average thickness for any locality. But beyond the fact that the cortex is somewhat thinner near the hemisphere-poles, and especially the occipital pole, than in the intermediate parts, no definite statement regarding the relative thickness of different parts can at present be made. Females have a very slightly less thickness of cortex than males (less than 1 per cent.), and the right hemisphere less than the left: the difference may amount to 7 per cent. (Donaldson). Weight. The results obtained by Sims, Clendinning, Tiedemann and J. Reid showed the maximum weight of the adult male brain, in a series of 278 cases, to be about 1810 grammes (64 oz.), and the minimum weight about 960 grammes (34 oz.). In a series of 191 cases, the maximum weight of the adult female brain was 1585 grammes (56 oz.), and the minimum 880 grammes (31 oz.). In a very large proportion the weight of the male brain ranges between 46 oz. and 53 oz., and that of the female brain between 40 oz. and 47 oz. Similar statistics have been published by Peacock, R. Wagner, Bischoff, Huschke, Boyd, Weisbach and others. The mean weight at from 20 to 40 years of age was found by Boyd to be 48 oz. (1360 grammes) for the male, and 43 oz. (1230 grammes) for the female brain. Although many female brains exceed in weight particular male brains, as a general fact it may therefore be affirmed that the adult male encephalon is on an average heavier by 4 oz. or 5 oz. than that of the female (or about 9 per cent.). The appended table, which has been compiled from the observations of R. Boyd (Phil. Trans. 1860), shows in grammes the mean weights at different ages in the It would appear from the above that the brain is absolutely heavier between 14 and 20 years of age than at any other period of life, and that at the age of 80 it has lost about 90 grammes, or rather more than 3 ozs., i.e., about of its whole weight. The figures obtained by Broca are somewhat higher than these, e.g., between the ages of 30 and 35, in the male, an average of 1421 grammes (50 oz.); in the female, 1269 grammes (45 oz.). According to the same statistics, the weight of the brain attains its maximum, not before the age of 20, as found by Boyd, but between 25 and 35 in the male and a little earlier in the female. This agrees with the results of Peacock. The two hemispheres of the same brain, although hardly ever of exactly the same weight, show no constant difference, the one half preponderating just about as often as the other, and the average difference being only about 5 grammes (Braune). There is no evidence that the right hemisphere is the heavier in left-handed people. It has frequently been found that the brains of distinguished men have a brain-weight above the average, sometimes markedly so, but the rule has many exceptions. The converse is by no means true. The relative weight of the encephalon to the body is liable to great variation; nevertheless, the facts to be gathered from the observations of Clendinning, Tiedemann, and Reid, furnish the following general result. In a series of 81 males, the average proportion between the weight of brain and that of the body at the ages of twenty years and upwards, was found to be as 1 to 365; and in a series of 82 females, to be as 1 to 36.46. The results of Bischoff's observations give 1 to 35.2 in the female. In these cases the deaths were the result of more or less prolonged disease; but in six healthy individuals dying suddenly from disease or accident, the average proportion was 1 to 41. The proportionate weight of the brain to that of the body is much greater at birth than at any other period of extra-uterine life, being, according to Tiedemann, about 1 to 5.85 in the male, and about 1 to 6.5 in the female. From various observations, it further appears that the proportion diminishes gradually up to the tenth year, being then about 1 to 14. From the tenth to the twentieth year the relative increase of the body is most striking, the ratio of the two being at the end of that period about 1 to 30. After the twentieth year the general average of 1 to 36.5 prevails, with a further trifling decrease in advanced life. Influence of stature on brain weight.-According to J. Marshall, the proportion of entire brain (in ozs.) to each inch of stature, is for the male sex 0.708; in the female 0.688. This relative preponderance in the male is due entirely to preponderance of cerebral development; the average stature-ratio for cerebrum alone being 0.0619 oz. in the male per inch of stature, and 0.599 oz. in the female, whils the stature-ratios of cerebellum, pons and medulla oblongata, are similar in the two sexes. The following tables have been compiled by Marshall from the data furnished by the observations of R. Boyd upon the brains of 1150 sane persons, viz. :—598 males and 552 females. They show the average weights in ozs. of the encephalon and its several parts at certain periods of life and in individuals having certain differences of stature: STATURE 69 INCHES AND UPWARDS. Cerebellum. 146 20-40 49.72 43.43 5.29 337 40-70 48.15 42.1 5.09 .96 115 70-90 46.92 41.19 4.8 *93 46. 