152 PHYSIOLOGICAL SERIES. 



[In this specimen the olfactory bulb has been removed, :ml 

 the irregular cut surface of its peduncle can be seen upon 

 the ventral surface of the anterior pole of the hemisphere.] 



At the concave margin of the postcomnnssural part of 

 the fascia dentata there is a strand of white fibres the 

 fimbria or fornix. This consists of a group of fibres 

 collected from or going to the hippocampus. In all 

 Marsupials and placental mammals the fimbria projects as 

 a prominent crest ; but in both Monotremes it consists 

 merely of a slight thickening of the edge of the alveus, 

 which does not project to form a ridge. Anteriorly 

 its fibres appear in this specimen to pass bodily into 

 the dorsal commissure, but many of them do not do 

 so. A considerable proportion of the fibres of the 

 fornix bend downward behind the ventral commissure as a 

 compact bundle (the anterior pillar or column of the 

 fornix), which enters the optic thalamus and proceeds 

 toward the corpus mammillare : others again enter the 

 precommissural area, and are known as the precommissural 

 fibres (of Huxley). These fibres can be satisfactorily seen 

 only in fresh specimens or in histological preparations 

 (compare Journ. of Anat. & Phys. vol. xxxii. fi^. 6, p. 36). 



In this specimen the fissures and sulci may be studied 

 with advantage. 



The rhinal fissure is extraordinarily deep in the Mono- 

 tremes, and especially in Tachyglossus, as the coronal section 

 (fig. 42) shows. .It pursues a very tortuous course on the 

 lateral, ventral, and caudo-mesial surfaces of the hemisphere. 

 Just before it leaves the lateral to reach the basal surface 

 of the hemisphere, it gives off a short horizontal branch as 

 deep as itself. This branch is probably produced by factors 

 analogous to those which cause the so-called u Sylvian 

 fissure" in other mammals ; it is, in other words, a kink 

 produced by the downward growth of the neopallium 

 behind it, and, as such, has a claim to be called the 

 " Sylvian fissure" equal to that of, say, the Cat's brain. 

 At the same time there is no reason for regarding it as 

 a strict homologue of the similarly-named fissure of other 

 mammals. The same term is used simply as a matter of con- 

 venience in the same way that if i- applied to the Edentate, 



