MOKPIIOLOGT OF THE BRAIN IX THE MA.M.MALIA. 893 



calary sulcus, aud in some of tlic smaller Marsupials, such as Trlchosurm, there is no 

 intercalary sulcus, the simple calcarine furrow alone being present. 



It may further he ohjected that a true calcarine sulcus can exist only w lien there is a 

 posterior cornu of the lateral ventricle. If so, the Lemurs cannot have a calcarine 

 sulcus, whei'cas the Seals and tlie Camel possess such a furrow. 



It is quite impossible to frame any definition of a calcarine sulcus or of an "occipital 

 lobe" which can he raised as a harrier between the Primates and the other mamnials, 

 such as Cunningham would have us erect. The calcarine sulcus and the calcar avis arc 

 not the exclusive property of the Primates, but are the common heritage of all tlie 

 Metatheria and Eutheria. 



In most mammals this sulcus becomes joined to the intercalary, a furrow of little 

 morphological importance; V)ut in the Primates the backward prolongation of the corpus 

 callosum so alters the direction of the calcarine that it forms an acute angle with the line 

 of the intercalary, so that for purely mechanical reasons the two furrows fail to unite, 

 and the calcarine becomes confluent with the retrocalcarine, which is another furrow of 

 quite secondary importance. 



It is not without consifleiable significance that these phenomena occur in an equally 

 pronounced form in the Lemurs as in the Apes, in sjjite of the fact that the mechanical 

 conditions favouring such a .sundering of the calcarine-intercalary junction are at least 

 equally marked in many Carnivoi-es, in which the separation does not occur. It Avould, 

 however, be unwise to attach too great an importance to this iact, because we 

 liud the calcarine sulcus separated from the intercalary in the Anteaters, Sloths, and 

 Pangolins. 



The fact that no trace of the parieto-occipital sidcus is found in the Hapalida; and 

 several of the smaller Cebidte seemed to suggest the possibility of that furrow being a 

 purely Simian feature, which had become evolved in the Apes only. I was therefore 

 inclined at first to follow Flower's teaching, and regard the paracalcarine sulcus of the 

 Lemurs as one of the limbs of the bifid extremity of the retrocalcarine sulcus of the Apes. 

 But the fact that it branches off from the point of union of the calcarine and retrocalcarine 

 sulci shows that it occupies a position analogous to that of the lower extremity of the 

 parieto-occipital sulcus in the higher A2:)es. 



But are we justified in calling this furrow parieto-occipital ? Most wi'iters do so 

 without the .slightest hesitation. 



In order to settle this point I have examined the brain in every genus of Monkeys, 

 and have found that in the higher Apes, where the true parieto-occipital sulcus of 

 Human Anatomy can alone be said to exist, this furrow is com[)o.sed of two and 

 frequently more elements. In the majority of cases, however, there are two sulci, 

 dorsal aud ventral, which overlap to a considerable extent, the intervening area of cortex 

 usually becoming submerged so as to obscure the dual natiu'c of the resultant furrow. 

 Tlic ventral furrow I have called "paracalcarine," Ijucause it pursues a course alongside 

 the calcarine sulcus in its most typical form, e. y. in Slmia. The dorsal sulcus is 

 obviously due to the bias a\ hich is given to the developing cortex near the dorsal edge by 

 the intraparietal sulcus. This furrow becomes so sharply bent that it forms an acute 



