216 Condition in the Anthropoid Ape [CHAP. 



The sulcus rectus is of greater importance. A furrow constantly present in the ape's brain, traversing 

 the frontal lobe in a horizontal direction, its homology is not so clear. According to some (Eberstaller 

 and Herve) it is the equivalent of the human sulcus frontalis medius, according to others (Cunningham, 

 Gratiolet) it represents the sulcus frontalis inferior. Into this discussion, even if I were qualified, I have 

 no desire to enter. The sulcus is clearly subject to great variations and I merely wish, for purposes of 

 orientation, to give an account of the parts as they appear in my three specimens. Now in all three 

 hemispheres, a deep sulcus is seen springing from the vertical portion of the sulcus precentralis inferior and 

 occupying a position and following a course which certainly corresponds very closely with that taken by 

 the sulcus frontalis inferior, or sulcus froutalis secundus as it is sometimes called, of the human brain. 

 For this and other reasons, I look upon and shall hereafter call this sulcus, the sulcus frontalis inferior. 

 In the oraug's hemisphere and in the chimpanzee's hemisphere of the right side, this sulcus runs forwards 

 for about 2 cm. and terminates blindly. The main division of the sulcus rectus lies at a higher level, 

 and, in my opinion, just in the position which the sulcus frontalis medius would occupy in the human brain. 

 In the left chimpanzee hemisphere my sulcus frontalis inferior is not isolated but forms the hinder portion 

 of one long continuous sulcus rectus. It will thus be apparent that if I were driven to give an opinion 

 on the homologies of the simian sulcus rectus, 1 would take a middle course and say, that the hinder 

 part, perhaps a third, represents the sulcus frontalis inferior, and the remainder the sulcus frontalis medius. 



Concerning the so-called anterior limb of the Sylvian fissure, it has been unequivocally demonstrated 

 by Cunningham and Elliot Smith on ontogenetic and phylogenetic grounds which there is no occasion for 

 me to repeat, that an anterior limb of the Sylvian fissure, sensu stricto, does not and cannot exist in 

 the anthropoid brain ; the furrow commonly mistaken for such is nothing but the superior limiting sulcus 

 of the island of Reil. In all my specimens, this sulcus is seen peeping out on to the surface immediately 

 anterior to the frontal operculum and, as I shall presently show, histology suggests that the view concerning 

 its honiology just given is correct. 



The long sulcus fronto-orbitalis so well-marked in all the higher apes is also clearly seen in my specimens 

 and cannot be mistaken for anything else. It is a fissure which for roe possesses great interest, but 

 in the meantime I would merely say that I have become convinced of the anatomical correctness of 

 regarding it, or at any rate its lower half, as the anterior limiting sulcus of the island of Reil. It therefore 

 follows that for a proper understanding of this part we must bear in mind that the exposed surface in 

 the anthropoid brain, lying between the anterior limiting sulcus (fronto-orbital) and the superior limiting 

 fissure (so-called anterior Sylvian), becomes submerged and forms a part of the insula in the human being. 



Continuing my description of the "intermediate precentral" area I will now show that the 

 distribution of this cortex is influenced in a remarkable manner by the disposition of sulci. 

 As may be seen in Plate XXII, the intermediate cortex after covering the lower end of 

 the ascending frontal gyrus spreads forwards and crosses the inferior precentral and inferior 

 transverse fissures, enveloping the whole of that block of substance included between what 

 I am calling the sulcus frontalis inferior above, and the superior limiting sulcus of the island 

 of Reil below. As this block seems to represent the pars basilaris of the frontal operculum, 

 we accordingly have a human reproduction. It is the further distribution which is peculiarly 

 interesting, and which helps to establish some of the above-mentioned homologies. Instead 

 of curling round the superior limiting fissure of the insula and covering the substance 

 intervening between this fissure and the sulcus fronto-orbitalis, it leaves this small area 

 entirely alone, and passing upwards and forwards invests the upper end of the fronto-orbital 

 sulcus; then turning abruptly downwards, and still following the fronto-orbital sulcus, it coats 

 the convolution forming its anterior wall, and is finally arrested well down on the inferior 

 surface by the sulcus orbitalis. 



Having expended much time and trouble over this undoubtedly difficult area, I think I have 

 given a correct account of the nature of its enveloping cortex, and curious though the distri- 

 bution at first sight may appear, all is readily explained if we view the parts from the standpoint 



