THE BR MX 679 



THE ENCEPHALON 



The encephalon, <!r brain, is the ])orti()n of the cerebro-spinal axis whieli is 

 lodged witliin the eranial cavity and constitutes (Vty weight) about 98 per cent, of 

 the whole. It consists of the cerebrum, the cerebeiluni, the i)ons Varolii, and the 

 medulla ol)longata. The medulla is continuous with the spinal cord at the decus- 

 sation of the i)yramids. The e!icei»halon, taken as a whole, is ellii»soidal or ovoidal 

 in form, presenting above a toleral)ly even convexity formed by the cerebral hemi- 

 spheres, but below a mor(^ irregular surface corresponding to the fossae in the base of 

 the. skull. The cerebrum comprises the cerebral hemispheres containing the lateral 

 ventricles (prosencephalon), the optic thalami with the third ventricle between 

 them ( thalamencephalon)^ and the mesencephalon. It occupies the upper 

 compartment of the cranial cavity, resting on a Hoor formed by the anterior and 

 middle cranial fossie and the tentorium cerebelli. The cerebellum occupies 

 the posterior cranial fossa, and is ])laced behind the fourtli ventricle. It is con- 

 nected l)y three pairs of peduncles or crura to tlie cereln'um, pons, and the 

 medulla respectively. The fourth ventricle is l^ounded in front by the pons and 

 medulla. 



Ill every part of the encephalon two ili>tinct kinds of nervous substance are to be met witli. 

 termed grey and white matter. The grey matter is ciiiefly disposed upon the surface, as in the 

 convolutions of the cerebiaim and cerebellunj ; but it is also met with in detached or partially 

 (letaclied masses or nuclei, such as the eorjjora striata and o])tic thalami in the cerebrum, the 

 nuclei of the medulla and p(jns, and the corpus dentatum of the cerebellum. It is composed of 

 groups of gangliotr cells, wliich possess the power of originating neiTous imjmlses; or of receiv- 

 ing impulses produced by the action of external stimuli on the end-organs of nerves ; or of modify- 

 ing and redirecting i^uch impulses. The white matter occurs in the greatest quantity in the 

 central j^arts of the cerebral hemispheres. It is composed of medullated nerve-fibres, which 

 conduct the impulses to and from the ganglion cells in the grey matter. Both grey and white 

 matter are supported by a delicate interstitial ti.ssue termed neuroglia. 



Base of the Brain 



Dissection. — The student is recommended to commence the dissection of the brain by a 

 general examination of the structures forming the base of that organ. He should carefxdiy 

 remove the remains of the membranes from the ba.se, and after studying that region shoidd ]»ro- 

 ceed in a similar manner to remove the membranes from the superior and lateral surfaces of the 

 cerebral hemispheres. 



The base of the brain presents for examijiation the inferior surfaces of the frontal 

 and temi)oro-s)dienoidal loVjes of the cerebrum ; the stnictures contained within and adjacent 

 to the interpeduncular space ; the anterior surfaces of the pons Varolii and medulla oblongata, 

 the inferior surfaces of the lateral hemis|iheres of the cerebellum ; and the superficial origins 

 of the cranial nerves. In front, the orbital surfaces of the frontal lobes are seen separated 

 from one another by the great longitudinal fissure, and indented by the triradiate and olfactory 

 .sulci, the latter occupied by the olfactory tract and bulb. Bchiml and somewhat laterally 

 the frontal lobes are marked off from the temporal lobes by the fissure of Sylvius, near the 

 commencement of which is the anterior perforated S|tace. This s]iace is of a grey colour, 

 and is formed by a part of the lenticular nucleus of the corpus striatum, which comes to 

 the surface at the base of the brain. Tiie perforations are causi-d by small arteries which 

 proceed from the middle cerebral arteiy to the corpus striatum. Each soace is linunded 

 internally liy one of the j)eduntlcs of the corpus callosum. The latter are white bands which 

 liass from the corpus callosum to the commencement of the fi.^sure of Sylvius. Between the 

 temporo-sphenoidai lobes is a large recess, which is bounded in front by the frontal lobes, and 

 bi-hind Vjy the pons Varolii ; within this is the remarkable arterial ana.stomosis known as the 

 circle of Willis, the crura cerebri, the interpeduncidar space, portions of the ontic tracts with the 

 optic commissure, the lamina cinerea. and portions of tlie third and flairtli jiairs of nerves. 

 The interpeduncular space is situated between the diverging cnini (-erebri. It correspomls below 

 to the deepest iiart of the cisterna pontis, and alcove to the posterior part of the flour of the third 

 ventricle. It contains (1) tlie posterior jierforateil space, and in front of this (2) a jiair of white 

 rounded eminences, the corpora albicantia or mammillaria, and i'.\) a conical greyish elevation, the 

 tuber cinereum. ending in a thin tulmlar process, the infundibnlum. The latter jiasses to the 



fituitary liody, from which it is detached in removing the brain by the ordinary method, 

 mmediately in front of the tuber cinereum is the optic commissure, from which tlie optic tracts 

 can be traced for a short distance backwards and outwards, winding round the crura cerebri, aud 



