156 PHYSIOLOGICAL SERIES. 



the narrow mesial slit-like third ventricle. At its caudal 

 cxtivmitv a blunt rounded sac is found representing the 

 pineal hotly, and in front of this the ganglion habenulae 

 and tiunia thalami are found on each side of the ventricle, as 

 in all other vertebrate brains. Perhaps the most significant 

 feature of this specimen is the absence of any protuberance 

 corresponding to the mesial geniculate body. In this 

 respect the Monotreme brain differs most markedly from 

 that of all other mammals. The absence of any markedly 

 projecting lateral geniculate body is not so peculiar, not 

 only because this body is not prominent among the lowlier 

 mammals, but also because the visual apparatus, of which 

 it forms an important part, is poorly developed in the 

 Monotremes. 



The quadrigeminal bodies are comparatively small and 

 flat as coinparedwith those of other mammals. 



The pons Varolii is prolonged into a forwardly-projecting 

 process or rostrum in the mesial plane, probably because 

 the nuclei pontis are scattered throughout a much greater 

 antero-posterior extent than the narrow lateral parts of the 

 pons occupy. This peculiar rostrum is distinctive of the 

 Monotremes, since it is also found in the Platypus. Im- 

 mediately in front of the rostrum note the interpeduncular 

 body, one of the oldest parts of the brain in the phylogenctic 

 sense. The rounded knob formed by the corpora mam- 

 millaria, the oculo-motor nerves, and the delicate optic 

 tracts conform to the usual mammalian type. 



In comparison with the corresponding organ in other 

 mammals, the cerebellum in the Monotremes presents 

 features so peculiar that no exact comparisons with that of 

 other mammals can be instituted with any degn 

 certainty. The fissura prima (prseclivalis of Human 

 Anatomy) is apparently placed very far back, so that the 

 greater part of the cerebellum, including the whole of tin- 

 anterior and the greater part of the dorsal surface, corre- 

 sponds to the anterior lobe of other mammals (the combined 

 lobus centralis and lobus culminis of Human Anatomy). 

 Thus the whole of the postclival region, which in the higher 

 mammals becomes so greatly expanded that it forms the I MI Ik 

 of the organ, becomes relegated to the caudo-vontral region 



