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enters the ventricle' becomes contracted to a very small extent of the base exterior to the crus. From this point begins the fold extending, as choroid plexus,' from one ventricle to the other by the fissure called 'foramen Monroianum' in Anthropotomy. On the interior surface of the hemisphere the pia mater is reduced to an epithelium, the cells of which are less flat in the lateral ventricles than in that continuation therefrom called third ventricle.' The part of the interhemispheral fissure overarched by the great transverse commissure is the 'fifth ventricle.' For other differentiated and definite parts in the archencephalous brainthe subjects of the bizarre' nomenclature of Anthropotomyreference may be made to the minute and exact monographs which have been published on that part of the human structure.

§ 209. Size of Brain.-The brain grows more rapidly than the body, and is larger in proportion thereto at birth than at full growth. But there is a difference in this respect in different Mammalian orders. The brain of the new-born Marsupial is less developed relatively than in higher Mammals, and grows more gradually or equally with the subsequent growth of the body.' So, in the degree in which a species retains the immature character of dwarfishness, the brain is relatively larger to the body: it is as 1 to 25 in the pygmy Petaurist, but is as 1 to 800 in the Great Kangaroo; it is as 1 to 20 in the Harvest Mouse, but is as 1 to 300 in the Capybara; it is as 1 to 60 in the little two-toed Ant-eater, and is as 1 to 500 in the Great Ant-eater. The brain weighs 6 grains in the Harvest Mouse (Mus messorius), and the same in the Common Mouse (Mus musculus); but the weight of the Harvest Mouse is 112 grains, whilst that of the Common Mouse is 327 grains. The brain of a Porpoise, 4 feet long, may weigh 1 lb. avoird. ; that of a Whale (Balanoptera) 100 feet in length does not exceed 4 lbs. avoird.2 In Artiodactyles the brain of a pygmy Chevrotain (Tragulus pygmæus) is to the body as 1 to 80; in the Giraffe3 it is as 1 to 800. In Perissodactyles the brain of the Hyrax is as 1 to 95, whilst that of the Indian Rhinoceros is as 1 to 764. The brain of the Elephant may be three times heavier than that of the Rhinoceros, but a full-grown male would probably weigh down four Rhinoceroses. In Carnivora the brain of the Weasel is to the body as 1 to 90; in the Grisly Bear it is as 1 to 500; in Quadrumana the brain of the

1 LXXV', p. 347, pl. vii, figs. 9-12.

2 SCORESBY, in a Balæna mysticetus of 65 feet in length, found the weight of the brain to be 3 lbs. 12 oz.

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Midas Marmoset is to the body as 1 to 20; in the Gorilla it is as 1 to 200.

But such ratios do not show the grade of cerebral organisation in the Mammalian class: that in the Kangaroo is higher than that in the Bird, though the brain of a Sparrow be much larger in proportional size to the body: and the Kangaroo's brain is superior in superficial folding and extent of grey cerebral surface to that of the Petaurist. The brain of the Elephant bears a less proportion to the body than that of Opossums, Mice, and proboscidian Shrews, but it is more complex in structure, more convolute in surface, and with proportions of pros- to mes-encephalon much more nearly those in the human brain. The like remark applies to all the other instances above cited.

The weight of the brain, without its membranes, in a fullgrown male Gorilla is 15 oz. avoird. I estimate that of the entire body as being nearly 200 lbs.: in the relatively larger brains of the small species of Quadrumana the convolutions are fewer, or may be absent, as in Midas.

In Man alone is a bulk of body, greater than in any Quadrumana save Gorilla, associated with a large size as well as with the highest stage of complexity of the cerebral organ. This is, perhaps, the most notable and significant fact in Comparative Anatomy.

The weight of the brain in the adult male averages about 49 oz. avoird., and ranges from about 35 oz. to 65 oz. In the adult female the weight of the brain averages about 43 oz. and a half, and ranges from 32 to 54 oz. The mean difference is thus about five ounces and a quarter. The brain has advanced to near its term of size at about ten years, but it does not usually obtain its full development till between twenty and thirty years of age, and undergoes a slight decline in weight in advanced life.'

