Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/43

Rh SYSTEM.] PHYSIOLOGY 33 ventricles may be seen a ganglionic mass corresponding to corpus striatum and optic thalamus. The optic lobes are relatively large and show considerable differen tiation of structure. Mammals, even the lower orders, not only show a general enlargement of the cerebral hemispheres, but we find a commissure, the corpus cal- losum, uniting them. This &quot;/ commissure is of small size, Fie;. IS. Typical brain of bird. A, view from above ; B, lateral view of a bisected brain. A. a, olfactory ; b, cerebral lobes ; c, optic or bigeminal lobes ; d, cerebellum ; e, medulla oblongata ; and /, spinal cord. B. a, cerebrum ; b, cere- and is confined to the fore part of the hemispheres in Monotrcmata (Ornithorhynchus, Echidna) and Marsupialia (kangaroos, &,c.), and in some of the Edentata (ant-eaters, sloths, &c. ), but it gradually extends farther and farther back as we ascend to the higher orders. The chief changes thus occur in the prosencephalon. In the lower orders of mammals the hemi spheres are comparatively small and simple, and do not present any division into convolutions, and very little distinction even of lobes. The cerebral hemispheres gradually grow backwards, cover ing mid -brain, cerebellum, and medulla oblongata, as we find in the higher Primates (monkeys, apes, and man). There is also a general enlargement of the brain and of the cranial cavity. The development of a posterior lobe only takes place in the higher orders, and in these also the enlargement of the frontal lobes brings the front of the cerebrum more and more over the nasal cavities, causing a development of forehead. This also explains how the olfactory bulbs in more highly-formed brains are thrown below the frontal part of the hemispheres, instead of originating at their anterior borders. But the internal arrangements of the brain also become more complicated. The fornix, already described, establishes, by its longitudinal commissural fibres, a connexion between the anterior and posterior lobes of the cerebrum. In the Monotrcmata and Marsiqnalia the mid-brain retains a bifid form, constituting the optic lobes, or corpora bigemina, but in all higher animals each is divided into two by a transverse groove, forming the corpora quadrigemina, of which the anterior pair is the largest. As we ascend also, we find the surface of the brain becoming more and more convoluted (see figs. 19 and 20). This is the general fact; but whilst the convolutions are most numerous and deepest in the highest orders there is no regular 3 gradation, as in each group there are very great variations in the degree of convolution (Allen Thom son). Thus in the Monotrcmata the Echidna has a more convoluted cerebrum than the Ornithorhynchus, whilst in the Primates the brains of the marmosets show a compara tively smooth non-convoluted sur- - .,.,. ,-, i FIG. 19. Rabbit s brain. 1, olfac- face, in striking contrast to the rich tory. 2&amp;gt; sur f ace O f cerebral hemi- convolutions seen on the brains of the higher monkeys and of the apes. It is important to note that the cerebellum also becomes more and more complicated as we ascend from the lower to the higher groups. At first merely a lamina or band, as seen in fishes and amphibia, it is a centrally differentiated body in crocodiles. In birds there is an indication of a division into three portions, a central and two lateral, whilst the central is by far the larger, the two lateral being feebly developed. In Monotrcmata the central portion is larger than the lateral, but, whilst it is larger in Marsupialia, Edentata, and Cheiroptera (bats, &c.), it is clear that the lateral portions are increasing in size so as to make the disproportion less. But in Carnivora (felines, hypena, otter, bear, &c. ) and in Unyulata (sheep, ox, camel, rhinoceros, horse) the lateral lobes, or hemispheres, of the cerebellum develop to a much greater size ; and in most of the Primates they are much larger than the median portion, which is now called the worm or &quot;vermiform pro cess.&quot; As regards the development of the spinal cord continuous with the medulla oblongata, it need only be said that it does not show any marked peculiarities of structure in different animals. The grey matter from which nerve-fibres originate and in which they end is found in the centre of the cord, and it is most abundant in the regions associated with the development of limbs. The white matter is external, and, in the cords of the higher animals, can be differentiated by fissures into columns, the special functions of which will be hereafter considered. The size of the cord is influenced by the masses of nerves given off from it, so that it attains its greatest thickness and development in the four higher Fig. 19. sphere ; 3, lateral ventricle, on the floor of which is seen the corpus voluted surface. Contrast the form of the cerebellum in the cat and the rabbit. In the cat the central lobe is small, whilst the lateral lobes are largely developed. wei f htof divisions of the vertebrates possessing limbs. Thus, too, are formed cervical, dorsal, and lumbar enlargements, contrasting with the more uniform and ribbon-like form of the cord in fishes, although even in these there are special enlargements corresponding to the points of exit of important spinal nerves. Sizeaiid Weight of Uruin. The gradual increase in the size of the brain, as Size and compared with that of the body, which is observed as we rise in the animal scale, has some intimate proportional relation to a corresponding increase of the nervous and mental endowments. Information as to the size of the brain may be obtained by direct measurement of dimensions and weight ; but as this is often difficult recourse may be had to the measurement of the capacity of the cranium, which contains, however, not