It should be noted that enlargement of the head can only take place during the years previous to synostosis of the skull bones. Leonard Guthrie (Harveian Lecture, March 17, 1910) writes, ‘I cannot find from any recorded cases of hydrocephalus acquired in later childhood and adult life that an increase in the size of the head has been any aid to diagnosis, and I believe it is true that internal hydrocephalus acquired after the sutures are set is hardly distinguishable from a non-localizable intracranial new growth giving rise to headache, vomiting, and optic neuritis.’ [15] The treatment for acquired hydrocephalus dependent on tumour formation is discussed elsewhere. This section deals with the congenital variety and with those cases of acquired hydrocephalus not due to obstruction by tumours. [16] _Review of Neurology and Psychiatry_, vol. ix, No. 1, p. 1. CHAPTER IV FRACTURES OF THE SKULL =General considerations.= Fractures of the skull do not form more than one-twentieth part of the fractures admitted annually into the hospitals, but, in spite of this relative infrequency of occurrence, the difficulties attendant on diagnosis, the numerous associated complications, and the all-important question of treatment, invest this subject with a special interest. The whole question of skull fractures is beset with difficulties, many of which, it is hoped, will be swept away in this and subsequent chapters. Brief allusion must first be made to some important points in connexion with the anatomical structure of the skull, such as bear relation to fractures and aid in the appreciation of the extent and mechanism of the fracture. The =vault= varies in density to a remarkable degree, not only in its several parts, but also in different individuals. Cases have now and again been recorded in which a very trivial blow, totally insufficient to produce any definite osseous lesion in the normal individual, has resulted in the production of a vault or basic fracture. Each case, therefore, must be judged on its own merits. The vault derives its strength from its shape and structure. The two tables are of equal strength, and, for the most part, separated from one another by a variable amount of diploic tissue. This diploe is most abundant in the frontal, parietal, and upper occipital regions. These parts are proportionately strong. Two regions are practically devoid of this inter-tabular buffer--the squamo-temporal and cerebellar (see Figs. 29 and 30). A recognition of this comparative weakness is of great practical importance in view of the fact that both these regions are liable to special lesions--injury to the middle meningeal artery in the first case, and, in the second, cerebello-medullary lesions. Nature’s ‘mistake’ in providing coverings unsuited to requirements has been compensated for in part by additional protection--the temporal and nuchal muscles.