THE BRAIN IN THE EDENTATA. 357 



direction. This mouth is closed by a very thin vertical film — the a.nterior medullary 

 velum — which proceeds upward to be attached to the upturned edge of the narrow bridge 

 of grey substance which connects the two posterior quadrigeminal bodies 



Upon being traced downward this medullary velum (called by the older anatomists 

 the valve of Vieussens) descends until it reaches the tegmentum, upon which it rests, 

 without, however, becoming attached to it. It proceeds backward, forming the anterior 

 part of the roof of the flattened fourth ventricle, and becomes attached to the ventral 

 surface of the cerebellum (figs. 4, 17, and 18). 



The aqueduct of Sylvius presents similar features in all the Ilynmcophayida;, and in 

 Cholcepus, Manis, and in the majority of mammals included in the Carnivora, Ungulata, 

 Rodentia, Marsupialia, and allied Orders. 



In my specimen of Bradypus the wide anterior mouth of the aqueduct gradually 

 tapers as it proceeds backward, so that there is no tubular anterior constriction. Thus 

 the Sylvian aqueduct appears i-elatively larger than it does in other Edentates (fig. 17). 



In the Brddypodldce the posterior part of the roof of the mesencephalon becomes very 

 much elevated, for reasons which we shall consider immediately. As a result of this the 

 posterior infundibular expansion becomes greatly exaggerated. The aqueduct of Sylvius 

 opens out of the third ventricle by a fairly wide opening and rapidly expands, so that the 

 posterior opening becomes very wide and the direction of the canal very oblique. These 

 modifications become most pronounced in Chlamydophorm, although they are already 

 very well-marked in Basypus. 



In Mynnecoplwga and Tamandua the anterior corpora quadrigemina become relatively 

 smaller and flatter in comparison to the optic thalamus than is the case in Orycteropiis, 

 and the posterior quadrigeminal bodies, instead of being lower than the anterior, 

 now rise to a considerably higher level than any other part of the mesencephalon or 

 thalamencepbalon. In Tamandua, in addition to this, these elevated posterior quadri- 

 treminal bodies now become somewhat flattened betw'een the cerebellum and the cerebral 

 hemispheres. In the Bradypjodkla; the anterior quadrigeminal bodies become still 

 further reduced, but the posterior geniculate bodies, which in the Ant-eaters and the 

 Aard-vark are already very large, now attain enormous proportions. Compared with the 

 size of the corpora quadrigemina the optic thalamus is proportionately larger than it is 

 in the 3Iyrmecop]uigid(F and Orycteropus. I have unfortunately been unable to examine 

 these parts of the brain of Ilanis. But in the BasypodidcB we again find modifications 

 of this region. The anterior quadrigeminal bodies become even further reduced in size 

 than they are in the Sloths, but in addition the optic thalamus is extremely small. In 

 marked contrast we find a large and prominent pair of j)Osterior quadrigeminal bodies 

 which project upward as an elevated ridge between the cerebellum and the cerebral 

 hemispheres. The posterior geniculate bodies attain such large proportions that in a 

 profile view they would appear to be almost half the size of the optic thalamus (fig. 28). 



In Chlamydophorm these changes become much more pronounced. The anterior 

 quadrigeminal bodies become extremely reduced and the region of the cori^ora 

 quadrigemina converted into a transverse vertical plate which is interposed between 



SECOND SERIES. — ZOOLOGY, VOL. VII. 49 



