NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



692 



b. The upper and posterior lobes, connected 

 by the single commissure, to be sought for on 

 the floor of the posterior notch. 



2. On the inferior surface of the hemi- 

 spheres. 



a. The amygdala, united by the spigot. 

 /'. The biventral lobes. 



c. The slender lobes. 



The biventral lobes and the anterior laminae 

 of the slender lobes are united by the pyramid. 



d. The posterior inferior lobes, connected 

 by the short and exposed and the long and 

 hidden commissures. 



The flocks or lobules of the pneumogastric 

 nerve, (lobule of the auditory nerve, Foville,) 

 which are situate altogether anterior to the 

 hemispheres and attached to each crus, are 

 united by the posterior medullary velum, and 

 through it appear to have some connection with 

 the most anterior portion of the inferior vermi- 

 form process. 



A vertical section of either hemisphere of 

 the cerebellum or of its median lobe displays 

 its structure, and serves further to demonstrate 

 the subdivision into lobes above described. 

 When either hemisphere is cut in the vertical 

 direction, the surface of the section displays a 

 beautiful ramification of fibrous matter, the 

 smaller branches of which are enveloped by 

 laminae of grey matter. This appearance has 

 such a resemblance to the trunk of a tree with 

 its boughs and branches, that it early received 

 and has continued to retain the name of arbor 

 vita. The trunk of the tree is represented by 

 a central nucleus of white matter, from the 

 upper and lower surfaces of which branch oft', 

 some at a right, others at an acute angle, 

 several laminae, each of which forms the parent 

 stem of a number of other branches. Each of 

 the primary branches is the foundation or cen- 

 tral stem of a lobule. Laminae of fibrous mat- 

 ter are seen branching from both sides of it 

 immediately after its separation from the nu- 

 cleus. Sometimes the primary branch bifur- 

 cates, and each division of it forms the stem 

 of what may be called a sub-lobule. The ul- 

 timate branchings are covered by a layer of 

 grey matter. If we suppose that one of the 

 primary branches is composed of a certain 

 number of laminae of fibrous matter, the se- 

 condary ramifications from it will in a great 

 degree correspond. In most instances these 

 secondary branches subdivide into two or 

 more tertiary ones, which, as well as the branch 

 from which they spring, are enclosed in grey 

 matter. (Figs. 380, 386.) 



A vertical section of the median lobe gives 

 quite a similar appearance to that of the 

 hemispheres. The central nucleus breaks up 

 into primary branches, which become the centre 

 of the lobules of which it consists. ( Figs. 386, 

 393.) 



The ramifications of the central nucleus, 

 whether of the median lobe or of the hemi- 

 spheres, separate from it only in the vertical 

 plane or from before backwards ; in the latter 

 direction, however, to a very slight extent. 

 Hence these branches are directed only up- 

 wards, or downwards, or backwards. The 



fibrous matter of the median lobe is continuous, 

 without any line of demarcation, with that of 

 the hemispheric lobules. By reason of this 

 disposition of the fibrous matter, the surface 

 which is exposed by a horizontal section 

 through the entire cerebellum, presents a very 

 different appearance from that which results 

 from a vertical section. It consists of a plane 

 of fibrous matter bounded on the sides and 

 behind by a narrow cortex of grey matter. 



The white matter consists exclusively of 

 fibres, chiefly of the tubular kind and of all 

 degrees of size. These, in the more distant 

 ramifications, penetrate the vesicular matter of 

 their grey cortex, and form some unknown con- 

 nection with its elements. The grey matter 

 consists of three layers, readily distinguishable 

 by the naked eye from their difference of co- 

 lour. The external layer is the darkest, and 

 consists chiefly of granular and vesicular mat- 

 ter. The next or intermediate layer is of a 

 light colour, and is composed of a stratum of fine 

 nucleus-like particles. The third layer has the 

 greatest thickness, and is immediately in con- 

 tact with the fibrous matter; it is intermediate 

 in point of colour to the other two, and con- 

 sists of numerous vesicles of the caudate kind, 

 especially with branching processes and nerve- 

 tubes of all sizes. The dark colour of the 

 external layer is doubtless owing in a great 

 measure to the great numbers of capillary ves- 

 sels which enter it; the greater paleness of the 

 inner stratum is to be attributed to the inter- 

 mixture of the white fibres, whilst the light 

 colour of the middle stratum is intrinsic. From 

 the usual dependent position of the cerebellum 

 in the dead body, it always appears to contain 

 more blood than the cerebrum. 



Corpus denttitum. If, in making a vertical 

 section of either hemisphere of the cerebellum, 

 the incision be made so as to leave two-thirds 

 of the hemisphere on its outside, a peculiarity 

 will be observed on the surface of the section 

 which deserves a separate consideration. The 

 central white nucleus is interrupted by a very 

 remarkable undulating line of vesicular matter, 

 which is convex towards the posterior margin 

 of the hemisphere, but open in front towards 

 the crus cerebelli. 



This constitutes the corpus dentation or 

 rhomboideum of the cerebellum. It presents a 

 remarkable resemblance to the structure of the 

 same name which is met with in the olivary 

 body of the medulla oblongata. It is evidently 

 a capsule of vesicular matter which is enclosed 

 in the inner third of the substance of the central 

 white nucleus of the cerebellar hemisphere, 

 being nearer its superior than its inferior sur- 

 face. The peculiar undulating arrangement of 

 it doubtless has reference to the accommoda- 

 tion of a certain extent of surface in a limited 

 space. The fibrous matter enclosed by it seems 

 derived from the processus cerebelli and from 

 the restiform body. 



The central stem of fibrous matter to which 

 the several lobules, both of the hemispheres 

 and the median lobe of the cerebellum, adhere, 

 ( crus cerebelli,) is formed by three bundles of 

 fibres, each situate on a difi'erent plane. These 



