704 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPIIALON.) 



Of the ventricles of the brain. The third 

 ventricle results from the apposition of the 

 lateral halves of the brain along the median 

 plane, and the lateral ones from the folding 

 inwards, above and below, of the convoluted 

 surface of each hemisphere. They must not 

 therefore be regarded as cavities hollowed in 

 the substance of the brain : on the contrary, 

 their walls must be viewed as part of the 

 cerebral surface, and the eminences which 

 project from them as convolutions. The cor- 

 pora striata and optic thalami are from their 

 structure entitled to be considered in this light, 

 and still more the hippocampi, which, how- 

 ever, are somewhat complicated by the addi- 

 tion of the layers of white matter derived from 

 the fornix. 



The distinction between the lateral and the 

 middle or third ventricle results from the de- 

 velopement of the corpus callosum and of the 

 fornix, which form horizontal strata by which 

 the ventricles are closed in above ; and the 

 extension of the anterior pillars of the fornix 

 downwards, and the close application of the 

 free margin of the body of the fornix to the 

 optic thalami, assign more complete limits to 

 the third ventricle. 



The fourth ventricle is also evidently formed 

 by the lateral adaptation of the symmetrical 

 halves of the medulla oblongata. The iter is 

 obviously a continuation of it closed behind 

 by the quadrigeminal bodies and their con- 

 necting fibres. This ventricle remains open in 

 the embryo, uncovered by any portion of the 

 encephalon until the full developement of the 

 cerebellum causes it to extend over it. 



The fifth ventricle must be viewed as ori- 

 ginally part of the third, which has been 

 closed off by the full developement of the 

 septum lucidum and fornix, and the union of 

 their lateral halves along the median plane. 



All these cavities are lined by a delicate 

 membrane nearly allied to, if not identical 

 with, serous membranes. It is covered by an 

 epithelium, ciliated according to Purkinje and 

 Valentin, beneath which are delicate fibres of 

 areolar tissue exactly of the same kind as those 

 found in connection with serous membranes. 

 I have never seen any basement membrane. 

 This membrane is reflected around the pro- 

 cesses of pia mater which are found in the 

 ventricles, and in this respect presents an ad- 

 ditional point of analogy to the serous mem- 

 branes, the portion which lines the walls of 

 the ventricles corresponding to the parietal 

 layer, and that which adheres to the pia mater 

 resembling the visceral layer of those mem- 

 branes. It is the reflection of this membrane 

 from the walls to the enclosed pia mater which 

 serves to shut off the ventricular cavity from the 

 sub-arachnoid space, at the anterior part of the 

 horizontal fissure, and at the inferior extremity 

 of the fourth ventricle. If any communication 

 take place between the intra-ventricular and 

 sub-arachnoid fluid, it must be, as already 

 remarked, by transudation through this mem- 

 brane. 



Of the circulation in the brain. Haller cal- 



culates that the human brain receives rather 

 more than one-fifth of the whole blood of the 

 body. Whether this calculation be correct or 

 no, it is certain that an organ of such great size, 

 of such high vital endowments, so active, and 

 which exerts so considerable an influence upon 

 all other parts of the body, must necessarily 

 require a large supply of the vital fluid. Four 

 large arteries carry blood to the brain, namely, 

 the two internal carotids and the two vertebrals. 

 Each carotid penetrates the cranium at the 

 foramen on the side of the sella Turcica, and 

 almost immediately divides into three branches, 

 the anterior and the middle cerebral arteries 

 and the posterior communicating artery. 



Ti\\e anterior cerebral arteries supply the inner 

 sides of the anterior lobes of the brain : they 

 ascend through the great longitudinal fissure, 

 and pass along the upper surface of the corpus 

 callosum, giving oft' branches to the inner con- 

 volutions of both hemispheres of the brain. 

 These arteries anastomose with each other just 

 beneath the anterior margin of the corpus cal- 

 losum by a transverse branch, called anterior 

 communicating artery. The middle cerebral 

 arteries, the largest branches of the carotid, pass 

 outwards in the fissures of Sylvius, and supply 

 the outer convolutions of the anterior lobes, 

 and the principal portion of the middle lobes. 

 At the inner extremity of each fissure of Sylvius 

 numerous small branches of these arteries pene- 

 trate, to be distributed to the corpus striatum. 

 The choroid arteries which supply the choroid 

 plexus sometimes arise from these arteries, but 

 also occasionally come from the carotid itself. 

 The posterior communicating artery is an ana- 

 stomotic vessel, which passes backwards along 

 the inner margin of the middle lobe on the base 

 of the brain, and communicates with the poste- 

 rior cerebral artery, a branch of the basilar. 



The vertebral arteries, having passed through 

 the canals in the transverse processes of the 

 cervical vertebra?, enter the cranium through 

 the occipital foramen towards its anterior part. 

 In their ascent they incline towards each other 

 in front of the medulla oblongata, and at the 

 posterior margin of the pons they coalesce to 

 form a single vessel, the basilar, which extends 

 the whole length of the pons. 



The vertebral arteries furnish the anterior 

 and posterior spinal arteries, and the inferior 

 cerebellar arteries. These last vessels arise 

 from the vertebrals very near their coalescence ; 

 they pass round the medulla oblongata to reach 

 the inferior surface of the cerebellum, to which 

 they are principally distributed. 



From the basilar artery numerous small ves- 

 sels penetrate the pons. At its anterior extre- 

 mity it divides into four arteries, two on each 

 side. These are, the two superior cerebellar, 

 and the two posterior cerebral arteries. 



The superior cerebellar arteries pass back- 

 wards round the crus cerebri, parallel to the 

 fourth nerve, and divide into numerous branches 

 on the upper surface of the cerebellum, some 

 of which anastomose with branches of the infe- 

 rior cerebellar artery over the posterior margin 

 of the cerebellum. Some branches of these 



