THE BRAIN AND SPINAL CORD. 



209 



(Fig. 171) is developed in the lamina terminalis — the primitive 

 anterior wall of the fore-brain. The commissure passes between 

 the temporo-sphenoidal lobes. These lobes represent the posterior 

 ends of the cerebral vesicles. At first they are mere dilatations 

 behind the foramen of Monro. The commissure crosses in the 

 lamina terminalis below and rather anterior to the foramen of 

 Monro. This is the earliest and most primitive of the cerebral 

 (pallia!) commissures (Elliot Smith). 



corp. callos. 



lam. term, 

 ant. comm. 

 lam. term, 



sulcus arcuatus 

 prim, callosal gyrus, 

 "ransuerse or choroid, fis. 

 roof plate of 3rd uentricle 



pineal 

 aqueduct Sylvius 

 cerebellum 

 4th uent. 



3rd uent. 



Fig. 171. — Mesial Aspect of the human Foetal Brain during the 4th month. (After 



Minot.) 



(2) The Corpus Callosum, the great commissure between the 

 cerebral hemispheres, forms late ; except in the higher mammals 

 it is smaller in size than the anterior commissure. It reaches its 

 fullest development in man, and in him it commences to form at 

 the end of the third month. 



To understand its development the student must be familiar 

 with the mesial aspect of the brain during the third and fourth 

 months. On this aspect he should note — 



(1) The sulcus arcuatus (Fig. 171), which indents the mesial 

 wall of the fore-brain. This fissure in the adult brain becomes 

 (a) the callosal sulcus between the corpus callosum and callosal 

 gyrus (gyrus fornicatus) (Fig. 172); (b) the hippocampal fissure, 

 which indents the posterior extremity (descending horn) of the 

 cerebral vesicle and causes the hippocampus major (Fig. 172). 

 The calcarine fissure may also be a derivative of it (Elliot Smith). 



