MORPHOLOfiY OF THE BRAIN L\ TUK .A!.\M MALTA. 329 



At tlu> luitero-mesial aspect of tlic uatil'orin ciiiiiiencc (as the posterior part oi' the 

 pyriform lohe is often called) there is an elliptical area hounded laterally by a shallow 

 i;roove. Retzins has called liiis the " livrus luiiaris " and has sliown how remarkably 

 constant it is in 1h(> ^ra-nnialia *. I am not aware that anyone lias ever considered the 

 significance of tliis little body (which I liave studied not only in most Mammals, but 

 also in the Reptilia) ; its nature is at once revealed in the Lemur's bi-ain by a section at 

 rig'lit angles to its long- axis. It is clearly nothing else than the surface of the nucleus 

 amygdalte and is not therefore strictly a ])art of the pyriloi-m lobe. 



At llie mesial border of the |)yrif()rni lobe we tiiul a peculiar depressed grey band, 

 whicli is erroneously labelled "gyrus uncinatus" by Tlatau and Jacobsohnf. It is 

 merely the low or end of the fascia dentata, whereas the uncinate gyrus is represented by 

 the pyriform lobe itself. These writers adopt a very umisual and utterly misleading 

 nomenclature Avhen they call the anterior part of the pyriform lobe by the name " gyrus 

 hi))p()canipi '" (p. 195). 



In most Lemurs the rhinal fissure, which is the lateral boiuidaryof the pyriform lobe, 

 becomes abnost wholly obliterated. This also happens in such Chiroptcra as Pferopus 

 and Ci/i/onijc/('/-/s. I have, liowever, been fortunate enough to obtain the brain of a 

 Lemvrfiiicnft, in which the rliinal fissure is retained in a form as distinct as it is in most 

 mammals (figs. |. and (5). I hav(> also in my possession another brain of the same genus 

 in which the whole of the rhiiial llssure is visible, although it is very shallow ; and also 

 the cranial cast of yet another in which it is cpiite distinct. These three examples are, 

 however, exceptional. 



The fissure is distinctly visible upon the lateral aspect of the first-mentioned brain 

 (fig. (5) pursuing a course (fx'om the cleft between the olfactory bulb and the hemisphere) 

 backward with a very slight upward inclination. Just behind the Vcillecula Sylvii it 

 suddenly bends downward at an obtuse angle and curves horizontally backward to a 

 point midway between the vallecula Sylvii and the posterior margin of the hemisphere. 

 Behind this point there is no line of demarcation betAveen the pyriform lobe and the 

 neopallium. 



In Monkeys no part of the rhinal fissiu-e is visible upon the lateral aspect of the 

 hemisphere : the relatively greater size of the neopallium and the much smaller 

 absolute dimensions of the pyriform lobe shift it wholly on to the ventral, or even 

 mesial, surface. 



In the brain of most members of the genus Lemur, all that remains to represent the 

 rhinal fissure is the angle and olten only the posterior limb of the angle (fig. 7). In 

 such cases, however, a careful examination enables one to distinguish the pyriform lobe 

 from the neopallium, and thus determine the situation from which the fissure has been 

 obliterated. 



The olfactory tract (fig. 4) ends in tlie vallecula Sylvii in a distinct " tubercle of the 



* Gu.slav Jictzius, " Windungon dcs Khincncephalous," Biolog. Unters., N. F. viii. p. 2 (1898). The condition which 

 he represents in the liodent DasijprocUi, Taf. 7. fig. 10, closely resembles that of the Lemue. 

 t Haudli. d. .Vriat. u. vergleich. .\nat., licrlin, IS'JO, Taf. o. fig. 2. 



49* 



