162 THE CEREBRAL HEMISPHERES. 



ON THE CAUSATION OF THE GYRI AND SULCI OF THE BRAIN. 



Various explanations have been offered to account for the convoluted form of the cortex. 

 These are based partly on mechanical considerations, e.g., the resistance offered by the larger 

 blood-vessels and undue growth of the brain as compared with that of its enclosing skull- 

 capsule, partly on physiological or physiologico-mechanical considerations, which suppose 

 that there is a relative increase of functional activity of certain parts as compared with others, 

 resulting in an increased growth of those parts, and hence their projection in the form of gyri. 

 These theories, however, are quite insufficient to account for the convoluted formation, since 

 it can be shown (1) that the blood-vessels for the most part do not correspond with the fissures, 

 nor do they lie in the depth of the fissures, when they happen to coincide with them ; (2) 

 that the brain does not fill the skull at the time the permanent fissures make their appearance ; 

 (3) that in the animal series there is no direct relationship between intellectual development 

 and cerebral convolution. Further, it may be stated that no theory which will not also 

 account for the fissures and lamina? of the cerebellum as well as the convolutions of the 

 cerebrum can be regarded as satisfactory. 



It has. however, lately been pointed out by Jelgersma that a simple mathematical 

 explanation exists for the existence of a convoluted surface. The one feature which mammals 

 with smooth brains have in common is smallness of body ; whereas those mammals which 

 possess convoluted brains are invariably found to be of relatively large size (Dareste). In 

 other words, small mammals have smooth brains, however high they may be on the animal 

 ladder, and in spite of their possessing a high degree of intelligence (e.g., certain monkeys), 

 whilst large mammals have convoluted brains, although their intellectual development may 

 be relatively low (e./;., cetaceans). " The grey cortex of the brain, which in members of the 

 same species maintains a tolerably constant thickness, increases by surface extension. 

 Further, with every advance in the growth of the grey matter, there must be a proportionate 

 increase of the subjacent white matter. The geometrical law involved is simply this that 

 in the growth of a body the surface increases with the second, but the interior with the third 

 power of the radius. From this it is evident, seeing that the proportion of internal white 

 matter and external grey matter is in all cases a uniform one. that in the evolution of a large 

 animal out of a small animal a disproportion between the grey capsule and the white core of 

 the cerebrum must result. This is compensated for by the extended cortex placing itself in 

 folds or puckers. Jelgersma further points out that the extent of the cerebral surface 

 depends upon two factors, namely, (1) the absolute quantity of the grey matter, and (2) the 

 thickness with which this is spread over the surface. The absolute quantity of grey matter 

 present is determined by the bulk, or by the psychical endowments of the animal, or by both 

 of these factors together. On the other hand, although the thickness of the grey cortex is 

 very much the same in the same species, it differs considerably in different animal groups ; 

 and it follows from the theory which he has advanced that the more sparsely the grey 

 substance is spread over the surface of the white matter, the richer will be the convolution 

 type. In the cetacean cerebrum the grey cortex is exceedingly thin, and it is due to this that 

 the surface shows such an extreme condition of complexity." ' 



Cases in which there is a congenital absence of the corpus callosum are characterized by a 

 peculiar type of convoluted surface, the fissures and convolutions showing a strong tendency 

 to radiate from the Sylvian fossa. This is partly due, according to Cunningham, to a retention 

 of certain of the primitive fissures which appear about the third or fourth month, are due to 

 unfoldings of the whole thickness of the cerebral wall, and are mostly quite transitory ; but 

 they cannot all be thus accounted for. There is often in these cases an intricacy of pattern 

 displayed which is comparable to that of the cetacean hemisphere, and may be very possibly 

 produced in a similar manner. 



The same law by which the formation of the cerebral convolutions is accounted for, 

 likewise explains the development of the cerebellar folia, and of the wavy outline of the 

 corpora dentata of the cerebellum and olives. 



Jelgersma's theory is not complete in so far that it does not explain why the convolutions 

 should tend to assume certain patterns in certain groups. It is not improbable that these 

 differences may be determined by variations in the relative functional importance of 

 different parts, producing a corresponding variation in the extent of grey matter which has 

 to be provided for, and relative increase of this can only be obtained by local puckering 

 (Cunningham). There is no doubt, however, that the formation of the various cerebral 

 patterns, fairly constant for the same species, is scarcely susceptible of any very simple 

 explanation, and that for the present we must rest satisfied with the statement of the fact. 



1 D. J. Cunningham, Address delivered at the opening of the section of Anatomy and Physiology at 

 the annual meeting of the British Medical Association, 1890. 



