1899.] BEAIlSr OF THE GORILLA, 75 



the divergences from this artificially created normal. Por this 

 purpose I shall select the brain belonging to the Eoyal College of 

 Surgeons. 



On the left side of this brain (fig. 1) there is a short prce- 

 centrcdis superior roughly parallel to the fissure of Eolando. From 

 it extends forwards the sulcus frontalis superior, divided into two 

 by a break and apparently ending anteriorly in a fork ; but a short 

 furrow belonging to this system arises between the extremities of 

 the fork and extends forward for a short distance. Again, parallel 

 with the fissure of Eolando, but below the sulcus pracentnilis 

 superior, is the sulcus prwcentralis inferior. Of this furrow the 

 ramus Jioi izontalis is very oblique and communicates with the 

 fissure of Eolando. Prom the mesial extremity of the ramus 

 horizoutalis arises the very short sulcus frontalis meclius. 



From about the middle of the praecentralis inferior arises the 

 sulcus frontalis inferior, -which is quite as extensive a furrow as the 

 frontalis inferior. It is roughly parallel to it. The first portion 

 of the furrow forks exactly as does the frontalis superior, and again 

 in the same way the distal part of the furrow arises between the 

 fork. The sulcus fronto-orlitalis is continuous with the Sylvian 

 fissure below, and bounds the anterior side of the (here exposed) 

 island of Eeil. In front of this is the Y-shaped sulcus fronto- 

 marginalis ; the stem is perpendicular to the long axis of the 

 hemisphere ; finally the pre-Sylvian fissure completes the triangular 

 boundary of the island of Eeii. 



The right half of this brain shows the following principal 

 differences : — The frontalis medius is much longer ; the ramus 

 horizontilis does not communicate with the fissure of Eolando. 

 The frontalis inferior does not communicate with the prcecentralis 

 inferior. 



The prcecentralis sujyerior nowhere differs greatly from the 

 arrangement which obtains in the brain that has just been 

 described. It never is continuous with the praecentralis inferior as 

 is the case with a Chimpanzee's brain in my possession. 



The frontalis sup>erior in other brains shows variations in the 

 degree and manner in which it is broken up into segments. Some- 

 times it is a continuous fissure ; this was the case with both sides 

 of one brain and with one side of another. In the right hemi- 

 sphere of a third brain the first part of this fissure became deflected 

 to the right and joined the prsecentralis inferior (fig. 6). 



Prcecentralisinferior. — In three hemispheres (belonging to different 

 brains), in addition to the College of Surgeons' brain already 

 referred to, the ramus horizontalis cut the Eolandic fissure. 



The sulcus frontalis medius is not a prominent feature of any of 

 the brains at my disposal. The smallness of its size in the 

 College of Surgeons' brain has been commented upon already ; it 

 is present and also small in only one hemisphere out of the four 

 remaining brains examined by me. In the rest I can find no 

 vestige of it. It might possibly be held that the furrow marked 

 F.p.i, in fig. 6, is i-eally a portion of the medius. But I think that 



