MORPHOLOGY OF THE BRAIN IX THE MA-M>rALIA. 331 



approximately vertically from the patch of fascia deatata on the ventral surface to the 

 posterior end, or splenium, of the corpus callosum. 



In the greater part of its extent tliis hippocampal formation conforms to tlio plan 

 which we may call characteristically mammalian (such as we find, for instance, in the 

 brain of Tamandua [vide tlie " Brain in the Edentata," op. ait. svpra, iiijs. 10 & 23, 

 p. 317)]. There is, as in Tamandua, a well-defined sub-splenial flexure, and the wrinkled 

 band of fascia dentata curves around on the under surface of the corpus callosum in the 

 characteristic manner and then tapers as it merges in the circumcallosal hippocampal 

 vestiges. A little hemispherical area of " hi])pocampus nudus " fills up the concavity of 

 the arc formed by the sub-splenial bending of the fascia dentata, just as I have represented 

 it in Tamandua [op. cU. p. 320, fig. 23). In all of these respects the hippocampal 

 formation conforms to the typical mammalian condition, which I have descril)ed in 

 detail in the above-quoted work. 



In a recent memoir Zuckerkaudl has briefly referred to the "hippocampus nudus" 

 (his " BalkenAvindung ") in the Lemurs *. He describes it as being " zapfenformige "' 

 in Lemur catta, L. varius, L. monyoz, and L. coronatus, and " kurze zapfenformige " in 

 Chiromys. 



The loAver extremity of the hippocamj)al formation presents a peculiar modification 

 not found in most mammals. Several years ago I called attention to the peculiar fact 

 that a part of the hipjjocamjjus is rolled in such a manner that it becomes completely 

 inverted: the surface which was originally dcej) is then superficial, and clce versa 

 (Jouru. of Anat. and Pliys. vol. xxxii. p. 28). A strip of this "inverted hippocampus" 

 separates the fascia dentata from the fimbria in the Lemur, as also happens in most 

 mammals; but this becomes greatly expanded at its lower extremity to form a "hippo- 

 campal tubercle," such as is found in Orycleropus (" Tiie Brain \xi the Edentata," op. cit. 

 fig. 23, p. 315). A hippocampal tubercle of similar constitution occurs in all the Apes, 

 and has been accurately described in the human brain by Giacomini and Hetzius 

 (compare Journ. Anat. and Phys. vol. xxxii. pp. 28 & 29). 



In the Prosimian brain the tubercle vai-ies considerably in size and prominence, and in 

 the brain of Nycticebus practically vanishes ; it is never so j)rominent as it is in the 

 Apes, althovigh its constitution is the same. In some Insectivores, such as Macroscelides, 

 I have seen a condition closely resembling that of the Lemurs, and have also seen 

 instances of the same phenomenon in Orycteropus and in the brain of a Manatee. This 

 peculiar condition is apparently inherited by all the Primates, both Lemurs and Apes, 

 from some primitive Insectivore-like ancestor, which has also transmitted it to the 

 primitive Aard-vark and that peculiar Eocene stock which has persisted with little 

 change as the Sirenia. 



The corpus callosum is very long and has a very plump caudal extremity or 

 splenium, and a very thick and fully-recurved genu. As a whole, it closely resembles 

 that of the Platyrhine Apes. The greatly attenuated psalterium, or hippocampal 

 commissure, resembles the common mammalian type, such as is found, for example, in 



* E. Zuckerkandl, " Ecitr. /ur An;it. dcs Reichcentrums," Sit/.uiigsli. Akad. Wiss. Wicn, ilatb.-nat. CL, Ed. cix. 

 Abth. iii. I'JUO. 



