MENTAL DEVELOPMENT 299 



consciousness occurs if the blood supply fails for a few seconds, 

 hence we understand why the superficial cortex of the brain is 

 pinkish and receives so abundant a supply of blood. 



Now if we look at a child's brain before birth at an early 

 period, the surface is quite smooth and there is no internal white 

 matter. As the embryo grows, primitive folds and fissures 

 appear, and a month or so before birth we have a brain 

 characteristic of the species ; at birth we have the brain of 

 the individual ; the convolutional pattern formed by the folds 

 and fissures (as with the physiognomy) may bear a resem- 

 blance to other individuals, but will exhibit features which 

 differ from other individuals (Plate IV, figs. I, 2). No two 

 patterns are identically similar any more than two faces are 

 identical ; but just as the faces of relatives are likely to be 

 similar, so Karplus showed that the pattern of the brains of 

 infants who were related exhibited similarities; and Dr. Edgar 

 Schuster, at my suggestion and from material with which I 

 provided him, has carefully investigated and recorded the 

 similarities in the brains of adult relatives. 



Now we may ask : Why should the brain exhibit these folds 

 and fissures ? The blood vessels which supply the brain lie in 

 the fissures and are thereby protected from pressure ; but probably 

 economy of space determines the balance between the dynamic 

 forces which determine the growth of the skull and the growth 

 of the brain, and by throwing into folds the grey matter, its 

 area is increased enormously without increasing the size of the 

 head. A very small head means a small brain and mental 

 deficiency, but the simpler the convolutional pattern (that is, the 

 fewer the folds and fissures) the less will be the extent of the 

 grey matter and consequently the fewer the number of neurones. 

 It is not surprising, therefore, to find that not only are the brains 

 of idiots and imbeciles deficient in the relative proportion by 

 weight of the cerebral hemispheres to the rest of the brain, but 

 the convolutional pattern is simple, consequently the superficial 

 area of cortex is diminished (vide Plate IV, figs. 3, 4). The degree 

 of amentia or congenital absence of mind is proportional to the 

 failure of superficial extent of the grey matter of the cortex — 

 the anatomical basis of mind. Savage man has a superficial 

 area three times that of the gorilla, but a microcephalic idiot's 

 brain weighed onlyeight ounces (Plate IV, fig. 3). Not infrequently 

 an idiot or imbecile has a large head caused by distension of the 