47.08 41.01 5'09 1 41.15 4.9 +9 .9 44.15 38.6 .9 It will be seen from these that although there is an increase of brain-weight with body-stature, this increase does not keep pace, pari passu, with the stature. That is to say, taller persons, although they have absolutely more brain substance, have relatively less than shorter persons. This is true for either sex. Nevertheless the proportion of brain to the stature remains larger in the male both at the mean height of both sexes, and at nearly corresponding heights. Marshall further finds from a minute analysis of these results of R. Boyd, that in the case of males of mean height, the weight in ounces of the cerebrum may be obtained by simply dividing the number of inches of height by 1·6, or in grammes by multiplying the number of centimetres of height by 7. For females, the same formula as that employed for the male can be used, but the result must be multiplied by Thus 30 The weights as calculated from these formulæ are found by Marshall to correspond very nearly with the observed weights for definite statures as recorded in Boyd's tables. The correspondence is most complete for statures near the mean, the observed weights being slightly defective at the higher, and excessive at the lower statures.' Most of the estimates of brain weight in different races have been obtained as the result of measuring the cubic contents of the skull cavity (compare Vol. II., p. 83, and Manouvrier, loc. cit.). In this way it is estimated (Davis), that the Chinese have an average brain weight of about 1330 grammes (approaching that of the European); the Sandwich islanders one of 1300 grammes; the Malays and North American Indians one of 1265 grammes; the negro 1245 grammes; the native Australians 1185 grammes. The Hindus have also a small brain weight (probably in relation to the small prevailing stature), viz.: 1190 grammes. Amongst Europeans the Latin races have a somewhat less brain weight than the Teutonic and Sclavonic races; here also in all probability the influence of stature is apparent. Weight of the several parts of the encephalon.-The proportionate weight of the cerebellum (inclusive of the pons and the medulla oblongata) to that of the cerebrum is, in the adult, as 13 to 87 (Huschke). The cerebellum is both absolutely and relatively somewhat heavier in the male than in the female. In the new-born infant the ratio of the weight of the cerebellum to that of the whole brain is strikingly different from that observed in the adult. Huschke found the weight of the cerebellum, medulla oblongata, and pons together in the new-born infant, as compared with that of the cerebrum, to be in the proportion of 7 to 93. Meynert found the proportions between the frontal, parietal, and conjoined occipital and temporal lobes to be 41·5 : 23·4; and 35.1 (in both the male and female). Weight of the spinal cord.-Divested of its membranes and nerves, the spinal cord in the human subject weighs from 1 oz. to 12 oz. (average 30 grammes. Schwalbe.) Its proportion to the encephalon is about 1 to 43. 1 For further discussion of the proportion of stature to brain weight, the reader is referred to a paper by le Bon in the Revue d'Anthropol., 1879, and to one by the late Prof. J. Marshall. F.R.S.. in the Journal of Anatomy and Physiology, July, 1892. THE MEMBRANES OF THE BRAIN AND SPINAL CORD. The cerebro-spinal axis is covered by three membranes, named also meninges. They are:-1. An external fibrous membrane, named the dura mater, which lines the interior of the skull, and forms a loose sheath in the spinal canal; 2. An internal areolar and vascular tunic, the pia mater, which closely covers the brain and spinal cord; and 3. An intermediate non-vascular membrane, the arachnoid, which lies over the pia mater, the two being in some places in close connection, in others separated by a considerable space. Some authors describe only two meninges, considering the arachnoid and pia mater to constitute one (leptomeninx), and the dura mater the other (pachymeninx). THE DURA MATER. The dura mater is a very strong dense inelastic fibrous tunic of considerable thickness (5 mm. or more in the cranium, less in the spinal canal). Its inner surface, turned towards the brain and spinal cord, is smooth and lined with Fig. 123. SECTION THROUGH THE PLACE OF EXIT OF A SPINAL NERVE-ROOT THROUGH THE DURA MATER. (Key and Retzius.) a, bundles of the nerve-root becoming collected into a single bundle as they emerge; b, dura mater; c, arachnoid; d, a reticular lamella of the arachnoid reflected along the nerve-root; s, subdural space; s', s', subarachnoid space. epithelium (endothelium), which was formerly regarded as a parietal reflection of the arachnoid membrane, this having been generally looked upon as a serous membrane. The space between the dura mater and arachnoid was formerly in like manner regarded as the sac of the arachnoid, but is now conveniently termed the subdural space. The outer surface of the dura mater is connected with the surrounding parts in a somewhat different manner in the cranium and in the spinal canal. In the cranium it adheres