154 BRAIN OF THE OPOSSUM. 



are divided into two oval masses, become much thicker below, and as they approach the 

 crura, the lowest part projecting forwards. The 7iates are much smaller, are also divided 

 into two lobes, have a flattened oval shape, and rest upon the more advanced portion of the 

 testes to which they seem only appendages. 



The optic thalami, nates and testes might be described superficially as a single mass of 

 cerebral substance divided by grooves into three pairs of lobes, the groove between the 

 nates becoming suddenly deeper at its foremost point so that its continuation separates the 

 optic thalami from each other, and thus forms the third ventricle. On each side of the en- 

 trance to the third ventricle is a raised lip on which the peduncles of the pineal body rest. 

 The connections of the thalami with the fornix have already been described. 



The passage from the third to the fourth ventricle becomes suddenly enlarged, under the 

 optic lobes, and if a transverse section be made through these it assumes the dimensions of 

 a ventricle under the testes. 



The optic tracts reaching the sides of the optic thalami, spread out in a fan-shaped man- 

 ner covering the whole surface. A few fibres were traced into the nates and into a small, 

 flattened, circular body, which occupies the place of a corpus geniculatum. 



The Cerebellum consists of a middle lobe which, seen from behind, has nine convolutions, 

 some of which do not extend beyond its borders ; of two lateral lobes, each with six or 

 seven convolutions, the upper and lower of which extend across the median line ; and of 

 the slender appendages on each lateral lobe usually seen in Marsupials, each of which has 

 three or four minute convolutions. The middle lobe forms a continuous ridge around nearly 

 the whole circumference of the cerebellum, being interrupted only on its hinder edge over 

 the fourth ventricle. Besides the sulci seen on the general surface, there are two somewhat 

 remarkable ones, which can only be seen by examining the anterior or that face of the organ 

 which is in contact with the vertical portion of the testes. One of these detaches the mid- 

 dle lobe in front, as far as the base of the cerebellum, from the posterior, middle and 

 lateral lobes ; the entrance to it is from near the top of the cerebellum, and its direction, as 

 seen in a longitudinal section, is from above downwards and backwards. A second and 

 smaller one is seen below this and has a direction parallel to it. 



