-AroKPiioT.onv of the brain in the .afammalia. 30!) 



than in the Apes, Imt smaller than that of tlie othoi- Li'niurs. Although the hippociiuipii-^ 

 is an appendage, so to speak, of llie apparatus of smell ami (Iwiiidles almost to nothing 

 in smell-less animals, like tlie Odontoceti, the size of the hippocainpal formation does nol 

 seem to vary directly (as that of the pyriforiu lobe does) with tlie degree of osniatism. 

 The hippocamjnis is distinctly larger in all Lemurs than it is in the Apes, and Tui-.siii.s 

 shares this peculiarity of the Lemurs, although in a lesser degree than the other Lemurs. 

 Thus, if we conijiave the hippoeampal swelling in the lateral ventricle of Tarslns with 

 that of Hapiilf (the Ape which most nearly ajiproaches it in size), we cannot fail to he 

 struck with the contrast between its plump columnar form in the Lemur and its slender 

 proportions in the Ape. 



Tiie most that can be seen of the mammalian hippoeampal formation in a mesial view 

 of the hemisphere is a strip of fascia dentata and flmlnia, and a variable area of alveus- 

 covered "inverted hippoeam[)us *. In Tanuiis the fascia dentata and fimbria form two 

 hands of equal and uniform l)rea(ltli, not separated by any " inveited hippocampus." 

 These bands extend almost vertically upward, with only a very slight inclination forward, 

 and the fascia dentata passes directly to the splenium of the corpus callosum, without 

 the formation of any subsplenial flexure. Such a phenomenon does not occur elsewhere 

 among the Lemurs, but we find a parallel in tlie case of the Sloths f, in the brain of 

 Avhich there is, as in Tursiiis, an unusually short corpus callosum. 'J his is indicative of a 

 very primitive state. 



The supra- and pre-callosal vestiges of the cephalic extension of the hippoeampal 

 formation call for no speci-il mention, because in Tarsius, as in all Primates, these 

 structures are extremely insignificant and difficult to demonstrate. 'Uhe hippoeampal 

 tubercle is relatively large in Tarsiun, seeing that it involves about one third of the 

 length of the hippoeampal formation. 



The corpus callosum is remarkable (in an Order in which this commissure is dis- 

 tinguished for its great length) in being unusually short. It measures only 5-5 mm. 

 in a hemisphere 17 mm. long, whereas the corresponding proportions in a Lemur tiiongo- 

 are 17-5 : 39, in a Ni/cticchiis tardigradus 14 : 31, and in a Chironti/s 22 : 45. Its 

 thickness is remarkably uniform throughout, the splenial and genual extremities being 

 only very slightly thicker than the body, ^^'hereas in Lemur and most of the Prosimiaj 

 the splenium and genu are twice or even three times the thickness of l^he body. The 

 corpus callosum is also more obliquely placed than it is in Lemur. It exliibics a very 

 slight, regular, dorsally-directed convexity, and its slightly dependcmt anterior extremity 

 shows no sign of being bent backward to form a rostrum, such as we find in the xVpes. 

 In all these respects the brain of Tarsius approximates to the primitive condition 

 exhibited in the Insectivora. 



In consequence of the relatively slight backward extension of the corpus callosum, 

 the psalterium or hippoeampal commissure is short and plump. In all other Trosimia', 

 as in most mammals, the great bulk of the psalterium is collected at the upper end 



• Viilf .Tonni. .Vnat. and I'liyn. vol. xxxii. 1>!_)7. \'\k liO & 51. 



t •• Jlraiu in llic Edentata, " Trans. Linn. Sue. ser. 2, Zool. vii. lSi)0. 



54* 