only the brain but its accessories, such as membranes and blood-vessels. Details will be found in vol. i. p. 879. After considering the measurements of several thousand skulls made by differ ent observers, the late Dr Allen Thomson arrived at the conclusion that the cranial capacity is on the whole greater among the highly-civilized than among the savage races, and that there is even a very manifest difference to be found between persons of higher mental cultivation and acknowledged ability and those of the uneducated class and of inferior intellectual powers ; and he states further that the amount of this difference may be from 5 to 7 per cent, irt persons of the same race, and about double that range in those of different races. Thus, the average adult brain of men in Britain being taken at 3 Ib, or, more precisely, at 49J oz. avoir, (women, about 44 to 44$ oz.), at an average specific gravity of 1040, would give a bulk of 82-5 cubic inches of brain-sub stance ; 10 per cent, being deducted for loss by membranes, fluid, &c., the cranial capacity will be about 90 inches. Conversely, the weight of the brain may be calculated from the known cranial capacity. If, therefore, the brain of the uneducated class falls 2-5 oz. below the average, whilst that of the more culti vated persons rises to the same amount above it, or to 52 oz., we may regard these brain-sizes as corresponding with brain-bulks and cranial capacities of 78 and 87 cubic inches, and of 88 and 97 cubic inches respectively. The average brain-weight of an Australian aboriginal man is about 42 oz., corresponding to a brain-bulk of about 70 cubic inches, and a cranial capacity of about 78 cubic inches. There are, however, great variations in all races. Thus the brain of Cuvier, the great naturalist, weighed 05 oz. avoir., corresponding to a brain- bulk of 108 cubic inches and a cranial capacity of 118 cubic inches ; whilst, on the other hand, in Europeans the brain- weight has fallen as low as 32 oz., or a brain-bulk of 53 cubic inches and a cranial capacity of 63 cubic inches. The brains of the anthropoid apes gorilla, chimpanzee, and orang are all inferior to man in their dimensions. In the gorilla the brain does not attain more than a third of the weight of the average human brain, aiid in the chimpanzee and orang it does not reach a fourth, so that the ratio of brain-weight to body- weight in these animals may be as 1 to 100, whilst in man it ranges from 1 to 40 to 1 to 50. It is remarkable that in general among the largest animals of any group the brain does not reach a size proportionate to the greater magni tude of the other organs or of the whole body, so that in the smaller members of the same order a considerably greater proportional size of the brain is observed. Thus in the small marmosets the proportion of the brain-weight to the body-weight may be 1 to 20, or more than double the proportion in man. Similar facts are brought out in comparing the brains of cetaceans, pachyderms, dogs, &amp;lt;fec., as shown in the following table. Table of comparative sizes of Brain and Body, Examples. Brain- weight in oz. avoir. Internal cranial bulk in cub. in. Whole weight of the body inlb. Proportion of brain to body weight Average European man Child at birth 48 (3 It.) 12 10 1 34 21 144 (9 ft) 6 96 (6 ft) 16 85 to 88 22 19 i 6 300 11 650 30 140 50* 6 oz. 36 6,720 (3 tons) 94 134,400 (60 tons) 60 1 to 46 1 to 10 1 to 80 Ito 18 1 to 164 1 to 45 1 to 747 1 to 250 1 to 22,400 Ito 00 Marmoset Middle-sized dog Small dog .... Elephant. . Pig Whale 1. Porpoise Although the proportion of brain-weight to body-weight in a male child at birth is 1 to 10, yet so rapidly does the brain continue to grow during the early period of childhood that by the age of three years it has attained more than three- fourths of its full size, by the age of seven years it has reached the proportion of nine-tenths, and after this, only by slow and small gradations, it attains the full size between the ages of twenty and twenty -five years. 2 See PHRENOLOGY. From this survey of the comparative development of the brain Com- the following general conclusions can be drawn. parative 1. The first and essential portion of the cerebro-spinal axis is develop- the portion forming the spinal cord and medulla oblongata, inas- ment of much as it is found throughout the whole range of vertebrate brain, existence, and is connected with the reflex or automatic movements on which locomotion, respiration, and the circulation more or less depend, and with the simple sense of contact, or touch, or press ure. This portion is necessary to mere existence. 2. When higher senses are added, such as those of taste, smell, hearing, vision, portions of the anterior part of the cerebro-spinal axis are differentiated so as to form centres. The earliest and most important of these senses (next to touch) is vision, hence the high degree of development of the optic lobes even in the lowest forms ; to these are added the optic thalami, which may be regarded as the centres of tactile sensations involving appreciation of differences of touch as to softness, smoothness, hardness, &c., requiring in the periphery special terminal organs. Special centres for hearing, taste, and smell are not differentiated. It is remarkable that the organs relating to the sense of smell are most anterior and most closely related with the prosencephalon, indicating, apparently, that this sense is one of the earliest in appearance, and probably, along with vision and touch, one of the most necessary to existence. 1 The large cranial bulk in this instance is connected with the enormous size of the roots of the cranial nerves. 2 Many of the facts of this paragraph as to size and weight of brain are derived from an unpublished lecture by the late Dr Allen Thomson. XIX. - S
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