318 DR- G. ELLIOT SMITH ON 



As a description of tlie mere appearance of the hippocampal vestige in Myrmecophaga 

 this account is lucid enough, but if we literally iuterpret either it or the expression of 

 Turner (Avhich it somewhat ti'avesties) as a statemeat of the actual constitution of the 

 parts we shall gain a very erroneous conception. For, as tlie hij^pocamjjal formntion 

 surrounds the splenium, the fascia dentata dwindles and practically disappears, leaving 

 the diminutive remnant of the simple hippocampus proper upon the surface of the corpus 

 callosum. This hippocampal vestige is very rich in longitudinal inedullated fibres, which 

 in human anatomy are called the strice Lmicisii (as well as an immense variety of other 

 names). This explains the statement of Turner that the fascia dentata is prolonged into 

 tlie upper surface of the corpus callosum as a narrow band *, which, to the naked eye, it 

 appears to be. It also explains the statement, which Eorbes wrongly attributes to 

 Turner, that the fascia dentata is " continued forward as a thin layer of longitudinally- 

 disposed fibres." 



Histological examination enables us to exactly interpret the puzzling macroscopic 

 appearances and to state definitely that the hijipocampal formation, a complex of fascia 

 dentata and hippocamj)us (in the narrow sense), extends up to the splenium of the 

 corpus callosum and becomes continuous around tlie S[)lenium with a diminutive baud 

 which represents the dwindled hippocampus from which the fascia dentata has 

 practically vanished ; as this vestigial liippocampus contains a large number of 

 me;lullated nerve-fibres, it is often mistaken for a purely fibrous structure. 



Although tbe figure of the mesial surface of the brain of Cycloturus which Pouchet 

 gives + is lacking in detail, it suffices to show that a sulcus limitans pallii exists in a 

 form exactly analogous to that of the other two representatives of the Myrmecopjhagldcc ; 

 and the fact that his figure represents a vestigial hippocampus at all shows that this 

 structure is more prominent than is usual among Eutlieria. 



Flower has given a useful figure and a descrij)tion of the hippocampal region in 

 Choloepus X, but the corresponding region in Bradypus has not, so far as I am aware, 

 been properly figured or described. 



In the brain of both of the Sloths the hippocampus presents the typical features in 

 the greater part of its extent, such as we have already described in Tamandua, and 

 which we find in most mammals. But in the upper part of its extent the hippocampus 

 presents in both Bradypodidce certain features in common which distinguish it from 

 that of the other Edentates. We have already observed that in this family the corpus 

 callosum has not grown backward even to the same extent as it has in Orycteropus. As 

 a result the psalterium, as we have already seen, is short and sim[)le, and in addition the 

 upper part of the hippocampal formation has undergone very slight disturbance. In 

 both Sloths the fascia dentata appears to extend directly toward the splenium without 

 such a marked infrasplenial flexure as we find in the other Edentates. To the naked 

 eye no area of exposed hippocampus makes its appearance behind the splenium, and the 



* W. Turner, op. cit., ' Journal of Anatomy and Physiology,' vol. i. 



t G. Poucliot, op. cit., ' Journal do I'Anatomie et do la Physiologic,' tome vi. pi. iv. fig. 4. 



J W. H. Flower, oj). cit, Phil. Trans. 18G5. 



