104 LECTURE IV. 



the cranium presents only indistinct impressions of tlie 

 larger convolutions. The coarser the convolutions^ and the 

 broader the furrows which separate them^ the more distinct is 

 their impress on the internal surface of the cranium. The cast 

 of a skull by means of a plastic mass which retains its form, is 

 but an imperfect substitute for a view of the brain itself and 

 its convolutions. 



The convolutions contribute to increase the quantity of grey 

 substance. Just as in the secreting glands the secreting sur- 

 face is increased in size by the subdivision of the originally 

 simple bag into tubes, so the cerebral substance, by its com- 

 plicated windings, secures a surface considerably exceeding 

 that of the internal space of the skull. Now, if it be true 

 that the grey substance alone is the source of nervous action ; 

 if it be farther true that the superficial grey matter is inti- 

 mately connected with mental activity, whilst the internal grey 

 nuclei are rather connected with the phenomena of sensation ; 

 then it follows that the multiplicity of the convolutions is con- 

 nected with the development and increase of the intellectual ca- 

 pacity, the substratum of which is the increased quantity of 

 grey matter. The convolutions have been compared to the 

 figure which would be produced by forcing a bag, possessing a 

 larger surface than the interior of the skull, into the cranium. 

 The comparison may be pursued, and it may be said that the 

 more grey substance we force into a skull the greater the intellec- 

 tual capacity, which would lead to the inference that an animal 

 must be more intelligent in comparison with another in proportion 

 as the convolutions are more complicated, and the furrows deeper. 



If this principle is adopted in its crudity, a single glance at the 

 convolutions in the brain of the mammalian series will be sufficient 

 to overthrow it. It is true that in some of the lower mammals, 

 e.g., in the edentata and marsupials,no convolutions are observed, 

 whilst, with few exceptions, they exist in all carnivorous ani- 

 mals and largely in the apes. But, on closer examination, we 

 find that within the orders which possess convolutions their 

 development seems to be connected with the size of the body. 

 Now it certainly cannot be maintained that all larger animals 

 are more intelligent than the smaller, and when it is considered 



