NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPIIALON.) 



677 



The direction of the long axis of the third 

 ventricle is obliquely downwards and back- 

 wards. Its anterior extremity being on a 

 higher plane than its posterior, is therefore 

 likewise superior. 



Pineal gland. We may here conveniently 

 notice the position and connections of the 

 pineal gland. This body, rendered famous by 

 the vague theory of Des Cartes, which viewed 

 it as the chief source of nervous power, i.$ 

 placed just behind the third ventricle, resting 

 in a superficial groove which passes along the 

 median line between the corpora quadrige- 

 mina. It is heart-shaped, and of a grey co- 

 lour. Its apex is directed backwards and 

 downwards, and its base forwards and up- 

 wards. A process from the deep layer of the 

 velum interpositum envelopes it and serves to 

 retain it in its place. From each angle of its 

 base there passes off a band of white matter 

 which adheres to the inner surface of each 

 optic thalamus. These processes serve to con- 

 nect the pineal body to the optic thalami. 

 They are called the peduncles of the pineal 

 gland, also habence. In general they are two 

 in number, one for each optic thalamus. They 

 may be traced forwards as far as the anterior 

 pillars of the fornix. Posteriorly these pro- 

 cesses are connected along the median line by 

 some white fibres which adhere to the base 

 of the pineal gland, as well as to the posterior 

 commissure beneath, and which seem to 

 form part of the system of fibres belonging to 

 that commissure. A pair of small bands 

 sometimes pass off from these fibres, along 

 the optic thalami, parallel to the peduncles 

 above described. 



It appears, then, that the pineal gland has no 

 other connexion with the brain than that which 

 these hubeiKf or peduncles secure for it ; other- 

 wise this body might more appropriately be 

 regarded as an appendage to the pia mater, in 

 which it is involved, and from which it derives 

 its nutrition. 



Grains of sand, similar in every respect to 

 those previously described (p. 635) as con- 

 nected with the internal processes of the pia 

 mater, are found in the pineal body in a large 

 proportion of instances in the adult. They 

 seem to be accumulated as it were in a cavity 

 which is situate towards its base. Hence So- 

 emmering gave to this collection of sabulous 

 matter the name acervulus. When, however, 

 the sand is abundant, it may be found upon 

 the surface as well as in the centre. 



The anterior commissure. In examining 

 the third ventricle, a rounded cord of very pure 

 white matter is seen through the interval which 

 is left by the divergence of the anterior pillars 

 of the fornix in their descent to the base of the 

 brain. This band is transverse, and appears 

 to form a tangent to the convex border of 

 those pillars. It may be traced outwards on 

 either side through the anterior extremities of the 

 corpora striata into the white substance of the 

 middle lobes of the brain. A very little dis- 

 section is required to expose this cord in its 

 entire extent. It seems placed in a canal hol- 

 lowed in the cerebral matter. When exposed, 



its surface is perfectly smooth, indicating that 

 fibres do not pass from it to the wall of the 

 canal in which it lies. Examined in its whole 

 extent, it presents the form of a curve with an- 

 terior convexity, and becomes gradually flat- 

 tened and expanded towards each extremity, 

 its component fibres becoming divergent and 

 mingling with the white substance of that por- 

 tion of the brain. 



This system of fibres possesses the characters 

 of a commissure or bond of connection between 

 symmetrical portions of the brain on either side 

 of the median plane as distinctly as the corpus 

 callosum itself. 



The soft commissure. The cavity of the 

 third ventricle is partly occupied by a lamina 

 of a light grey matter, which extends between 

 the optic thalami of opposite sides. It forms a 

 transverse horizontal plane dividing the ven- 

 tricle into two portions, one above, the other 

 below it. Sometimes it is divided and disposed 

 as two planes. There is but little power of cohe- 

 sion between its particles, so that in the recent 

 state the separation of the thalami in the neces- 

 sary manipulations will frequently cause its 

 rupture. Hence the adjunct " soft" is appro- 

 priately applied to it, and by its connecting the 

 thalami of opposite sides, this structure may be 

 ranked with the other commissures. It does 

 not extend throughout the entire length of the 

 ventricle : both its anterior and posterior mar- 

 gins are concave and leave an open space be- 

 tween each extremity of the ventricle. 



Thus far our examination includes the topo- 

 graphical anatomy of the cerebrum proper. 

 The pineal body, indeed, scarcely lies within 

 the confines of that segment of the encephalon, 

 but from its internal relation to the third ven- 

 tricle and the optic thalami, it must be included 

 in the description of those parts. This body 

 rests on the upper surface of that segment of 

 the brain which lies intermediate to the ce- 

 rebrum, cerebellum, and medulla oblongata, 

 namely, the mesocephale. And we shall now 

 proceed to a brief notice of this part and its 

 connection to the other segments. 



The mesocephale. Four eminences are seen 

 immediately behind the third ventricle. A 

 transverse furrow separates them into an 

 anterior and a posterior pair, and a longitudinal 

 furrow along the median line divides the 

 right and left pair from each other. The pineal 

 body rests in the anterior extremity of the lon- 

 gitudinal depression. The anterior pair have 

 been long named the nates, the posterior the 

 tcstes. In the human subject the former are 

 the larger. In the inferior mammalia these 

 bodies are much more highly developed than 

 in man, and exhibit a more marked difference 

 of size. 



The posterior of the corpora quadrigemina 

 are apparently connected to the cerebellum by 

 two columns of white matter, one of which 

 passes into the central white substance of each 

 cerebellar hemisphere. These are the processus 

 ct'nlu'lti ad testes. They enter into the for- 

 mation of the crura cerebelli. Each of them 

 forms the superior layer of the crus cerebelli of 

 its own side. 