to the inner surface of the bones, and forms their internal periosteum. The connection between the two depends, in a great measure, on blood-vessels and small fibrous processes, which pass from one to the other; and the dura mater, when detached and allowed to float in water, presents a flocculent appearance on its outer surface, in consequence of the torn parts projecting from it. The adhesion between the membrane and the bone is more intimate opposite the sutures, and also at the base of the skull, which is uneven, and perforated by numerous foramina, through which the dura mater is prolonged to the outer surface. being there continuous with the pericranium. The fibrous tissue of the dura mater becomes blended with the areolar sheath of the nerves at the foramina which give exit to them. In leaving the skull, the dura mater is intimately attached to the margin of the foramen magnum, and below this to the cervical vertebræ as far as the third. Above the atlas it has an orifice on each side for the passage of the vertebral artery. Within the rest of the vertebral canal it forms a loose sheath around the cord (theca), and is not adherent to the bones, which have an independent periosteum. Towards the lower end of the canal, a few fibrous slips proceed from the outer surface of the dura mater to be fixed to the vertebræ; one such being especially well marked at the lower end, and seeming to join the anterior surface of the dura mater to the posterior common ligament of the vertebræ (anterior ligament of the dura mater, Trolard). The theca ends opposite the second sacral vertebra in the adult (see p. 6). The space intervening between the wall of the canal and the dura mater (epidural space) is occupied by loose fat, by areolar tissue, and by a plexus of spinal veins. Opposite each intervertebral foramen the dura-matral theca has two openings, placed side by side, which give passage to the two roots of the corresponding spinal nerve. It is continued as a tubular prolongation on each nerve (fig. 128), and is lost in its sheath. Besides this, it is connected with the circumference of the foramen by areolar tissue. The fibrous tissue of the dura mater, especially within the skull, is divisible into two distinct layers, and at various places the layers separate from each other and leave intervening channels, called sinuses. These sinuses, which have been elsewhere described (Vol. II.), are channels for venous blood, and are lined with a continuation of the endothelium of the veins. The division into two layers is most complete at the base of the skull, in the middle fossa, and in the neighbourhood of the cavernous sinus; on the outer side of this the Gasserian ganglion is included in a space (cavum Meckelii) between the two layers. Between the two cavernous sinuses the pituitary body is received into a depression of the membrane, which closely surrounds the organ in question, except where the infundibulum enters it. There is further a fissure immediately over the orifice of the aquæductus vestibuli, and here the prolongation of the membranous labyrinth of the ear, known as the saccus endolymphaticus, is received between the two layers. The dura mater also sends inwards into the cavity of the skull three strong membranous processes or partitions. Of these, one descends vertically in the median plane, and is received into the longitudinal fissure between the two hemispheres of the cerebrum. This is the falx cerebri. The second is a sloping vaulted partition, stretched across the back part of the skull, between the cerebrum and the cerebellum, named the tentorium cerebelli. Below this, another vertical partition, named falæ cerebelli, of small extent, passes down between the hemispheres of the cerebellum. Lastly, the portion of dura mater which stretches over the sella turcica, and pierced by a small hole for the infundibulum, covers the pituitary body, is sometimes spoken of as the operculum or tentorium of the hypophysis. The falx cerebri (fig. 129, 1) is narrow in front, where it is fixed to the crista galli, and broader behind, where it is attached to the middle of the upper surface of the tentorium, along which line of attachment the straight sinus is attached. Along its upper convex border, which is attached to the middle line of the inner surface of the cranium, runs the superior longitudinal sinus. Its under edge is free, and reaches to within a short distance of the corpus callosum, approaching nearer to it behind. This border contains the inferior longitudinal sinus. The tentorium cerebelli, or tent (fig. 129, 8), is elevated in the middle, and declines downwards in all directions towards its circumference, thus following the form of the upper surface of the cerebellum. Its inner border is free and concave, and leaves in front of it a shield-shaped opening, through which the isthmus encephali extends. It is attached behind and at the sides by its convex border to the hori zontal part of the crucial ridges of the occipital bone, and there encloses the lateral sinuses. Farther forward it is connected with the upper edge of the petrous portion |