THE NERVOUS SYSTEM 1449 



Weight of the Brain. — ^The average weight of the brain of the 

 adult male is about 48 ounces, and that of the adult female about 

 44 ovmces. 



Arteries of the Encephalon. 



Cerebral Part of the Internal Carotid Artery. — ^The internal carotid 

 artery, having pierced the roof of the cavernous sinus, internal to 

 the anterior clinoid process of the sphenoid bone, ascends between 

 the second and third cranial nerves to the inner end of the stem 

 of the fissure of Sylvius. Here it divides into its terminal branches, 

 namely, anterior cerebral and middle cerebral. 



Branches. — These are as follows : posterior communicating 

 anterior choroidal, anterior cerebral, and middle cerebral. 



The posterior communicating artery arises from the back part of 

 the internal carotid, and passes backwards to anastomose with the 

 posterior cerebral artery. It is usually of small size, but is occasion- 

 ally large, and it furnishes twigs to the cms cerebri and inner aspect 

 of the optic thalamus. 



The anterior choroidal artery arises from the back part of the 

 internal carotid close to its termination. It passes backwards and 

 outwards between the cms cerebri and the hippocampal gyms, 

 and it enters the lower and anterior extremity of the descending 

 comu of the lateral ventricle by passing through the choroidal 

 fissure. In its course it gives twigs to the hippocampal gyms and 

 cms cerebri, and it terminates in the choroid plexus of the lateral 

 ventricle. 



Anterior Cerebral Artery. — This is the smaller of the two terminal 

 branches of the internal carotid artery, and it has a more limited 

 distribution than the other terminal branch, namely, the middle 

 cerebral artery. It passes forwards and inwards, above the optic 

 nerve, to the commencement of the great longitudinal fissure, 

 where it is connected with its fellow of the opposite side by a short 

 transverse vessel, called the anterior communicating artery. There- 

 after it enters the great longitudinal fissure, along with its fellow, 

 turns round the genu of the corpus callosum, and passes backwards 

 over the upper surface of that body to the splenium, where it anas- 

 tomoses with the posterior cerebral artery. 



Branches. — ^These are arranged in two groups, antero-mesial or 

 ganglionic and cortical. 



The antero-mesial or ganglionic branches, smaU in size, pass 

 through the lamina cinerea, along with twigs from the anterior com- 

 municating artery, and supply the front part of the caudate nucleus. 



The cortical branches are as follows : (i) internal orbital, to the 

 internal orbital gyms, olfactory lobe, and gyms rectus ; (2) anterior 

 internal frontal, to the superior frontal gyms, the anterior two- 

 thirds of the middle frontal gyms, and the anterior part of the 

 marginal gyms ; (3) middle internal frontal, to the callosal gjn-us, 

 the posterior part of the marginal gyrus, and the upper part of the 

 precentral gyms ; and (4) posterior internal frontal, to the quadrate 



