Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/937

Rh BRAIN.] mammals has shown that the development of the hemi spheres bears a direct relation to the size of the crusta and its ganglia, whilst the development of the hemispheres is in inverse relation to the size of the tegrnenturn and its ganglia. The superior peduncles of the cerebellum connect that organ with the cerebrum. They arise in the grey matter of the vermiform process, ascend to the corpora quadri- gemina, and some fibres are even prolonged apparently into the tegmentum, and through it doubtless into the optic thalamus. b, The fibres which connect together the two hemispheres are called commissural fibres. The largest of these com missures is the corpus callosum, which, as has already been described, connects corresponding convolutions in the opposite hemispheres. As its fibres lie on a plane superior to those of the corona radiata, the two systems of fibres intersect with each other on their way to the convolutions. The anterior commissure, though often described as con necting the two corpora striata, yet, as Spurzheim pointed out half a century ago, passes through these bodies to the convolutions around the Sylvian fissure, and gives a root of origin to the olfactory nerve. The posterior commissure passes into the two optic thalami ; some of its fibres are said to extend into the tegmentum, and others into the sub stance of the hemisphere. c, The tracts which connect different convolutions in the same hemisphere are named arcuate fibres, orfibrce proprice. The arcuate fibres are situated immediately beneath the inner surface of the cortex of the hemispheres, and connect together the grey matter of adjacent convolutions. In some localities they are strongly marked, and havejreceived special names. The fasciculus uncinatus passes across the Sylvian fissure, traverses the claustrum and amygdala, and connects the convolutions of the frontal with those of the temporo- sphenoidal lobe. The fillet of the gyrus fornicatus extends longitudinally in that convolution, immediately above the corpus callosum, from its anterior to its posterior ends, and connects two different parts of its grey matter together. The longitudinal fibres of the corpus callosum, or nerves of Lancisi, also connect the anterior and posterior ends of the callosal convolution. The longitudinal inferior fasci culus connects the convolutions of the occipital with those of the temporal lobe. Longitudinal fibres lie on the inner surface of the septum lucidum, and extend into the gyrus fornicatus. The corpora quadrigemina are connected with the optic thalami by nervous tracts called brackia, and smaller tracts also connect the thalami with the corpora geniculata. The peduncles of the pineal gland connect that body with the fornix, and are probably continued into the optic thalamus. The tcEnia semicircularis is also at one end apparently con nected with the optic thalamus, but its posterior termination is not well ascertained. The great cerebral ganglia and the central masses of grey matter are centres of origin for sensori-motor nerves. The hemispherical ganglia, again, are the parts of the brain associated with the intellectual processes. The question has often been put, Are not the individual convolutions distinct organs, each endowed with special properties ? and various arguments based on physiological, pathological, and anatomical grounds have been advanced in support of this proposition. In connection with the anatomical branch of the argument it may be stated that the convolutions possess, not only in man, but in all animals with convoluted brains, great regularity both in position and arrangement; but specialisation of form is not in itself a sufficient test of specialisation of function. Again, though the convolutions have definite forms they are not disconnected from each 879 other, for the grey matter forms a continuous layer over the whole surface of the hemisphere. Hence a group of cerebral convolutions differs from a group of muscles, each member of which is undoubtedly a distinct organ, for each muscle is isolated from those around it by a definite invest ing sheath. As regards internal structure, evidence has already been given that all the convolutions are not con structed on precisely the same plan, and it has also been pointed out that the convolutions are not all connected in the same way with the great cerebral ganglia. These structural modifications unquestionably point to functional differences in the several parts in which they are found. But further, special connections through the arcuate fibres are established between certain convolutions and not be tween others, and it is possible not only that particular combinations of convolutions through an interchange of internuncial fibres may condition a particular state of intellectual activity, but that these combinations associate various convolutions together in the performance of a given intellectual act, just as in the muscular system several muscles are as a rule associated together for the performance of a given movement. A clue to the special functions of the convolutions may perhaps be obtained by studying their connections, just as the action of the members of a group of muscles is ascertained by examining the direc tion of their fibres and the attachment of their terminal tendons. MASS AND WEIGHT OP THE BRAIN. The human brain Weight of is absolutely bigger and heavier than the brain of any train, animal, except the elephant and the larger whales. It is also heavier relatively to the bulk and weight of the body than are the brains of lower animals, except in some small birds and mammals. Considerable variations, however, exist in the size and weight of the human brain, not only in the different races of mankind, but in individuals of the same race and in the two sexes. The heaviest brains occur in the white races. The average weight of the adult Euro pean male brain is 49 to 50 oz., that of the adult female 44 to 45 oz. ; so that the brain of a man is on the average fully 10 per cent, heavier than that of a woman. The greater weight of the brain in man as compared with woman is not in relation merely to his greater bulk, but is a funda mental sexual distinction ; for, whilst there is a difference of 10 per cent, in the brain weight, the average stature of women is, as Thurnam has calculated, only 8 per cent, less than that of men. Dr Boyd states that the average weight of the brain in the newly born male infant is 11*67 oz.; in the female only 10 oz. The exact age at which the brain reaches its maximum size has been variously placed at from, the 3d to the 8th years by different authors ; but it con tinues to increase in weight to 25 or 30, or even 40. After 60 the brain begins to diminish in weight ; in aged males the average weight is about 45 oz., in females about 41 oz. In some cases the adult brain considerably exceeds the average weight. The brains of several men distinguished for their intellectual attainments have been weighed : the brain of Cuvier weighed 64^ oz. ; of Dr Abercrombie, 63 oz. ; of Professor Goodsir, 57| oz.; of Spurzheim, 55 oz. ; of Sir J. Y. Simpson, 54 oz. ; of Agassiz, 5 3 4 oz. ; and of Dr Chalmers, 53 oz. But high brain weights have also been found where there was no evidence of great intellectual capacity. Peacock weighed four male brains which ranged from 62-75 to 61 oz.; Boyd, a specimen of 60 7 5 oz.; and Turner has recorded one of a boy aged fifteen which weighed 60 oz. In the brains of the insane high brain weights have also been observed. Bucknill met with a brain in a male epileptic which weighed 64J oz.; Thurnam, one which weighed 62 oz. ; and in the West Riding Asylum, out of 375 males examined, the weight of the brain in 30 cases was 55 oz. or upwards, and the highest weights were