CHAP.X.] CONVOLUTIONS OF THE BRAIN. 283 



difference is also observable in these respects in the human 

 brain. The convolutions of the female brain are not so deep 

 nor so thick as those of the male. Age, too, causes a marked 

 difference. The convolutions of the child just born, besides being 

 much more simple, and having fewer undulations, are less deep 

 and less thick than those of the adult ; and in old age, when 

 the brain has shrunk, the mental faculties being less vigorous 

 and active, the convolutions have become much smaller in every 

 dimension, and water is apt to accumulate in the intergyral 

 spaces. 



In man, the convolutions of the right and left hemispheres do 

 not present a perfect symmetry. It is important, however, to 

 notice, that careful examination will invariably display the same 

 essential convolutions on each side, although they present such 

 striking differences in detail that it is at times difficult to recog- 

 nize the likeness; and it is not a little remarkable, that, in 

 general, the lower the development of a brain, the more exact 

 will be the symmetry of its convolutions. Thus the brains 

 of all the inferior mammalia, even of those which make the 

 nearest approach to man, are exactly symmetrical. The imper- 

 fectly developed brain of the child exhibits a similar symmetry ; 

 and that of the inferior races of mankind, in whom the neglect 

 of mental culture, and habits approaching those of the brute, are 

 opposed to the growth of the brain, also presents a symmetrical 

 disposition of the convolutions. 



A convolution consists of a fold of the gray, or vesicular matter, 

 enclosing a process of the fibrous. The gray matter of neighbouring 

 convolutions is obviously continuous throughout at the bottoms of 

 the sulci, so that it forms one unbroken although undulating sheet 

 over the whole convoluted surface of the brain. That portion 

 of the gray layer which is in contact with the pia mater is purely 

 vesicular, i. e. unmixed with nerve-tubes, with the exception of a 

 few stray ones on the surface ; but blood-vessels penetrate it in 

 very great numbers. The more deeply seated portion, however, 

 contains very numerous tubular fibres, which become larger as 

 they approach the white matter. It is very plain, that a large 

 proportion of the constituent fibres of the white matter of the 

 convolutions penetrate the gray matter : these appear to enter it 

 more or less at right angles to that portion of the gray surface 

 with which they are more immediately in relation ; and, on the 

 other hand, they converge inwards towards the central parts of 

 the brain, the corpora striata and optic thalami. A large pro- 



