A<!(f<'it<fttiii 283 



A full histological examination lias revealed several points of more than passing interest. Thus 

 it is important to have again recognised in all these lower animals that strip of cortex imme- 

 diately investing the genu corporis callosi (Limbic A), possessed of special structural features, to 

 wit, the deep layer of peculiar chromophilous cells, so strongly reminiscent of what one saw in 

 the brain of man. We likewise again meet with the second variation, that in the postsplenial 

 region (Limbic B). In regard to both these sub-areas, I think it correct to say that the cortical 

 constituents present a better aspect of development in the lower animal than in Homo, and, 

 further, there seems to be no doubt that they are better represented in Sus than in either Canis 

 or Felis. 



In the foregoing work I inclined to the belief that there was more truth than is generally 

 supposed, in the suggestion credited to Broca, that the gyrus fornicatus may play some riila in 

 association with the olfactory sense, and going more into detail, I gave it as rny opinion that, 

 at any rate, the pregenual strip of cortex probably stood in direct connection with the inner 

 olfactory root. I think it will now be conceded, that to have proved the better development of the 

 same cortex in the macrosniatic brain is a point in favour of this belief. 



In reference to the remaining limbic cortex, as in Homo, it is readily recognised by its relative 

 poverty in nerve fibres and its correspondingly weak nerve cell representation, features which lead 

 to its easy differentiation from neighbouring areas. 



On the homologies of fissures traversing or bounding the limbic area there is little to write. 

 'When discussing the intercalary sulcus I ventured the opinion that it was a fissure which had 

 undergone retrograde changes in the course of cerebral growth, and I opposed Dr Elliot Smith's 

 statement, that it develops into the calloso-marginal. The genual fissure, rudimentary and 

 inconstant as it is in the brains of lower animals, is the one, as I have said, which I believe 

 to be the antecedent of the calloso-marginal ; while as regards the remaining sulcus on the mesial 

 surface, the rostral, it may either continue as such, or become the anterior portion of the calloso- 

 rnargiual, the so-called prelimbic sulcus. 



In the brain of Sus a small and very shallow sulcus may be observed, limiting the sub-area of 

 cortex marked Limbic A in the diagram ; this is known as the sub-limbic sulcus of Guldberg, or 

 the sub-singular arc, it seems to be specially characteristic of the Ungulate group, and in some 

 families forms a complete arc. The sulcus is not well seen in Carnivores, and it is doubtful whether 

 its remains can be identified in Homo. 



Finally, as the striae longitudinales mediates are not very much larger in these animals than in 

 Man, it is plain that they must have had their supposed existence as gyri very far back in the phylo- 



genetic scale. 



POSTCKUCIAL OR SENSORY AREA. 



As an addition to the histological homologies already detailed, I have now to mention some 

 results gained from a study of secondary degeneration, which, to my mind, further strengthen the 

 contention that the area, which I have called " Postcrucial or Sensory," is the equivalent of the 

 " Postcentral or Sensory " area in Primates. The results to which I refer were obtained by 

 Tschermak, from experiments on cats. The research consisted of tracing, by the method of Marchi, 

 the secondary degeneration following destruction of the nuclei gracilis et cuneatus, at the lower 

 end of the medulla. Since, for us, the chief interest in Tschermak's work centres on the fact that 

 he succeeded where others failed in following degenerated fibres right up to the cortex cerebri, 

 we can pass over his detailed and thorough account of the degeneration seen at lower levels, 

 and confine ourselves to the cerebral findings. He states that a number of diseased fibres enter 

 the corona radiata, principally vid the internal capsule, and, streaming outwards, impinge mainly 

 on the cortex of the gyru.s coronalis and adjacent parts of the gyrus ectosylvius (pars anterior), 



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