Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/938

Rh 880 ANATOMY [NERVOUS SYSTEJ: 61 oz. in a case of senile dementia, 60 oz. in a case of dementia, and 60 oz. in one of melancholia. No case has as yet been recorded of the weight of the brain in a woman possessing intellectual eminence; but Boyd met with a woman s brain as high as 55 25 oz., and many instances of upwards of 50 oz. in women where there &quot;was no evidence of high mental endowment. Skae, in a female monomaniac, observed a brain which weighed 61 ^ oz.; and of 300 females examined in the West Riding Asylum the weight of the brain in 26 cases was 50 oz. or upwards, the highest weights being 56 and 55 oz. in two cases of mania. The size and weight of the brain do not therefore, per se, give an exact method of estimating the intellectual power of the individual, and a high brain weight and great intellectual capacity are not necessarily correlated with each other. It seems certain, if the human brain, even amongst the most uncultivated peoples, falls below 30 oz., that this low weight is not merely incompatible with intellectual power and activity, but is invariably associated with idiocy or imbe cility; so that the human brain has a minimum weight below which intellectual action is impossible. Amongst the more cultivated races the minimum weight-limit of intelli gence is, however, in all probability higher than 30 oz. It has been placed by Broca at 32 oz. for the female, and 37 oz. for the male brain ; and Thurnam s numbers are almost the same. To how low a weight the brain in the microcepha- lous idiot may fall is well shown in a case recorded by Theile, where it weighed only 10 6 oz., in Gore s case of 10 oz. 5 grs., and in Marshall s case, 8^- oz. But instances are not wanting in which the brains of idiots have exceeded even 50 oz. Langdon Down observed the brain of a male idiot aged 22, which weighed 59J oz.; and J. B. Tuke has recently met with a brain of 60 oz. in a male idiot aged 37, the capacity of whose cranium was 110 cubic inches. In the West Riding Asylum tables the brain weights in 10 idiots were not less than 34 oz., and in 5 cases exceeded 40 oz. As yet the opportunities of weighing the brain in the coloured races of men have been but scanty. But from a very extensive series of observations made by Barnard Davis, not on the brains themselves, but on the cubic capacities of crania, from which an approximate estimate of the brain weight may be obtained with a fair measure of accuracy, the following facts are derived : The average weight of the male brain in the African races is 45 6 oz.; of the female brain, 42 - 7 oz. : the average weight of the male brain in the Australian races is 42 - S oz. ; of the female brain, 39 2 oz.: the average weight of the male brain in the Oceanic races, 46 5 oz. ; of the female brain, 43 oz. The conclusions which may legitimately be drawn from an analysis of Barnard Davis s observations are as follows : 1st, That the average brain weight is considerably higher in the civilised European than in the savage races ; 2d, That the range of variation is much greater in the former than in the latter ; 3d, That there is an absence, almost complete, of specimens heavier than 54 oz. in the exotic races, so that the higher terms of the series are not repre sented ; 4th, That though the male brains are heavier than the female, there is not the same amount of difference in the average brain weight between the two sexes in the uncultivated as in the cultivated peoples. No reliable determinations have as yet been made of the exact proportion, as regards bulk and weight, which the convolutions bear to the corpora striata, optic thalami, and corpora quadrigemina, but data are obtainable of the rela tive weight of the pons, cerebellum, and medulla to the entire encephalon. Between the ages of 20 and 70 the ratio of weight of the pons, cerebellum, and medulla, to the entire brain, is as 13 to 100, and this relative weight is virtually the same in both sexes. ORIGIN, ARRANGEMENT, AND DISTRIBUTION OF THE ENCEPHALIC NERVES. Several pairs of nerves, called Crania CRANIAL or ENCEPHALIC, arise from the under surface or nei ves base of the encephalon, and pass outwards through foramina situated in the floor of the cranial cavity. Continental anatomists usually enumerate twelve pairs of cranial nerves; but because in one locality two of these nerves He together and pass through the same foramen, and in another spot three of these nerves emerge together from the skull, British anatomists have restricted the number to nine pairs. These nerves are numbered from before backwards, in the order in which they are seen at the base of the brain. The names applied to the individual nerves, and their numerical designations, according to both the Continental and British methods, are given in the following table : Continental. British. Olfactory Nerves, 1st pair 1st pair Optic Nerves, 2d 2d ,, Oculo-motor Nerves, 3rd 3rd ,, Trochlear Nerves, 4th 4th, , Trifacial or Trigemiual Nerves, ... 5th f&amp;gt;th ,, Abducent Nerves, 6th (3th ,, Facial Nerves (Portio dura), 7th ) ,, Auditory Nerves (Portio mollis), 8th Glosso-pharyngeal Nerves, 9th Pneumogastrie Nerves (Vagus), 10th 8th ,, Spinal Accessory Nerves, llth Hypoglossal Nerves, 12th Oth ,, These nerves maybe arranged in three groups according to the presence or absence of motor and sensory fibres. First group. Sensory nerves, or nerves of special sense : a, olfactory, the nerve of smell ; b, optic, nerve of sight ; c, auditory, nerve of hearing. Second group. Motor nerves : Ci, oculo-motor, the prin cipal nerve of supply for the muscles of the eyeball; b, trochlear, the nerve for the superior oblique muscle ; c, abducent, the nerve for the external rectus ; d, portio dura, the nerve for the facial muscles of expression ; e, spinal accessory, the nerve which gives a motor root to the pneumogastric, and supplies the sterno-mastoid and tra- pezius muscles ; /, hypoglossal, the nerve for the muscles of the tongue. Third group. Mixed nerves : a, trifacial, distributed to the muscles of mastication, the skin of the face, various mucous membranes, and to the anterior and lateral surfaces of the tongue, where it may play the part of a nerve of the special sense of taste ; b, glosso-pharyngeal, distributed to the mucous membrane of the pharynx, to certain palato- pharyngeal muscles, and to the mucous membrane of the back of the tongue, where it acts as a nerve of the special sense of taste; c, the pneumogastric, conjoined with the internal division of the spinal accessory, is distributed to several muscles, mucous membranes, and internal organs. The consideration of the 1st group of cranial nerves may appropriately be deferred until the organs of sense, in which they terminate, are described. The anatomy of the motor nerves is as follows : The Oculo-motor or third nerve springs out of the inner Motoi surface of the crus cerebri. When its fibres are traced into cramo the crus, some are seen to pass to the nerve cells of the ne locus niger, whilst others sink into the corpora quadri gemina, and extend as far as the Sylvian group of large nerve cells. The nerve, after it has emerged from the crus, runs forwards in the outer wall of the cavernous sinus, and enters the orbit through the sphenoidal fissure. It supplies the leva-tor palpebraj superioris, the superior, inferior, and internal recti muscles, and the inferior oblique. It contributes the short or motor root to the ciliary ganglion, and through it influences the iris and ciliary muscles within the eyeball. It also communicates with the cavernoua plexus of the sympathetic.