306 SCIENCE PROGRESS 



owe their mental deficiency to inborn germinal defect. Mentally 

 defective children of this type may be born to normal parents, 

 but the chances of such occurring are extraordinarily less than 

 if a parent is feeble-minded, epileptic, or insane, or exhibits 

 other signs of the neuropathic inheritance. (2) Mental deficiency 

 from other causes occurs in 25 per cent, of the cases, and this 

 includes pre-natal, natal, and early post-natal conditions. The 

 pre-natal conditions are those associated with disease of the 

 mother especially from such poisons as syphilis (giving rise to 

 congenital syphilis), lead and alcohol, injuries, falls and de- 

 pressing conditions by which the developing offspring is 

 imperfectly nourished, and absence of the thyroid gland, which 

 gives rise to myxoedematous cretinism. Natal or post-natal 

 causes are difficult labour, fevers and poisoning in early infancy, 

 which cause arrest of the development of the brain cortex ; its 

 damage may also be occasioned by rupture of blood vessels and 

 tumours. It is extraordinary how well the brain is protected 

 from injury and failing nutrition of the body. In starvation all 

 the tissues of the body waste away, yet the brain loses hardly 

 any weight at all. Donaldson at the Wistar Institute has clearly 

 shown by a large number of experiments on white rats that the 

 growth of the brain is hardly at all impaired by insufficient food. 

 He took litters of white rats and divided them into two groups; 

 one group he fed well, the other insufficiently. Although there was 

 a great difference in the weight of the bodies of the two groups, 

 the brains showed hardly any appreciable difference ; proving 

 that all the tissues of the body may suffer in order that the brain 

 may grow. This shows that the neurones have normally a great 

 inborn specific energy, as they should have, for they are perpetual 

 cellsof thegreatestimportance forthe preservation of the common- 

 weal of the social organism of the body. All the neurones are 

 present at birth with all their latent potentialities; some are fully 

 developed ; the majority, especially the neurones of the grey 

 matter of the surface of the brain, are in their infancy ; those 

 which in the process of evolution have been the latest to appear 

 — the association neurones — will be the latest to complete their 

 growth by extension of their processes. I have said these cells 

 are perpetual cells ; by this I mean that in a healthy brain they 

 are endowed with a durability to function during the life of the 

 individual. Unlike the cells of the body generally, neurones 

 destroyed cannot be replaced. They are the master-cell- 



