MORPHOLOGY OF THE BKAIX IX THE .M.\M>rALIA. 4.07 



oi- of the Cai-nivoi-e TlceiTa ov the Ung-iilatc Procaria, with tin; orbital sulcus of such h 

 Prosimian bmiu as that of (iaUKjo or ^\q\\ Leinnr, cue cannot deny that the two sulci are 

 morphologically identical. If we examine a larg'c series of iunuan brains, it will be found 

 that the stable portion of the orbital sulcus consists of a deep oblique sulcus, the posterior 

 end of which approaches close to and often joins the outer limb of bifurcation of tlu^ 

 olfactory sulcus, just as the presylvian sulcus of the Carnivore's joins the rliinal fissure ; 

 but the simjile linear form of the orbital sulcus of Man is usually disguised by a scries 

 of inconstant branches, so that it seems to lose the simplicity of form it possesses in most 

 Apes. If this fundamental part of the human orbital sulcus be compared with the pre- 

 sylvian sulcus of the larger members of the other Mammalirm Orders, such, for example, 

 as the Camel and the Seals, a demonstration of the identity of these two furrows will b(; 

 afforded which is quite as striking as the similarity of the presylvian sulcus of DolicholtH 

 to the orbital of Galago. Such instaucas might be multiplied without limit. 



So that, if we take into consideration all the facts which a study of the brain in all 

 mammals yields, there is a mass of evidence a)uounting to absolute demonstration that 

 the sulcus variously called pi'esylvian, supraorbital, orbital, and triradiate in different 

 mammals is fundamentally one and the same furrow. 



Between the orbital sulcus and the lower end of the Sylvian fissure we occasionally 

 find a furrow in the Lemurs and the New- World Ajies, and generally in the 01d-"\Vorld 

 i\j)PS : this is the frouto-orbital sulcus. AVith regard to this homology in the Lemurs, 

 I quite airree w ith Zi( hen. r>ut, on the other hand, the evidence which I have been al)le 

 to collect points conclusively to the truth of the suggestion of Eberstaller and Cunningham, 

 that the fronto-orhital sulcus of the Apes [and Lemurs] is the anterior limiting sulcus of 

 the insula of the human brain. 



In most Ungulates, in many Carnivores, and in the Bradypodida: and .\fiin)tecupluMiii, 

 a small oblique sulcus makes its appearance in the position exactly corresponding to that 

 occupied by the fronto-orhital sulcus in the Primates : it is called the diagonal sulcus 

 (Krueg), and is placed between the orbital and the suprasylvian su1<m. It is highly 

 improbable that a small oblique sulcus such as the diagonal (especially in the Ungulates 

 and in Bradi/pus) should develop in the analogous position in three different Orders so 

 as to exactly reproduce the features of the fronto-orhital sulcus of the Lemurs and Apes 

 without being homologous. The probability that " diagonal," " fronto-orhital," and 

 " anterior limiting" are merely different names for the corresponding furrows in different 

 mammals is immeasurably greater than the reverse. 



The evidence upon which Professor D. J. Cunningham chiefly relied to clinch the proof 

 of the identity of the fronto-orbital and anterior limiting sulci, was the distriljution of 

 the claustrum. This consists of the deeper part of the cortex which composes the island 

 of Beil. In the human brain the clausti-um becomes separated from the rest of the 

 cortex by a definite medulla ly layer in a region coextensive with the insula, /. e. so far 

 forward as the anterior limiting sxilcus. In the Chimpanzee [and the same holds also for 

 the Orang and Gorilla] the claustrum extends as far forward as the fronto-orbital sulcus. 

 In the Cercopithecidie and Cebid^c it is unfortunately the fact that we cannot vise such 

 evidence with the same degree of definiteness, because the anterior extremity of the 



SECOND SERIES. — ZOOLOGY, VOL. Till. ^'J 



