584 THE PINEAL GLAND. 



ular cineritious neurine, and the whole folded and plaited, so 

 as to present on the exterior the appearance of sulci and con- 

 volutions, and to be susceptible of being packed away, in the 

 narrow compass of the walls of the cranium. In either view 

 of the case, each convolution will consist of four layers, two 

 exterior and cineritious, two*interior and medullary : the latter 

 of which may readily be separated by a jet of water in the 

 middle line, so as to allow the doubling forming the convolu- 

 tion to be straightened out. 



The corpus callosum, or commissura magna cerebri, is' 

 evidently formed in part by a layer of white fibres, which, 

 proceeding from the upper and outer margin of the thalamus 

 and corpus striatum, turns inwards and passes transversely 

 over to the middle line of the body, to unite in some way that 

 is not fully understood, with a similar band from the opposite 

 side ; forming an arch or vault over the thalamus and corpus 

 striatum, which constitutes the roof of the lateral ventricles. 

 The remaining part of the structure of the corpus callosum 

 consists of a more external and thicker layer of white fibres, 

 which evidently come in from the hemispheres of the cere- 

 brum, as may be seen in an attempt to separate these parts 

 from above outwards and downwards. 



On the top of the tubercula quadrigemina, is seen a small 

 conoidal mass of cineritious matter, called the pineal gland, 

 (conariurn,) which usually after the seventh year of life, con- 

 tains some particles of gritty or sabulous matter. From it, 

 extends a small band of medullary matter called its peduncle to 

 each thalamus. The use of this organ is not known. Descartes 

 considered it, absurdly enough, the seat of the soul, after other 

 uses had been found for the corpus striatum, fornix, and other 

 parts, for which at different periods the same high office had 

 been assigned. Solly is disposed to consider it with its peduncles 

 a commissure between the thalami, and which he calls the 

 pineal commissure. Above this body, and between the tuber- 

 cula quadrigemina and back part of the corpus callosum, is the 

 great lateral fissure of Bichat, where the pia mater covering the 

 cerebellum and posterior lobes of the cerebrum is extended into 

 the ventricles in the interior of the brain. A portion of it enve- 



