668 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHAI.ON.) 



that great nervous mass which is situate above 

 this plane, it will be obvious, even to the most 

 superficial observer, that it admits of a con- 

 venient subdivision into certain great segments, 

 each of which, although extensively connected 

 with the neighbouring ones, may yet be capable 

 of acting as an independent centre, and, in short, 

 possesses the anatomical as well as physiolo- 

 gical properties of a ganglion. And on a more 

 minute investigation the number of gangliform 

 segments will be found to be greater than the 

 observation of the mere surface of the ence- 

 phalon would lead us to conclude. The sub- 

 division, however, which it is most conve- 

 nient for the purpose of description to adopt, 

 is that already stated at page 650, into, 

 1, the medulla oblongata, which is immedi- 

 ately continuous inferiorly with the spinal cord. 

 This segment has certain characters of struc- 

 ture which decidedly indicate its ganglionic 

 nature; several nerves of considerable size and 

 of great physiological importance are implanted 

 in it, and its external anatomy very clearly in- 

 dicates its distinctness from the spinal cord 

 inferiorly and from the other encephalic seg- 

 ments above, of which that next in order pro- 

 ceeding from below upwards, is, 2, the meso- 

 cepfiale. To this mass, so called because of its 

 intermediate position between the other seg- 

 ments, the term isthmus has been also very 

 appropriately applied, as it is the connecting 

 link between all the encephalic segments. In- 

 ferior and posterior to it is placed, 3, the cere- 

 bellum, which has very intimate relations to the 

 medulla oblongata as well as to the segment 

 last described, but much less extensive ones to 

 that which forms by far the largest proportion 

 of the encephalon, namely, 4, the cerebrum, 

 which therefore occupies the principal portion 

 of the cranial cavity. 



The distinction between these different seg- 

 ments is very obvious on an examination of the 

 surfaces of the brain, which indeed ought to 

 be the first step to be taken by the anatomist. 

 To discover how they are connected to each 

 other and to the spinal cord, how the corres- 

 ponding portions on opposite sides of the 

 mesial plane are associated together, what fibres 

 are common to all the segments, and what 

 peculiar to some, and, lastly, how the grey 

 matter is related to the white, these are the 

 chief objects to be attained in the dissection 

 of the brain. No one method of dissection 

 will suffice for this purpose. The anatomist 

 should first make himself familiar with the 

 simple topographical anatomy of the brain, 

 that is, with all those parts in it which possess 

 such characters of form or structure as may 

 entitle them to be regarded as distinct and de- 

 serving of separate description, and have ob- 

 tained for them a special appellation. The 

 form, size, general structure, and relations of 

 these parts should be carefully noted. And 

 this method of examination is equally applica- 

 ble to the dissection of each segment of the 

 encephalon. But the, most convenient way in 

 which it can be conducted for ordinary prac- 

 tical purposes, is to commence with the cere- 

 bral hemispheres, and having studied their 



general structure as displayed on a horizontal 

 section, to examine the extent and connections 

 of the fibres which connect the right and left 

 hemispheres with each other (the corpus cal- 

 losum) ; then to open the ventricles, examine 

 their shape and extent, and note the various 

 particulars connected with the numerous parts 

 which are brought into view by exposing those 

 cavities. The dissector may next observe how 

 certain of the parts concealed by the lateral 

 ventricles are connected with the mesocephale 

 (the optic thalami for instance), and, having 

 been already acquainted with the various pro- 

 minences which are seen upon the surface of 

 the latter, he may by vertical, or transverse, or 

 horizontal sections, investigate the manner in 

 which the white matter of this segment con- 

 nects itself with that of the neighbouring ones. 

 In examining the cerebellum, the larger fis- 

 sures afford sufficient indication for a con- 

 venient subdivision of the organ, and by hori- 

 zontal or vertical sections at various parts of it 

 the connexion of th grey and white matter 

 may be displayed, and of the latter to the me- 

 socephale and medulla oblongata. The me- 

 dulla oblongata has upon its surface various 

 lines or fissures which denote the proper limits 

 of its constituent columns, and which will be 

 sufficient guide to the dissector in tracing the 

 extent and connexions of each. Transverse 

 and longitudinal sections also afford useful in- 

 formation respecting the structure of this seg- 

 ment of the encephalon and the relations of its 

 parts. 



Such is the mode of dissection from above 

 downwards, against which it has been greatly 

 the fashion of late years to declaim with much 

 vehemence. But, however the advocates of 

 a particular theory may object, there can be 

 no doubt that this method is by far the most 

 useful for all practical purposes. It enables the 

 anatomist, without difficulty, to study the pro- 

 minent parts or landmarks (so to speak) of the 

 brain, without a knowledge of which it is in 

 vain to attempt any other mode of dissection. 

 And for pathological investigations it is the 

 only method which can be conveniently adopted. 

 It is plain, therefore, that all who are desirous 

 of becoming acquainted with the anatomy of 

 this organ should begin by making dissections 

 in this way. An additional advantage is found, 

 in this mode of investigation, from its great ap- 

 plicability to the dissection of the brains of the 

 lower animals, of the Mammalia and Birds 

 especially, for the purpose of comparing them 

 with the brain of the human subject. 



The method of our celebrated countryman 

 Willis was very much the same as that above 

 described. He removed the membranes from 

 the posterior lobes of the hemispheres, and 

 thus separated the latter from the subjacent 

 parts, and by raising them as far forwards 

 as possible he was enabled to observe the con- 

 nections of the cerebral hemispheres with the 

 mesocephale, and the attachments of the fornix 

 behind. He also must have studied the sub- 

 stance of the hemisphere by horizontal section. 

 By then dividing the posterior parts of the 

 hemispheies horizontally along the plane of the 



