v] Vixii<>-S<iiKori/ Area fit the Aiith>-o/><>i</ Aj>e 123 



Analysing these discrepancies we find, first, that the mode of termination of the 

 calcarine fissure in the anthropoid brain is peculiar; for in both my chimpanzee hemispheres 

 there is no sign of bifurcation at the posterior extremity, and in the orang's hemisphere 

 only the slightest suspicion of a fork is noticeable, hence we see no tissura extrema of 

 Seitz. Secondly, as Cunningham indicates, the tail of the fissure instead of pointing straight 

 to the occipital pole, as in the human brain, is directed more forwards so as to form a 

 sharper angle with the parieto-oecipital fissure (in one of my chimpanzee hemispheres, 

 however, the direction of the fissure exactly resembles that in the human brain). In the 

 third place, we observe on the postero-lateral surface of the hemisphere a sulcus from 3 

 to 4 cm. long, which lies in the same plane and is almost continuous with the calcarine 

 fissure ; it is a sulcus of considerable depth and obviously has a special significance, not only 

 because it is deposited almost exactly in the middle of the field showing a calcarine type of 

 cortex, but because the distribution of this type of cortex seems to bear a close relation 

 to and to be influenced by the position of this sulcus, just as its distribution on the 

 mesial surface of the hemisphere is influenced by the calcarine fissure. At first I was 

 inclined to regard this sulcus as the homologue of the human sulcus lateralis occipi- 

 talis of Eberstaller, but further investigation and above all things a knowledge of the 

 histology of the cortex of these parts caused me to demur from that view, and I now 

 think that the fissure is either the equivalent of a sulcus known as the <: external cal- 

 carine fissure," or, and this is more likely, it represents a dislocated " fissura calcarina 

 posterior." 



Of the external calcarine fissure Cunningham gives the following account : 



" It is placed very obliquely along the lower border of the cerebrum, and corresponds on the outer 

 surface of the hemisphere with the calcarine fissure on the mesial face. When transverse sections are made 

 through the occipital part of the cerebral hemisphere, the external calcariue fissure is seen to be a deep 

 infolding of the hemisphere wall, and the bulging which it forms into the ventricular cavity lies exactly 

 opposite, and may be actually in contact with the calcar avis. The external calcarine fissure appears very 

 early. It can be distinguished in a large number of cases amongst the primitive transitory furrows, and at 

 this period it is sometimes continuous around the occipital pole of the hemisphere with the precursor of 

 the true calcarine fissure. This connection, where it exists, is always obliterated about the fourth month. 

 In the human brain the external fissure is transitory. It is effaced towards the end of the fifth month. 

 It is not so constant as the external perpendicular fissure of Bischoff and in many cases it fails completely. 



Although evanescent in the brain of man, there is some reason to believe that it has a permanent 

 representative in the brain of the ape. On the outer surface of the occipital lobe of most apes a deep 

 fissure runs horizontally forwards from the occipital pole, and comes to an end a short distance behind the 

 free anterior lip of the occipital operculum. This fissure is placed on the outer face of the hemisphere in 

 an exactly corresponding position to the calcarine fissure, and its anterior end, in most of the numerous 

 specimens which I have examined, just falls short of the posterior horn of the lateral ventricle 1 ." 



While the histological evidence which I can now supply lends some support to Cun- 

 ningham's reasoning that the transitory external calcarine fissure of the human cerebrum is 

 homologous to the permanent, deep, horizontal fissure on the outer face of the occipital lobe 

 in the anthropoid ape, it does not wholly prove the correctness of his argument, simply 

 because the external calcarine fissure is unrepresented in the adult human brain. One point, 



1 I observe that Elliot Smith in his earlier papers speaks of this simian external calcarine fissure as the lateral 

 occipital, but it is not to be confused with the human lateral occipital sulcus of Eberstaller, which is <niite another 

 element. In his later papers Elliot Smith removes the confusion by introducing the name sulcus intrastriatus lateralis. 



162 