The brain, without dura mater, of an Australian female, of 5 feet 3 inches high, weighed 32 oz.; that of a Bushwoman, 5 feet high, is estimated, in LIII",2 at 30.75 oz. In European females the brain has been found as low in size; but the requisite observations to determine the range and the average of cerebral development have hitherto been made only on Europeans. The weight of the brain of the male Hottentot, 3 lbs. 2 oz. avoird., dissected by WYMAN, encourages the expectation of analogous If the capacity of a cranium in cubic inches be ascertained, a fair and instructive notion of the weight of the brain may be obtained by estimating that of a cubic inch of it at 259-57 grains. 3 XLIX", L", LXI”.

2 LVIII'.

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results. The human brain is exceeded in weight by that of the Elephant and the Whale, but is absolutely heavier than in all other animals. In the proportionate size of the cerebrum to the cerebellum the human brain surpasses that of all Mammalia: it is as 8 to 1.

The brain in some individuals distinguished for intellectual power has been found of unusual size, and remarkable for the number and depth of the cerebral convolutions: the brain of Cuvier weighed upwards of 64 oz. The superficies of the cerebrum of the mathematician Gauss was estimated by Wagner at 341 square inches, while that of an ordinary wage-man was 291 inches.

We know not the size of brain in the Melanian inventor of the 'throwing-stick,' or of that of the deductive observer of the properties of the broken branch bent at the angle of the boomerang.' Such benefactors of their race were, perhaps, as superior to ordinary Australians in cerebral development, as the analogous rare exceptions in intellectual power have been found to be among Europeans.1

§ 210. Membranes of the Brain.-The encephalon, like the myelon, is immediately invested by an areolo-vascular tunic called 'pia mater:' it adheres to and follows all the foldings of the surface, is continued into the ventricles, and there forms processes called 'velum interpositum' and 'choroid plexus.' It is the area on which the vessels undergo the requisite degree of diminution for penetrating the cerebral substance; and, when withdrawn, the proportion of such vessels pulled out of that substance gives the flocculent appearance of the inner surface of the membrane which Anthropotomy calls tomentum cerebri.'

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The movements of the brain are served by a delicate serous sac, called the arachnoid.' The outermost membrane, called dura mater,' adheres to the inner surface of the cranium, and consists of a dense inelastic fibrous tissue. It sends a process or duplicature inwards between the cerebrum and cerebellum called tentorium,' and a second between the cerebral hemispheres called 'falx.' In the Ornithorhynchus a bony plate extends from the cranium into the falx (vol. ii. p. 323, fig. 204, B). A ridge of bone extends a short way into the tentorium in some marsupials: it is thin in Kangaroos and Phalangers, thick in Thylacines, but of less extent here than in the Wolf, (vol. ii. p. 504). In the Cachalot a bony plate projects from the 1 Tables of size and weight of Mammalian brains will be found in XII, XL!", XXXII".

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superoccipital into the back part of the falx': the tentorium receives a bony plate in many Delphini. In Seals both the tentorium and hind part of the falx are ossified, and a thick ridge enters the fore and under part of the falx between the rhinencephalic fossæ. The tentorium is ossified in the Carnivora to the extent, and in the families, noted in vol. ii., where the conditions. of such bony plate are discussed at p. 506.3 A short tentorial ridge projects anterior to the cerebellar fossa of the petrosal in Lemur macaco. The tentorial margin of the petrosal is slightly produced in Cebus, and to a greater extent in Ateles. In other Quadrumana, as in Man, the sole ossification co-extended with part of the dura mater is that called crista galli' in Anthropotomy. An unossified process from the middle of the posterior border of the tentorium, extending from the internal occipital crest, projects into the notch between the hemispheres of the human cerebellum, and is termed falx minor' and falx cerebelli.'

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§ 211. Nerves of Mammals.-The olfactory nerves are absent in all the Cetacea save those with baleen, in which they are few and small; they are present in all other Mammals, and are sent off in greater number from their cerebral centre-the rhinencephalon -than in lower Vertebrate classes." The Ornithorhynchus is the 1 XLIV. p. 442.

