_13_, Cerebro-spinal artery; _14_, middle spinal artery; _15_, basilar
artery; _16_, posterior cerebellar artery; _17_, anterior cerebellar
artery; _18_, internal auditory artery; _19_, posterior cerebral
artery; _20_, deep cerebral artery; _21_, stump of internal carotid
artery; _22_, anterior choroid artery; _23_, anterior meningeal
artery; _24_, middle cerebral artery; _25_, artery of corpus
callosum; _26_, anterior communicating artery; _1_, _1′_, _1″_,
olfactory tracts; _1‴_, olfactory peduncle; _2–12_, cranial nerves;
_a_, olfactory bulb; _b_, trigonum olfactorium; _c_, lamina
perforata anterior; _d_, fossa transversa; _e_, pyriform lobe; _f_,
cerebral peduncle; _g_, tractus transversus; _h_, corpus mammillare;
_i_, tuber cinereum; _k_, lateral fissure (of Sylvius); _l_,
presylvian fissure; _m_, pons; _o_, pyramid; _p_, facial eminence;
_q_, corpus restiforme; _r_, cerebellum; _s_, middle peduncle of
cerebellum. (After Ellenberger Baum, Top. Anat. d. Pferdes.)
]
The =posterior= or =retrograde branch= (Ramus descendens) of the
occipital passes up through the foramen transversarium of the atlas and
joins the vertebral artery. It gives branches to the great oblique
muscle of the head, which covers it.
The =anterior= or =occipital branch= (Ramus occipitalis) passes through
the alar (antero-external) foramen of the atlas and supplies the muscles
and skin of the poll, anastomosing with the deep cervical artery and its
fellow of the opposite side. In the alar furrow it gives off the
=cerebrospinal artery=, which enters the spinal canal through the
intervertebral (antero-internal) foramen of the atlas, perforates the
dura mater, and divides into cerebral and spinal branches. The =cerebral
branch= unites with that of the opposite side to form the =basilar
artery=, and the =spinal branch= similarly forms by union with its
fellow the =middle spinal artery=.
The =basilar artery= (A. basilaris cerebri) passes forward in the median
groove on the ventral surface of the medulla and pons and divides into
the two posterior cerebral arteries. The collateral branches of the
basilar are:
1. Medullary branches (Rami medullares), ten or twelve in number,
distributed to the medulla oblongata.
2. Posterior cerebellar arteries (Aa. cerebelli caudales) which pass
outward around the medulla behind the pons to the cerebellum, to which
they are distributed after giving twigs to the medulla and pons.
3. The small auditory artery (A. auditiva interna) accompanies the
eighth nerve to the internal ear. It often arises from the posterior
cerebellar.
4. Anterior cerebellar arteries (Aa. cerebelli nasales). These are very
variable in number and origin. There are often two or three on either
side and they frequently arise from the posterior cerebrals. They pass
outward in front of the pons and supply the anterior part of the
cerebellum.
The =posterior cerebral arteries= (Aa. communicantes caudales) diverge
at an acute angle and join the posterior communicating branches of the
internal carotid arteries on the inferior surface of the cerebral
peduncles. They are connected by a transverse branch and by a network of
fine twigs which form often a rete mirabile.