THE TELENCEPHALON. 



1191 



ventricle, where its remains are seen as the definite choroid plexus. The second furrovv, the 

 hippocampal fissicre, appears shortly after and above the choroidal on the mesial surface of the 

 pallium. Its primary position is marked by an invagination affecting the entire thickness of 

 the cerebral wall (Fig. 1028), which, therefore, appears on the inner aspect of the wall of the 

 pallium as an arched longitudinal ridge, the later hippocampus. At first open on the mesial 

 surface, the fissure subsequently becomes almost entirely filled by the dentate gyrus and in the 

 fully developed brain is scarcely seen. 



The central sulcus or the fissure of Rolando is usually the first of the permanent furrows 

 to appear on the outer surface of the hemisphere. As a rule, it is recognizable during the last 

 week of the fifth month, although its appearance may be delayed until a month later (Cunning- 

 ham). When laid down as two separate furrows, as it not infrequently is, the lateral one is the 

 longer and usually the deeper. Subsequently the two parts become united into a continuous 

 sulcus, although very rarely tlie primary condition may persist and the Rolandic fissure be 

 interrupted by a superficial gyrus. During the fifth month, on the mesial surface of the hemi- 

 sphere, also appear the calcarine and the parieto-occipital fissure. The first of these is often 

 mapped out by two or even three separate parts, of which the front one is complete and, as tlie 

 anterior limb of the calcarine fissure, produces the elevation known as the calcar avis. The 



Fig. 1029. 



Superior collicuhis Pineal body 



III tier\-e 



Median geniculate body 

 'Lateral geniculate body 

 Occipital lobe 



Parietal lobe 



Pallium 



Inferior coUiculus -V 



IV nerve 

 Mammillary body 



Cerebellum 



Rhombic lip 



■Frontal lobe 

 ■Temporal lobe 



Sylvian fossa 



Rhinenceohalon 



- Optic stalk 

 \ Infundibulum 

 \ Pituitary bodv 



V nerve 



Pons 



V'lII nerve 



Spins 



Reconstruction of brain of human embryo of five weeks (13.6 mm.); external lateral aspect. 



Drawn from His model. 



Xii. 



other parts subsequently unite to form the posterior limb of the calcarine fissure. When first 

 formed the parieto-occipital fissure is usually distinct from the calcarine, with which, however, 

 it soon becomes confluent. Towards the end of the fifth month the collateral fissicre appears 

 on the inferior surface of the hemisphere. The inferior and the superior precentral sulcus may 

 usually be distinguished, the lower slightly in advance of the upper, during the early weeks of 

 the sixth month, and about the same time the superior temporal and the olfactory sulcus. The 

 middle of the sixth month marks the appearance of the postcentral and occipital limbs of the 

 interparietal sulcus and the first suggestion of the orbital furrows and the calloso-marginal 

 sulcus, as well as the junction of the inferior frontal with the lower precentral sulcus. Towards 

 the close of the same* month are added the superior frontal, the itiferior temporal and the 

 occipital sulci. The seventh month witnesses the extension and deepening of the fissures already 

 formed and the union into continuous sulci of parts which before were separate. During the 

 succeedingmonth, the surface of the hemisphere and the brain-case once more come into inti- 

 mate relation, from which it follows that the rounded elevations marking the convolutions can 

 no longer unrestrictedly expand, but from now on must accommodate themselves in their growth 

 to the inner surface of the cranium. In consequence of this limitation, the convolutions become 

 less rounded and more closely packed, and the free surface of the hemisphere conforms with the 

 interior of the cranium. Increased complexity in the details of the convolutions arises from the 



