176 DISSECTION OF THE BRAIN. 



crus and hemisphere of the cerebrum to the velum interpositum ami the 

 choroid plexus. 



From the foregoing examination of the offsets of the vertebral arteries 

 and the basilar trunk, it appears that about half the^ encephalon viz., the 

 medulla oblongata, the pons, the cerebellum, and the po&tejciocJJturd of the 

 cerebrum receives its blood throuh the branches of the subclavian 



The INTERNAL CAROTID ARTERY ( ! ) terminates in branches for the re- 

 maining part of the cerebrum. The vessel emerges from the cavernous 

 sinus (p. 33) inside the anterior clinoid process, and divides at the inner 

 end of the fissure of Sylvius into cerebral and communicating arteries. At 

 th^ base of the brain the carotid lies between the second and third nerves, 

 but nearest the former. 



Branches. In the skull the carotid gives off the ophthalmic offset, before 

 it ceases in the following terminal branches to the cerebrum. 



a. The anterior cerebral artery (' 2 ) supplies the inner part of the cerebral 

 hemisphere. It is directed forwards to the median fissure between the 

 halves of the large brain ; and as it is about to enter, it is united to its fel- 

 low by a short thick branch the anterior^ f.ommiinicaLinfl ( 3 ). Then 

 passing into the fissure, it bends round the anterior part of the corpus 

 callosum, so as to be placed on the upper aspect in the natural position of 

 the brain, and is continued backwards distributing offsets nearly to the 

 posterior extremity of the hemisphere. 



It gives off numerous branches, and some of them supply the base of the 

 cerebrum, thus : 



Near the commencement it furnishes small branches to the part of the 

 brain (anterior perforated spot) prmf.i'orunns^n the innerjjnd of. the fissure 

 Q Sylvius : and it distributes some branches to the under part of the 

 frontal lobe. 



b. The middle cerebral artery ( 4 ) is the largest offset of the carotid, and 

 ramifies over the outer side of the hemisphere. Entering the fissure of 

 Sylvius, the artery divides into many large branches, which issue at the 

 outer end of that groove, and, spreading over the ^Yfpfpml qf^fRr" of the 

 hemisphere, inosculate with the other two cerebral arteries at the front, 

 the back. an_d the upper part of the brain. Only a few fine offsets require 

 special notice : 



A set of small branches arise at the inner end of the fissure of Sylvius, 

 and enter the cerebral substance through the substantia perforata antica. 



c. The posterior communicating artery () is a small twig, which is 

 directed backwards parallel to, and on the inner side of the third nerve, to 

 join the p^tpr^or cqrebflal artery (of the basilar) near the pons. 



d. The choroid artery (anterior) ( 5 ) is small in size, and arises either 

 from the trunk of the carotid, or from the middle cerebral artery : it passes 

 backwards on the other side of the preceding, and finds its way between 

 the hemisphere and the crus cerebri to the ^hr^u! pl^vna of the lateral 



Circle of Willis. The arteries at the under part of the brain are united 

 freely both on their own side and across the middle line in an arterial 

 anastomosis, the circle of Willis. On each side this circle is formed by 

 the trunk of the internal carotid giving forwards the anterior cerebral, and 

 backwards the posterior communicating artery. In front it is constructed 

 by the converging anterior cerebral, and the anterior communicating 



