NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPIIALON.) 



689 



inserting the handle of a knife along the pos- 

 terior margin of the cerebellum, this fissure 

 may be shewn to pass forwards to a consider- 

 able depth, and to communicate on each side 

 with the semilunar fissure, whilst it is inter- 

 rupted in the middle posteriorly, by the notch. 

 Its inner surface is lined by a process of pia 

 mater, which sinks into it. 



The right and left cerebellar hemispheres 

 exhibit a general symmetry, which is, however, 

 not always perfect, as a manifest difference is 

 sometimes observable in their sizes. And a 

 corresponding want of symmetry may be fre- 

 quently seen in the right and left fossae of the 

 occipital bone. 



Both the hemispheres and the median lobe 

 are composed of an assemblage of laminae 

 closely applied to each other. Each lamina 

 consists of a thin layer of white or fibrous 

 matter, between two of grey or vesicular sub- 

 stance, which are continuous along the outer 

 margin of the former. Thus the exterior of the 

 cerebellum consists of a stratum of vesicular 

 matter, which forms a cortex to the enclosed 

 white or fibrous substance. The lamina are 

 separated from each other by fissures, and they 

 are covered by pia mater, which adheres closely 

 to them, and penetrates to the floors of the fis- 

 sures. 



The laminae are collected into sets on the 

 superior as well as on the inferior surface. Each 

 set forms a lobe. Each lobe is surrounded by 

 a deep fissure, which separates it from the next 

 adjacent lobes. 



It is necessary to distinguish the fissures 

 which separate the laminae from those by which 

 the lobes are bounded. The former are very 

 shallow : the latter are deep, and penetrate 

 quite to the central stem of the hemisphere. 



By removing the pia mater carefully from 

 the surface of the hemispheres, and from the 

 deep fissures, the shape and boundaries of the 

 lobes may be clearly demonstrated. Or if a 

 vertical section of a hemisphere be made, the 

 deep fissures may be readily distinguished from 

 the superficial ones which separate the laminae; 

 and in this way also the lobes may be demon- 

 strated. 



The floor of each deep fissure is formed by 

 white matter. And as the deep fissures inter- 

 vene between the lobes, laminae of the lobes 

 constitute their walls, and the superficial fis- 

 sures which separate these laminae open into 

 them. 



On the superior surface of the cerebellum 

 two principal lobes may be distinguished. 

 These are the square lobe and the posterior 

 superior lobe, according to the nomenclature 

 of Reil, whose descriptions cannot be sur- 

 passed in minuteness or accuracy. (Fig. 390, 

 A, P.) 



The anterior margin of the square lobe over- 

 hangs the semilunar fissure ; its posterior mar- 

 gin is a little behind the level of the floor of the 

 posterior notch. By careful separation of its 

 laminae or by a vertical section, it may be 

 shewn to consist of eight lobules, each having 

 a stem of fibrous matter derived from the cen- 

 tral one of the hemisphere. 



VOL. III. 



The posterior superior lobe (P, Jig. 390,) 

 forms the posterior part of the superior surface 

 of the cerebellum ; its posterior margin is that 

 of the hemisphere; the horizontal fissure sepa- 

 rates it from the posterior inferior lobe. It is 

 separated from its fellow of the opposite side by 

 the posterior notch. 



On the inferior surface of each hemisphere 

 the following lobes are readily distinguishable. 

 (Fig- 391.) We enumerate them, passing 

 from before backwards. 



1 . The amygdala, so called from its resem- 

 blance to an enlarged tonsil. Tin's and its 

 fellow of the opposite side form the lateral 

 boundaries of the anterior extremity of the 

 valley, and are in great part covered by the 

 medulla oblongata. 



2. Behind the amygdala is the biventral 

 lobe, wedge-shaped, narrow towards the valley, 

 wide towards the semilunar fissure. Its laminae 

 are curved with their concavity forwards and 

 inwards, and it is united with its fellow of the 

 opposite side by laminae which cross the val- 

 ley forming part of the inferior vermiform 

 process. 



3. The slender lobe, which consists of a few 

 laminae curved parallel to the posterior ones of 

 the biventral lobe. 



4. The inferior and posterior lobe, which 

 extends to the posterior edge of the hemi- 

 sphere. The inner margin of each of these 

 lobes constitutes the lateral boundaries of the 

 posterior notch. 



Such is the constant disposition of the supe- 

 rior and inferior surfaces of the cerebellum. 

 A defect of symmetry is sometimes apparent 

 in the inequality of corresponding lobes; but 

 those above enumerated are always present. 

 So definite an arrangement must obviously have 

 some physiological import. What that may 

 be it is impossible even to conjecture, and 

 we must be, for the present, content with a 

 concise statement of the facts of the anatomy. 

 Some analogy exists between this arrangement 

 and that of the convolutions on the surface of 

 the brain, many of which exhibit a constancy 

 of position and form quite as remarkable. 



The median portion of the cerebellum is also 

 composed of laminae, which are continuous with 

 those of the hemispheres, but their arrangement 

 on the superior and inferior surfaces is so diffe- 

 rent as to demand a separate description. On 

 the superior surface the lamina? are separated 

 from each other by fissures, in the same way as 

 those which constitute the hemispheres, and 

 they are collected into sets forming lobes which 

 correspond to and connect those of the lateral 

 hemispheres. These laminae are curved, their 

 anterior margin being very slightly convex 

 (fig- 390). The edges of these laminae, as they 

 lie in close apposition, resemble the segments or 

 rings of a worm ; whence the term vermiform 

 has been applied to this as well as the inferior 

 surface of the median lobe. The laminae lake 

 for the most part a vertical direction, with the 

 exception of the anterior and posterior ones, 

 which pass gradually to the horizontal, the free 

 margins of the former being directed forwards 

 and those of the latter backwards. The pos- 



2 Y 



