NEUROLOGY 



I 



of Monro. This sulcus ends anteriorly at the medial end of the optic stalk, and in 

 the adult brain is retained as a slight groove extending backward from the inter-- 

 ventricular foramen to the cerebral aqueduct. 



At a very early period in some animals before the closure of the cranial part of 

 the neural tube two lateral diverticula, the optic vesicles, appear, one on either 

 side of the fore-brain; for a time they communicate with the cavity of the fore-brain 

 by relatively wide openings. The peripheral parts of the vesicles expand, while 

 the proximal parts are reduced to tubular stalks, the optic stalks. The optic vesicle 

 gives rise to the retina and the epithelium on the back of the ciliary body and iris; 

 the optic stalk is invaded by nerve fibers to form the optic nerve. The fore-brain 

 then grows forward, and from the alar laminae of this front portion the cerebral 

 hemispheres originate as diverticula which rapidly expand to form two large 

 pouches, one on either side. The cavities of these diverticula' are the rudiments of 

 the lateral ventricles; they communicate with the median part of the fore-brain 

 cavity by relatively wide openings, which ultimately form the interventricular 



Choroidal fissure 

 Hypophysis 



Recessus infundibuli 



Tuber cinereum 



Corpus mamiliare 



Cervical .flexure 

 FIG. 653. Interior of brain of human embryo of five weeks. (From model by His.) 



foramen. The median portion of the wall of the fore-brain vesicle consists of a 

 thin lamina, the lamina terminalis (Figs. 654, 657), which stretches from the 

 interventricular foramen to the recess at the base of the optic stalk. The 

 anterior part of the fore-brain, including the rudiments of the cerebral hemi- 

 spheres, is named the telencephalon, and its posterior portion is termed the 

 diencephalon; both of these contribute to the formation of the third ventricle. 



The Diencephalon. From the alar lamina of the diencephalon, the thalamus, 

 metathalamus, and epithalamus are developed. The thalamus (Figs. 650 to 654) 

 arises as a thickening which involves the anterior two-thirds of the alar lamina. 

 The two thalami are visible, for a time, on the surface of the brain, but are subse- 

 quently hidden by the cerebral hemispheres which grow backward over them. 

 The thalami extend medialward and gradually narrow the cavity between them 

 into a slit-like aperture which forms the greater part of the third ventricle; their 

 medial surfaces ultimately adhere, in part, to each other, and the intermediate 



