October i. 1903] 



NATURE 



541 



of the outer surface of the cranium to its inner surface 

 and to the brain. This question has engaged the attention 

 of experts as well as the " man in the street " since the 

 time of Gall and Spurzheim, and one might naturally sup- 

 pose that the last word had been said on the subject. This, 

 however, is far from being the case. All anatomists are 

 agreed that the essential function of the cranium is to form 

 a box for the support and protection of the brain, and it is 

 generally conceded that during the processes of development 

 and growth the form of the cranium is modified in response 

 to the stimulus transmitted to it by the brain. In fact it 

 is brain growth that determines the form of the cranium, 

 and not the skull that moulds the brain into shape. This 

 belief, however, need not be accepted without some reserv- 

 ations. Even the brain may be conceived as being in- 

 fluenced by its immediate environment. There are probably 

 periods of development when the form of the brain is 

 modified by the resistance offered by its coverings, and there 

 are certainly stages when the brain does not fully occupy 

 th" cranial cavity. 



At an early period in the phylogeny of the vertebrate skull 

 the structure of the greater part of the cranial wall changes 

 from membranous tissue into cartilage, the portion persist- 

 ing as membrane being situated near the median dorsal 

 line. In the higher vertebrates the rapid and early ex- 

 pansion of the dorsal part of the fore-brain is so marked 

 that the cartilaginous growth fails to keep pace with it, 

 and more and more of the dorsal wall of the cranium re- 

 mains membranous, and subsequently ossifies to form 

 membrane bones. Cartilage, though constituting a firmer 

 support to the brain than membrane, does not possess the 

 same capacity of rapid growth and expansion. The head 

 of a young child is relatively large, and its skull is dis- 

 tinguished from that of an adult by the small size of the 

 cartilaginous base of the cranium as compared with the 

 membranous vault. The appearance of top-heaviness in the 

 young skull is gradually obliterated as age advances by the 

 cartilage continuing slowly to grow after the vault has 

 practically ceased to enlarge. These changes in the shape 

 of the cranium are associated with corresponding alter- 

 ations in that of the brain, and it appears to me that we 

 have here an illustration of how the conditions of skull 

 growth may modify the general form of the brain. 



Whatever may be the precise influences that determine 

 skull and brain growth, there can be no doubt but that 

 within certain limits the external form of the cranium serves 

 as a trustworthy guide to the shape of the brain. State- 

 ments such as those by Dr. J. Deniker (" The Races of 

 Man," p. 53) " that the inequalities of the external table 

 of the cranial walls have no relation whatever with the 

 irregularities of the inner table, and still less have any- 

 thing in common with the configuration of the various parts 

 of the brain," are of too general and sweeping a character. 

 Indeed, various observers have drawn attention to the fact 

 that in certain regions the outer surface of the skull 

 possesses elevations and depressions which closely corre- 

 spond to definite fissures and convolutions of the brain. 

 Many years ago Sir William Turner, who was a pioneer 

 in cranio-cerebral topography, found that the prominence 

 on the outer surface of the parietal bone, known to 

 anatomists as the parietal eminence, was situated directly 

 superficial to a convolution of the parietal lobe of the brain, 

 which he consequently very appropriately named " the con- 

 volution of the parietal eminence." Quite recently Prof. 

 G. Schwalbe has shown that the position of the third or 

 inferior frontal convolution is indicated by a prominence 

 on the surface of the cranium in the anterior part of the 

 temple. This area of the brain is of special interest to all 

 students of cerebral anatomy and physiology, since it was 

 the discovery by the illustrious French anthropologist and 

 physician, M. Broca, that the left inferior frontal convolu- 

 tion was the centre for speech, that laid the scientific 

 foundation of our present knowledge of localisation of func- 

 tion in the cerebral cortex. This convolution is well known 

 to be much more highly developed in man than in the 

 anthropoid apes, and the presence of a human cranial 

 speech-bump is usually easily demonstrated. The faculty 

 of speech, however, is such a complicated cerebral function 

 that I would warn the " new " phrenologist to be cautious 

 in estimating the loquacity of his friends by the degree of 

 prominence of this part of the skull, more particularly as 



NO. 1770, VOL. 68] 



there are other and more trustworthy methods of observ- 

 ation by which he can estimate this capacity. 



