596 DEVELOPMENT OF THE FCETAL BRAIN. 



wards and to the sides, form the hemispheres. On the front of 

 this cavity, will be seen already formed the anterior part of two 

 large commissures of the brain, the corpus callosurn and the 

 fornix. On separating the two thalami, we find them as yet 

 not united at their middle parts so as to constitute the middle 

 commissure of the third ventricle, or commissure mollis. By 

 cutting into the mass of the*tubercula quadrigemina, we find it 

 a sac ; the cavity of which opens below by a small orifice into 

 the fourth ventricle, which is continuous with that of the 

 medulla spinalis, and above has a similar communication with 

 the third ventricle. 



In the foetus of four months, the medulla spinalis is yet very 

 large in proportion to the bulk of the brain and encloses a 

 cavity with which the posterior and median fissures commu- 

 nicate. It is distinctly composed of longitudinal fibres and has 

 formed on its bulb or medulla oblongata little elevations, which 

 represent the corpus pyramidale and the corpus restiforme. Be- 

 tween these bodies a middle or lateral band is seen but on which 

 there is as yet no appearance of the corpus olivare. The cor- 

 pora restiformia which are extended backwards and outwards 

 on each side to form the cerebellum, have connected with 

 their cerebeller extremities the ganglion of the cerebellum or 

 corpus dentatum. The cerebellum is small and shows none 

 of its horizontal folds. The great hollow mass of the tubercula 

 quadrigemina has become altered in form, and contracted in 

 the middle, diminishing somewhat its great cavity, which is 

 subsequently to become a mere channel between the third and 

 fourth ventricles called the aqueduct of Sylvius. In front of 

 this contraction the small bodies forming the tubercula quad- 

 rigemina proper, begin to appear, and behind, the rudiment of 

 the pons varolii is visible. The thickness of the membranous 

 walls of this mass is about a line and three quarters behind, 

 but is not more than a third of a line in front. The hollow 

 hemispheres of the cerebrum, are considerably more developed 

 backwards and upon the sides, but as yet do not cover the 

 tubercula quadrigemina or the pons. On separating the two 

 sacs of the hemispheres, they will be found united in front so 

 as to form the front part of the corpus callosum, which is as 