2 Ib. No. 2500, p. 453.

A more extensive series of comparisons of the interior of the skull has tended to rectify the physiological view entertained at the period of the publication of the posthumous edition of the Leçons d'Anatomie Comparée,' of Cuvier, vol. ii. p. 290; vol. iii. p. 155. XLIV. p. 722.

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Anthropotomists still describe the connections and course of the 'crura rhinencephali' as the origins of the olfactory nerve; although they recognise that, unlike other nerves, a large proportion of grey matter is mixed with the white fibres,' &c. (LXII". vol. ii. p. 583, 1866), and might rectify the notion by many weightier anatomical conditions. Some even maintain the view by such remarks as the following: As it is known that in the first development of the ear the peripheral part or vestibular expanse, as well as the rest of the acoustic nerve, is originally formed by the extension of a hollow vesicle from the first or hindmost fœtal encephalic compartment, so in the case of the crus cerebri, although the peripheral or distributed part (crus rhinencephali or olfactory nerve) is of separate origin from the hemispheric bulb, this latter part is comparable in its origin with the acoustic vesicle.' I have paraphrased the argument of the editors of LXII" (vol. ii. p. 584), to show that development, as a vesicle in connection with nervous centres, is no ground of homology or homotypy. Whenever a false homology has to be maintained, the earliest and obscurest phenomena of embryonal development are usually resorted to in support of such view.

The terminal expansion of the acoustic nerve is in an organ which begins as 'a follicle or hollow vesicle;' the terminal expansion of the optic nerve is also in a vesicle; and the true olfactory nerves expand terminally on what began as a follicle or vesicle, which form is retained, little altered, in Fishes. The vascular pituitary membrane supporting that expansion is the homotype of the choroid supporting the retina. No doubt the cerebellum is at first a vesicle, as is the optic lobe, and the hemisphere, and the olfactory lobe; and each may claim to be regarded as the

sole known instance of the olfactory nerve quitting the skull by a single foramen, as in Birds and Lizards (i. e. one from each rhinencephalon). In the Echidna the contrast in the vast number of nerves and the concomitant extent of the 'cribriform plate' is extraordinary. Those from the grey tract proceed to 'Jacobson's organ.' The number of olfactory nerves and extent of the pituitary surface on which they spread is very great in Marsupials. In the Insectivora the Hedgehog is most remarkable in this respect. Both Herbivorous and Carnivorous Gyrencephala have numerous olfactory nerves: some of the Phocide show this character in excess. The number of the olfactory nerves decreases, with the diminished size of the rhinencephalon, in Quadrumana, up to Man, where they seldom exceed twenty in number, and are least in proportion to the size of the body. They become flattened and expanded where they spread upon the vascular pituitary membrane.

The optic nerves are smallest in the Moles (Tulpa), largest in the Giraffe. They arise from the bigeminal bodies, chiefly from the nates and optic thalami, in Lyencephala and in some Lissencephala, to which origin are superadded in other Lissencephala and in Gyr- and Archencephala, fibres from the corpora geniculata, along the tract marked d, fig. 68. In the groups in which the eyes are relatively largest, Ungulata and Rodentia, e. g., the larger proportional size of the homologue of the optic lobes, fig. 68, a, is significant of its important relationship with the origin of the nerves of vision: the 'thalami' do not show the like increase; their larger size in Quadrumana and Bimana relates more to their function as recruiting ganglia of the prosencephalon. The optic nerves, nevertheless, seem to be derived more wholly from the thalami' in Man than in most lower Mammals, whence the Anthropotomical name of those parts. This character is shown in the foetal brain at the fourth month, fig. 125, where c shows the optic

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tract quitting the thalamus, e:

125

CCIL.

the optic lobe, f, has not yet origin of optic nerves. Foetal brain at four months. undergone its subdivision into 'nates and testes.' The liberated nerves bend downward and

homotype of the eye-ball, on the ground taken, in LXII" for viewing the olfactory bulbs as nerves, and not as encephalic lobes. The grand old anatomists had truer views of these 'processes of the brain,' as on some other points, than their successors

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