THE FOEM-KELATIONS OF THE HUMAN BRAIN. 201 



tion to the fissura hippocampi, which really belongs to the mesial surface. 

 On the under surface of the frontal lobe lie the sulci orbitales and olfactorii. 

 The convolutions between them, regarded as continuations of the frontal 

 gyri, are designated by the names of those frontal gyri with which they are 

 respectively continuous. 



The cortex of the basal surface of the frontal lobe borders on that gray matter 

 at the base of the brain which belongs to the olfactory apparatus. We shall have 

 occasion to consider this gray matter later. Two small elevations situated near the 

 median line and extending out from this gray matter dorsally, the gyrus rectus and 

 the (JU>'us suicallosiis lying behind this, belong, perhaps, to the olfactory apparatus. 

 At all events, the latter of the two gyri ai'ises from the outfolding produced by a, 

 bundle of fibers passing along under it in this situation, which bundle passes from 

 the terminations of the olfactory radiation up over the septum into the fornix. It 

 is that very bundle to which, in the lower vertebrates, I thought it necessary to 

 attribute so great significance for the interpretation of the mesial cortex of the brain. 

 See Figs. 76 and 100: Tr. cortico-olfactorius septi. 



At the base of the brain the temporal and occipital lobes cannot be 

 separated from one another. Longitudinally-directed fissures, in smaller 

 number, traverse the region common to both lobes, which, in general, is 

 included in the temporal lobe. The middle temporal gyrus extends only a 

 short distance toward the base; that which is visible belongs almost en- 

 tirely to the inferior, or third, temporal gyrus. This is separated by means 

 of a rather superficial fissure, — which is almost always interrupted several 

 times, — the sulcus temporalis inferior, from a long, spindle-shaped convolu- 

 tion, the gyrus fusiformis, — a gyrus invariably well defined. This gyrus 

 borders directly on the long gyrus hippocampi. It is separated from the 

 hippocampal gyrus by a long, deep fissure, a fissure appearing very early in 

 embryonic life, the fissura coUateralis. The collateral fissure extends over 

 the entire under surface of the brain from the occipital lobe to the apex of 

 the temporal lobe. 



The fissures of the brain may be very easily fixed in mind by studying 

 them on the developing brain instead of on that of the adult. At the same 

 time, several facts, very interesting from a morphological stand-point, are 

 disclosed as an additional compensation. 



If the very young brain is examined, which is shown in Fig. 30, it is 

 seen that a fissure runs along the greater part of the inner edge at the place 

 where the wall of the forebrain passes over into the thin velum interpositum, 

 which consist of epithelium only. The two walls of the fissure are formed 

 Just by this epithelial plate. His has named it the fissura chorioidea. Later 

 in life it is filled up by the vessels growing into it, and it is then no longer 

 demonstrable, because its walls form the covering of the plexus chorioideus. 



Even during the second and third months, a second fissure is met with, 



