MOIiPlIOLOGY OF TllH HHAIX 1\ TJIE \l A^IMATJA. :;71 



commissure is relatively large. Aiul this is yet a further indication oi" the primitive state 

 of the braiu ol' Tarsim. 



The anterior part of the mesial surface of the h-nuis[)here is quite smooth and flattened 

 against the correspond ing surface of the other heuiispliere. This flattened area. i)roloQg<'d 

 backward above the corpus callosum, extends for more than 6 mm. in the caudal 

 direction beyond the splenium of the corpus callosum as a baud less than 3 mm. Avide. 

 The whole of this flattened area is entirely devoid of sulci, no trace of the intercalary 

 (calloso-margiual), genual, or rostral sulci, which are found in some Lemurs, being 

 detected here. Ziehen says* IJiat in the region abcne tlie corpus callosum there is a 

 deep furrow on the mesial surface, ^\ liieii is probably nul a vascular groove, but is a true 

 fissure running parallel to the corpus callosum. In all of the four hemispheres of Tamlns 

 which I have examined this so-called sulcus a of Ziehen is nothiug else than the 

 impression of the anterior cerebral artery, as it oblitjuely crosses the surface of the 

 hemisphere, after having l)ent around the genu of tlie corpus callosum. A similar 

 impression may l>e fouml in any mammalian l)rain, as I have already pointed out in tlie 

 case of Oj-iiil]ioi-]ii/iicluiis\. 



In a cerebral hemisjdiere 17 mm. long, 8 mm. (or almost half of its length) lies behind 

 the splenium of the corpus callosum. Such a marked caudal or (as it is commonly 

 called) occipital extension of the hemisphere is found nowhere else among the Lemurs, 

 and a parallel for it must be sought among the Apes, where such an exaggerated 

 backward prolongation is present in the Hapalidre and tiie smtdler Cebidae. It is a 

 si<Tniticant fact that this Pithecoid condition is found in a brain wbich in manv other 

 respects exhibits more primitive features than the majority of the Frosimite. We have 

 already seen that a narrow upper strip of this postsplenial area of tlie mesial surface is 

 flattened against tlie corresponding surfacs of the othei" hemisphere. The rest of this 

 region is deeply excavated in a \e\'\ irregular manner in adaptation to the shape of the 

 corpora quadrigemiua and cerebellum, wiiich are jjartly overlapped by the occijjital 

 prolongation as l)y a cap. 



Immediately below (or to the lateral aspect of) the flat area there is a deep cup-shaped 

 depression about 3 mm. in diameter, placed about 2 mm. beliind the splenium. This 

 is produced by the anterior quadrigeminal body. In this depression there is a triradiate 

 arrangement of deep siilci diverging froLu its centre. The calcarine sulcus extends 

 downward and forward toward the hippocampal Assure ; the retrocalcarine extends 

 horizontally backward ; and the paracalcarine extends upward, but does not quite reach 

 the flat area. The other two sulci, however, just minage to cross the rim of the cup- 

 shaped depression. 



By comparison with otlier Pi'osimian brains, it will be shown later on that tlie 

 sulci a and y prolxxbly re[)resent the true calcarine sulcus (/ c. that called " anterior 

 calcarine" by Cunninghaui), and that the sulcus /3 is the i-etrocalcarine (Cunningham's 

 "posterior calcarine"). All of these sulci are very deep and completely involute the 

 mesial wall of the very large pithecoid posterior cornu of the lateral ventricle. In other 



* Archiv f. Psycli. P.d. xxviii. p. l.tcO. 



t " Purtbcr Observations on the Braiu in thu ilonotromuUi," Journ. Auut. and I'll}--., vol. xxxiii. 



