6

6

they form the superior longitudinal commissure,' fig. 122, o; and fibres are traceable from both extremities to the perforated space,' figs. 82, 120, x. The dissection, fig. 122, shows also the longitudinal fibres extending from the anterior to the inferior and posterior lobes, and forming the external longitudinal commissure,' e, above which are seen part of the radiating fibres, s, interlacing with those of the corpus callosum, e; which is overarched by the outermost of the superior longitudinal commissural fibres, o.

Above

Dissection of the left hemisphere of the brain, from the inner side. XXXIII.

these are shown the fibres which mainly form the convolutions, but which include not only the radiating' fibres, but those of the 'transversely commissural' and 'longitudinally commissural' kinds: they terminate in or blend with the grey matter which forms the outer crust of the hemisphere. In a section of this substance in a recent brain, a white line is seen to separate it into two layers, as in fig. 124. More closely scrutinised, the following strata have been defined from the surface downward :- a thin superficial white layer, a thick reddish grey layer, the intermediate white layer, a thicker grey layer, a third thin white layer, and the deepest grey layer receiving the radiating fibres of the white or medullary cerebral neurine.'

In the contemporary Reports of my Hunterian Course of Lectures, 1842, the chief conclusions of the comparative anatomy of the superficial grey substance in

124

The anterior commissure-the most constant of the transverse system is relatively largest in Lyencephala, figs. 69, 73, c. In the human brain a similar transverse section of it shows its insignificant dimensions, fig. 123, a. Traced transversely, in them, it passes, as in a special canal, across the lower part of the corpora striata, bends backward, and expands as it radiates into the middle of each hemisphere. It indicates the small part of the human cerebrum which is homologous with the main part of that of birds and marsupials. But the increase of the mammalian over the avian brain begets the added structures for association of added parts, already described. In Man, each anterior pillar of the fornix, after leaving the 'thalamus,' descends and is bent upon itself before ascending, the bend projecting at the base of the brain, behind the 'infundibulum,' as the 'corpus albicans,' or 'mammillare,' fig. 128, m.

In the Lissencephala, where a corpus callosum is first established, it might seem, in a dissection from below, that the outer fibres of the radiating cone' curved over the lateral ventricle, and were constricted lengthwise as they ran into each other across the interhemispheral fissure, as in the dissection of the Beaver's brain, fig. 78: but it is deceptive. There is no actual continuity of any of the ascending radiating fibres of the crus cerebri with those which spread out in transverse curves from the corpus callosum. The two systems are everywhere closely interlaced; but the fibrous character of the commissural series is lost,

mammalian brains was summarised by the Reporter for the Medical Times,' as follows:-'A symmetrical arrangement, more or less regular or complex, can always be traced between the foldings of the two hemispheres, and the more regular in proportion to the simplicity of the convolutions: the foldings of the cerebral substance follow likewise, both in the embryonic development of a complex brain, and in the progressive permanent stages presented by the mammalian series, a regular determinate law: some convolutions being more constant than others, and these being traceable through the greatest number of brains, and recognisable even in the human brain, where, at first sight, they are obscured by so many accessory convolutions.' The Lecturer then demonstrated, in a considerable number of prepared brains of different animals, and in a large scries of diagrams, in which the corresponding convolutions in the brains of different animals were marked by the same colours, the facts establishing this important generalisation.-The Medical Times, Nov. 12, 1842, vol. vii. p. 101. Report of 13th Lecture, delivered May 16th, 1842.

under the microscope, before it quits the ventricular wall to descend, with the radiating fibres, into the crus. From this stage in the mammalian series the great transverse commissure grows with the growth and complexity of the hemisphere. It consists mainly of white or fibrous neurine, but some grey matter (nucleus lenticularis') is superadded to the inferior fibres external to the radiated cone, and between this and the island of Reil' there is also a thin layer of grey neurine (nucleus tæniæformis').

Always maintaining its closest connection with the part of the fornix called lyra,' or hippocampal commissure, whence its development began, the increasing body of transverse fibres extends forward and upward, with a bend or genu,' fig. 123, c, o, . corresponding in extent with elevation and expansion of the front lobes of the cerebrum. In Man its narrow anterior beginning is connected by the lamina cinerea' with the optic commissure, receives a small part of the grey substance of the thalamus, and sends off two bands, called peduncles of the corpus callosum,' which, diverging, pass backward across the 'perforated space' to the lower part of the sylvian fold. The corpus callosum, expanding as it rises, bends backward, and presents on its upper surface a medial longitudinal groove, called 'raphe,' bounded laterally by the white striæ longitudinales: 'it terminates behind in a slightly down-bent, thickened, free border or 'pad.' Some way in advance of this the attachment of the under surface of the corpus callosum to the fornix begins, and, as the hemispheres increase in the placental series, so does the extent of the filmy inner walls of the lateral ventricles (septum lucidum,' Anthro., fig. 123, b) between the body of the fornix and the great superadded transverse commissure, the fibres of which extend over the roof of those ventricles. The most intelligible illustrations of the comparative anatomy of this interesting part of the cerebral structure is obtained by dissecting and exposing the lateral ventricle from the outer side, as in the views of the brains of the Opossum, Kangaroo, and Ass, showing the relative proportions of the hippocampus, and of the part of the inner wall distinct therefrom, called 'septum lucidum,' in LXX', pl. vii. In fig. 5, the vascular fold of pia mater called 'choroid plexus' is shown passing beneath the fore part of the 'tænia hippocampi' through the canal of communication between the lateral ventricles, in both marsupial and placental brains. The supraventricular neurine, being folded upon its stem, the cavity is a reflection of the external surface, and is lined by a continuation of the pia mater, although the fissure by which it

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 brain— the 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 Balana mysticetus of 65 feet in length, found the weight of the brain to be 3 lbs. 12 oz.

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.1

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.3 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'.