(1) The results of the rupture of a cerebral vessel--_e.g._, from asphyxia at birth or a delivery by forceps. Blood has been poured out into the nervous tissue. The latter has been destroyed over a greater or less extent, and there is found in its place a cyst filled with sero-sanguineous fluid. (2) The obstruction of an artery--_e.g._, by septic thrombosis--has prevented the blood from reaching a part of the brain, with similar results to those mentioned. (3) In other cases are found the more or less extensive changes produced by meningitis or meningo-encephalitis. The inflammation of its envelopes has interfered with the brain, and consequently with its functions. (4) An increased secretion of cerebro-spinal fluid has led to a compression of the nervous system or a distension of its cavities, notably of the lateral ventricles of the cerebral hemispheres, and has led to a separation of the bones of the cranium, thus producing the large globular head of hydrocephalus. (5) There may be found simply defects of development whose causes are known (microcephalus, or extreme smallness of the cranium relatively to the face; microgyria, or marked thinness of the convolutions). (6) Lastly--and this is frequently the case in the worst degrees of deficiency--the post-mortem, and even microscopic examination of the organs may show no change at all. Let us add that the nature of the lesions just mentioned does not seem to have any relationship to the condition of the mental faculties. An anatomico-pathological grouping of the cases and a grouping according to the mental condition, far from being parallel, are frequently decidedly different. On the other hand, the extent of the lesions is of more importance. Diffuse lesions affect the mind more than those which are circumscribed--that is to say, limited to a certain part of the brain--as if the mental functions required the co-operation of the entire cerebral cortex. One will often find, for example, sound judgment in the subject of a marked paralysis, whilst it is very rare to find that good intelligence co-exists with any degree of microcephalus. Let us emphasise the last fact we mentioned, the absence of any lesion. Some authorities have maintained that all conditions of mental deficiency and want of balance found in children are connected with definite diseases of which they are the symptoms. The question is unsettled. For our own part we adopt the following provisional statement: Mental deficiency and want of balance are peculiar mental conditions which it is often impossible to connect with definite pathological changes. Thus, we do not know of any medical treatment which is likely to act upon the preceding lesions when they are present, and we do not think it is even possible to act upon them.