After having given off the communicating artery, the carotid sends a branch to the choroid plexus, the arteria choroidea ; the artery passes backwards and outwards, enters the tractus opticus, supplies the pia 'pater of the middle lobe of the brain and the optic thalamus and, entering the inferior cornu of the lateral ven tricle, spreads out its branches in the choroid plexus.
After having given off the choroid artery, the internal carotid divides always at an obtuse angle, and at the internal extremity of the fis sure of Sylvius, into two branches, the an terior and the middle cerebral, of which the latter is much the larger vessel : sometimes the lateral communicating artery arises at the place of this division, and forms with these branches a sort of tripod.
The anterior cerebri, also called the artery of the corpus callosum, is always smaller than the media cerebri; it passes upwards, forwards, and inwards to the fissure wiiich separates the anterior lobes of the cerebrum, passing over the optic nerves, and inferior to the internal origin of the olfactory: on entering the above mentioned fissure, it approaches closely to the corresponding branch of the opposite side, with which it communicates by a large and very short transverse branch, called the anterior communicating artery, by which the circle of Willis is completed anteriorly : sometimes this branch is double, and occasionally we find it partially double, in consequence of a forking of one of its extremities ; its place is sometimes supplied by a fasciculus of small branches ; it gives off', especially when it is unusually long, a number of small twigs, which pass upwards and backwards to the septum lucidum, fornix, and corpus callosum.
From the place of this communication the trunk of the anterior cerebri passes forwards under the corpus callosum, giving off consi derable branches to the inferior and internal part of the anterior lobe of the cerebrum; it then turns round to the anterior extremity of the corpus callosum, mounts up on the internal surface of the hemisphere of the cerebrum, and divides into many branches, the anterior and superior of which supply the convolutions on their internal surface, while the posterior take a lower course along the upper sur face of the corpus callosum, at the posterior extremity of which they take an ascending direction. All these branches extend to the
superior surface of the cerebrum, and anasto mose with those of the media cerebri and the posterior cerebri, which is furnished by the ver tebral.
Besides these large branches into which the arteria callosa divides superiorly, it gives off from its inferior and concave side a vast number of smaller branches, which penetrate the corpus callosum.
Sometimes, instead of being connected by the communicating branch, the anterior cerebral arteries of opposite sides unite, forming a single trunk, which runs forward for some little dis tance and then divides into a right and left branch ; this junction is the more remarkable, on account of its analogy to the union of the two vertebral arter:es in forming the single trunk of the basilar on the median line.
The media cerebri, from its greater size com pared with the anterior branch, appears, as it were, the continuation of the trunk of the carotid ; it passes outwards and backwards, in the fissure of Sylvius, and divides into two branches, the subdivisions of both of which are distributed over the pia mater of the anterior and middle lobes of the brain, anastomosing in front with the anterior cerebri, and behind with the posterior cerebri from the basilar : this artery at first gives branches at the base of the brain to the pia mater on the crus cerebri ; one of these, larger than the others, enters the inferior cornu of the lateral ventricle, where it is lost in the choroid plexus.
The anterior and middle cerebral arteries are not always similarly disposed on opposite sides ; it not unfrequently happens, as Haller has remarked, that the two large trunks of the middle cerebral arteries are given off by the right carotid, and the two anterior from the left carotid, while the three others come from the right : considering these anomalies with that of the union of the two cerebral already mentioned, we here find a very remarkable repetition of many of the varieties exhibited by the mode in which the trunks that spring from the arch of the aorta take their origin.