches which are distributed over the posterior surface of the occipital bone, sup plying the occipito-frontalis and the scalp, to gether with the pericranium, and anastomosing, as already mentioned, with the opposite occi pital, posterior auris, and temporal. One of these branches frequently enters the cranium by the parietal hole, and spreads over the dura mater.
The occipital artery sometimes gives small twigs, which enter the cranium by the foramen lacerum posterius and the anterior condyloid foramen.
2d. A. posterior auris, v. auricularis pos terior, arises immediately after the occipital, in the substance of the parotid. gland ; it is generally a much smaller vessel than the latter, from which it is mostly separated by the stylo hyoid muscle : sometimes it comes from the occipital. It passes upwards and backwards under the parotid gland between the mastoid process of the temporal bone and the cartila ginous tube of the ear ; it first sends branches to the parotid gland, the stylo-hyoid muscle, the posterior belly of the digastric and the external ear ; it then gives off the stylo mastoid artery, which, among other branches to the external ear, gives off one to be dis tributed to the membrana tympani. Then the stylo-mastoid traversing the aqueduct of Fallopius finds its way into the cavity of the tympanum, on the lining membrane of which, and its prolongation into the mastoid cells, its branches are expended, where it anas tomoses with a branch of the middle menin geal, which enters the hiatus Fallopii, and arrives in the tympanum along With the chorda tympani nerve. Sometimes the stylo-mastoid artery comes from the middle meningeal.