NERVOUS SYSTEM. VERTEBRATA. 469 



the true calcarine sulcus, and this is shown by its adaptability to the 

 varying mechanical conditions prevalent in different Orders *. I u t }., 

 Primates both the retrocalcarine (Cunningham's ' posterior cal<- in 

 and the true calcarine (Cunningham's ' l anterior calcarine ") aulci 

 tend, as a result of the occipital extension of the hemisphere, to 

 become horizontal and in most cases become concurrent. In the 

 more rapidly expanding human brain it often happens that the two 

 sulci do not exactly meet, as they generally do in the Apes. Cun- 

 ningham is thus led to the belief that the human brain differs from 

 the Simian brain in possessing a retrocalcarine sulcus ; but there can 

 be little doubt that the so-called "calcarine" sulcus of the Ape* 

 is really a fusion of the retrocalcariue and true calcarine sulci, and 

 therefore does not materially differ from the human calcarine complex. 

 If the caudal extremity of this " calcarine complex " be studied in the 

 Apes it will be found to be exceedingly variable and unstable, so that 

 one cannot regard it as part of the true calcarine, which is an 

 exceedingly stable sulcus. 



By true " calcarine sulcus " I mean that depression which corre- 

 sponds to or produces the calcar avis. As Flower long ago pointed 

 out (Phil. Trans. 1862, p. 198, footnote), the presence of a posterior 

 cornu of the lateral ventricle is not necessary for the existence of a 

 calcar avis. Thus we find a free calcar in many hemispheres (those 

 of Orycteropus, Thylacinus, Pteropus, for example) in which there is 

 no posterior cornu. But in most mammals the calcar becomes hidden 

 by a great mass of fibres (compare most Carnivores and Ungulates), 

 and cannot therefore be said to exist as a projection in tlie ventricle ; 

 and yet in many large Carnivores (Phoca for instance) and Ungulates 

 (Camelus) a small posterior cornu of the lateral ventricle makes its 

 appearance, and with it a typical calcar again becomes exposed as in 

 the Primates. 



It would be strange indeed if the most constant and stable sulcus 

 of the mesial surface of the hemisphere of most mammals should 

 entirely disappear in the Primates, to be replaced by another sulcua 

 presenting identical relations to the lateral ventricle and a similar 

 developmental history, but without being homologous. There ia an 

 overwhelming mass of evidence to show that the vertical part of the 

 sulcus generally called " splenial " is the direct homologue of the 

 calcarine sulcus of the Primates. 



* At the same time the fact that it develops in the midst of the region 

 in which Vicq d'Azyr's stripe occurs in the Primates, and which represent* 

 the visual " centre," lends a special interest to this sulcus, which obviously 

 accommodates the expanding visual cortex. 