In addition to the prominences on the outer surface of 

 th^ cranium, corresponding to the convolutions of the 

 parietal eminence and the left inferior frontal convolution, 

 the majority of skulls possess a shallow groove marking the 

 position of the Sylvian point and the course of the hori- 

 zontal limb of the Sylvian fissure. Below these two other 

 shallow oblique grooves indicate the line of the cerebral 

 fissures which divide the outer surface of the temporal lobe 

 into its three convolutions, termed superior, middle, and 

 inferior. Most of these cranial surface markings are 

 partially obscured in the living body by the temporal muscle, 

 but they are of interest as showing that in certain places 

 there is a close correspondence in form between the external 

 surface of the brain and that . of the skull. There are, 

 however, distinct limitations in the degree to which the 

 various cerebral fissures and convolutions impress the inner 

 surface of the cranial wall, or are represented by inequalities 

 on its outer aspect. Thus over the vault of the cranium 

 the position of the fissure of Rolando and the shape of the 

 cerebral convolutions in the so-called motor area, which 

 lie in relation to this fissure, cannot usually be detected 

 from a cast of the cranial cavity, and are not indicated by 

 depressions or elevations on the surface of the skull, so 

 that the surgeons in planning the seats of operations neces- 

 sary to expose the various motor centres have to rely mainly 

 upon certain linear and angular measurements made from 

 points frequently remote from these centres. 



The cranium is not merely a box developed for the support 

 and protection of the brain, and more or less accurately 

 moulded in conformity with the growth of this organ. Its 

 antero-lateral portions afford attachments to the muscles 

 of mastication and support the jaws and teeth, while its 

 posterior part is liable to vary according to the degree of 

 development of the muscles of the nape of the neck. Next 

 to the brain the most important factor in determining 

 cranial form is the condition of the organs of mastication 

 — muscles, jaws, and teeth. There is strong evidence in 

 favour of the view that the evolution of man from micro- 

 cephaly to macrocephaly has been associated with the 

 passage from a macrodontic to a microdontic condition. 

 The modifications in the form of the cranium due to the 

 influence of the organs of mastication have been exerted 

 almost entirely upon its external table ; hence external 

 measurements of the cranium, as guides to the shape of 

 the cranial cavity and indications of brain development, 

 while fairly trustworthy in the higher races, become less 

 and less so as we examine the skulls of the lower races, of 

 prehistoric man, and of the anthropoid apes. 



One of the most important measurements of the cranium 

 is that which determines the relation between its length 

 and breadth and thus divides skulls into long or short, 

 together with an intermediate group neither distinctly 

 dolichocephalic nor brachycephalic. These measurements 

 are expressed by an index in which the length is taken as 

 100. If the proportion of breadth to length is eighty or' 

 upwards, the skull is brachycephalic ; if between seventy- 

 five and eighty, mesaticephalic ; and below seventy-five, 

 dolichocephalic. Such a measurement is not so simple a. 

 matter as it might appear at first sight, and craniologists 

 may themselves be classified into groups according as they 

 have selected the nasion, or depression at the root of the 

 nose, the glabella, or prominence above this depression, . 

 and the ophryon, a spot just above this prominence, as the 

 anterior point from which to measure the length. In a 

 young child this measurement would practically be the same 

 whichever of these three points was chosen, and each point 

 would be about the same distance from the brain. With 

 the appearance of the teeth of the second dentition and the 

 enlargement of the jaws the frontal bone in the region of 

 the eyebrows and just above the root of the nose thickens, 

 and its outer table bulges forwards so that it is now no 

 longer parallel with the inner table. Between these tables 

 air cavities gradually extend from the nose, forming the 

 frontal sinuses. Although the existence and significance 

 of these spaces and their influence on the prominence of 

 the eyebrows were the subject of a fierce controversy more 

 than half a century ago between the phrenologists and their 

 opponents, it is only recently that their variations have been 

 carefully investigated. 



