1464 



A MANUAL OF ANATOMY 



Palliuir 



Optic Thalamus - 



growth is forwards, upwards, and backwards from the position of the foramen 

 of Monro on either side. They soon cover the diencephalon, and, as their 

 backward growth continues, they cover in succession the mesencepalon and 

 rhombencephalon. The mesial wall of each vesicle is intimately related to 

 the lateral wall of the diencephalon on either side. 



As the vesicles of the cerebral hemispheres grow backwards they are still 

 separated by the interhemispherical fissure, and the roof of the diencephalon 

 lies at the bottom of this fissure over a limited area, the corpus callosum 

 and fornix eing as yet undeveloped. 



The cavity of each vesicle, which, as stated, is a lateral extension on either 

 side of the cavity of the telencephalon, is the rudiment of the corresponding 

 lateral ventricle; and the cavity of the telencephalon, mesially placed, forms, 

 as stated, the anterior portion of the third ventricle. The communication 

 between the cavity of the vesicle of the cerebral hemisphere and the cavity 



of the telencephalon persists 

 throughout life on either side! 

 It is the rudiment of each 

 foramen ol Monro, and is at 

 first of large size. 



The front part of the 

 telencephalon now presents 

 the anterior portions of the 

 developing cerebral hemi- 

 spheres right and left, and 

 between them is the thin 

 anterior wall of the telenceph- 

 alon. This wall represents 

 the terminal cephalic bound- 

 ary of the embryonic neural 

 canal, and it is known as the 

 lamina terminalls, which con- 

 nects the antero-mesial parts 

 of the developing cerebral 

 hemispheres. Inasmuch as 

 the cavity of the telen- 

 cephalon forms the anterior 

 portion of the third ventricle, 

 the lamina terminalis forms 

 the anterior wall of that 

 ventricle, the anterior pillars 

 of the fornix and the anterior 

 white commissure being subsequently developed in connection with it. The 

 roof -plate of the telencephalon forms the anterior part of the roof of the third 

 ventricle. 



The walls of the primitive cerebral hemispheres are very thin and consist 

 of neuro-epithelium, the cells of which multiply very freely, the cavity (lateral 

 ventricle) meanwhile diminishing. The innermost cells form the ependyma, 

 which lines the lateral ventricles, but the majority form an extensive super- 

 ficial mantle-layer, known as the neopallium, or mantle, which constitutes 

 the grey cortex. The neuroblasts of the mantle multiply freely, the ven- 

 tricular cavity meanwhile becoming diminished. Many of the axons of the 

 neuroblasts pass towards the centre, and they give rise to the white matter 

 which constitutes the medullary centre. The grey matter is not confined to 

 the cortex, but exists also in the basal ganglia of each hemisphere — namely, 

 the corpus striatum, claustrum, and amygdaloid nucleus of each side. 



The surface of each hemisphere-vesicle is at first smooth and unbroken. 

 At a very early period, however, a depression makes its appearance at the 

 lower part of each lateral surface of the vesicle. This depression, which is 

 triangular, is called the Sylvian, or lateral cerebral, fossa. The cortical sub- 

 stance over the floor of this fossa becomes much thickened, and its deep part 

 projects into the cavity of the vesicle (lateral ventricle), giving rise to the 



Third Ventricle 

 Middle Commissure 



Pineal Body _ 



Corpora ;' " 



Quadrigemina ', 



Valve of Vieussens 



Superior Cerebellar Peduncle 



XX Foramina of Monro 



Fig. 611. — Diagram of Primitive Fore- 

 Brain AND Contiguous Structures. 



