206 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



continuation of the alar lamina of the mid-brain, and the same 

 may also be said of the caudate nucleus. The sulcus of Monro 

 (Fig. 167), which runs from the opening of the aqueduct of 

 Sylvius to the foramen of Munro, on the lateral wall of the 3rd 

 ventricle, marks off the alar from the basal lamina of the thala- 

 mencephalon. A section across the 3rd ventricle and optic 

 thalami, at the end of the second month is shown diagramma- 

 tically in Fig. 168. The internal capsule has not yet appeared. 

 The cerebral vesicles already overlap and bury the optic thalami. 



The Cerebral Hemispheres. — In the 3rd week very soon 

 after the medullary plates have closed, a hollow bud, which almost 

 immediately divides into a right and left half, grows out from the 

 anterior superior extremity of the fore-brain. These two processes 

 form the cerebral hemispheres. It will be seen that the cerebrum 

 represents a super-addition to the neural-tube system. The 

 lateral ventricles with their horns represent the cavity of the 

 cerebral vesicles ; the foramina of Monro, which have a common 

 entrance to the third ventricle, represent the position at which 

 the primitive cerebral outgrowth took place. The walls of the 

 vesicles thicken and form the mantle of the brain. The anterior 

 horn represents the anterior extremity of the vesicle — the point 

 at which the olfactory lobe is produced ; the descending horn 

 represents the real posterior extremity of the vesicle ; the posterior 

 horn, although found in all mammalian brains, is a later diverti- 

 culum formed in connection with the growth of the occipital lobe. 



The primitive simple relationship of the cerebral vesicle which 

 holds for low vertebrates (see Figs. 154 and 155) and for the 

 first two months of foetal life, becomes obscured in the third by 

 the vesicles growing over the optic thalami and burying them. 

 The wall of the vesicle, which comes in contact with the optic 

 thalamus, adheres to the outer surface of that body (Figs. 168 

 and 169). Hence the optic thalamus comes to form part of the 

 floor of the body of the lateral ventricle and enters into the roof 

 and inner wall of the descending horn. 



Divisions of each Cerebral Vesicle. — A prolonged study of 

 the adult vertebrate brain has led Elliot Smith to divide the wall 

 of each cerebral vesicle into three primary parts : 



(1) Rhinencephalon (defined on p. 22 ; see Figs. 18 and 172). 

 It represents the oldest part of the cerebral vesicle and composes 



