Part 12
This has been the subject of a great deal of research, but the results are not altogether conclusive; it seems, however, that, although the male brain is 4 to 5 oz. heavier than that of the female, its relative weight to that of the body is about the same in the two sexes. An average male brain weighs about 48 oz. and a female 43-1/2 oz. The greatest absolute weight is found between twenty-five and thirty-five years of age in the male and a little later in the female. At birth the brain weighs comparatively much more than it does later on, its proportion to the body weight being about 1 to 6. At the tenth year it is about 1 to 14, at the twentieth 1 to 30, and after that about 1 to 36.5. In old age there is a further slight decrease in proportion. In many men of great intellectual eminence the brain weight has been large--Cuvier's brain weighed 64-1/2 oz., Goodsir's 57-1/2, for instance--but the exceptions are numerous. Brains over 60 oz. in weight are frequently found in quite undistinguished people, and even in idiots 60 oz. has been recorded. On the other hand, microcephalic idiots may have a brain as low as 10 or even 8-1/2 oz., but it is doubtful whether normal intelligence is possible with a brain weighing less than 32 oz. The taller the individual the greater is his brain weight, but short people have proportionally heavier brains than tall. The weight of the cerebellum is usually one-eighth of that of the entire brain. Attempts have been made to estimate the surface area of the grey matter by dissecting it off and measuring it, and also by covering it with gold leaf and measuring that. The results, however, have not been conclusive.
Further details of the brain, abundantly illustrated, will be found in the later editions of any of the standard text-books on anatomy, references to which will be found in the article on ANATOMY: _Modern Human. Das Menschenhirn_, by G. Retzius (Stockholm, 1896), and numerous recent memoirs by G. Elliot Smith and D.J. Cunningham in the _Journ. Anat. and Phys._ and _Anatomisch Anzeig._, may be consulted.
_Histology of Cerebral Cortex._
The cerebral cortex (see fig. 15) consists of a continuous sheet of grey matter completely enveloping the white matter of the hemispheres. It varies in thickness in different parts, and becomes thinner in old age, but all parts show a somewhat similar microscopic structure. Thus, in vertical section, the following layers may be made out:--
1. _The Molecular Layer (Stratum zonale)._--This is made up of a large number of fine nerve branchings both medullated and non-medullated. The whole forms a close network, the fibres of which run chiefly a tangential course. The cells of this layer are the so-called _cells of Cajal_. They possess an irregular body, giving off 4 or 5 dendrites, which terminate within the molecular layer and a long nerve fibre process or neuraxon which runs parallel to the surface of the convolution.
2. _The Layer of small Pyramidal Cells._--The typical cells of this layer are pyramid-shaped, the apices of the pyramids being directed towards the surface. The apex terminates in a dendron which reaches into the molecular layer, giving off several collateral horizontal branches in its course. The final branches in the molecular layer take a direction parallel to the surface. Smaller dendrites arise from the lateral and basal surfaces of these cells, but do not extend far from the body of the cell. The neuraxon always arises from the base of the cell and passes towards the central white matter, thus forming one of the nerve-fibres of that substance. In its path it gives off a number of collaterals at right angles, which are distributed to the adjacent grey matter.
[Illustration: From Cunningham, _Text-book of Anatomy_.
Fig. 15.--Diagram to illustrate Minute Structure of the Cerebral Cortex.
A. Neuroglia cells. B. " " C. Cell with short axon (N) which breaks up in a free arborization. D. Spindle-shaped cell in stratum zonale. E. Small pyramidal cell. F. Large pyramidal cell. G. Cell of Martinotti. H. Polymorphic cell. K. Corticipetal fibres.]
3. _The Layer of large Pyramidal Cells._--This is characterized by the presence of numbers of cells of the same type as those of the preceding layer, but of larger size. The nerve-fibre process becomes a medullated fibre of the white matter.
4. _The Layer of Polymorphous Cells._--The cells of this layer are irregular in outline, and give off several dendrites branching into the surrounding grey matter. The neuraxon gives off a number of collaterals, and then becomes a nerve-fibre of the central white matter.
Scattered through these three layers there are also a number of cells (_cells of Golgi_) whose neuraxon divides at once, the divisions terminating within the immediate vicinity of the cell-body. Some cells are also found in which the neuraxon, instead of running into the white matter of the brain, passes toward the surface; these are called _cells of Martinotti_.
The medullated nerve-fibres of the white matter when traced into the cortex are seen to enter in bundles set vertically to the surface. These bundles taper and are resolved into isolated fibres in the upper parts of the pyramidal layers. The fibres constituting the bundles form two sets. (a) The centrifugal fibres consist as above described of the fibre processes of the pyramidal and polymorphous cells. (b) The centripetal fibres ascend through the cortex to terminate within the molecular layer by horizontally running branches. As they pass through they give off a number of collaterals. The position of the cells from which these fibres arise is not known. In addition to the radially arranged bundles of fibres, networks are formed by the interlacement with them of large numbers of fine medullated fibres running tangentially to the surface. These are derived chiefly from the collaterals of the pyramidal cells and of the centripetal fibres. They form two specially marked bundles, one within the layer of the polymorphous cells known as the _inner band of Baillarger_, and another in the layer of large pyramidal cells called the _outer band of Baillarger_. This latter is very thick in the calcarine region, and forms the _white stria of Gennin_, while the inner band is best seen in the precentral gyrus. As both these strands cross the already mentioned radial bundles at right angles, they are regarded as specialized parts of an _interradial reticulum_ of fibres, but, nearer the surface than the radial bundles penetrate, tangential fibres are found, and here they are called the _supraradial reticulum_. In certain parts of the brain the fibres of this reticulum are more closely set, and form the _band of Bechterew_ in the superficial part of the small pyramidal cell zone.
[Illustration: From _The Museum Catalogue of the Royal College of Surgeons of England_.
Fig. 16.--Brain of _Petromyzon marinus_ (dorsal view). A, Brain; B, choroid plexus removed.]
For further information on the structure of the cerebral cortex, see A.W. Campbell, _Proc. R. Soc._ vols. lxxii. and lxxiv.
_Comparative Anatomy._
A useful introduction to the study of the vertebrate brain is that of the Amphioxus, one of the lowest of the Chordata or animals having a notochord. Here the brain is a very slightly modified part of the dorsal tubular nerve-cord, and, on the surface, shows no distinction from the rest of that cord. When a section is made the central canal is seen to be enlarged into a cavity, the neurocoele, which, in the young animal, communicates by an opening, the neuropore, with the bottom of the olfactory pit, and so with the exterior. More ventrally another slight diverticulum probably represents the infundibulum. The only trace of an eye is a patch of pigment at the anterior end of the brain, and there are no signs of any auditory apparatus. There are only two pairs of cerebral nerves, both of which are sensory (Willey, _Amphioxus_, 1894). In the Cyclostomata, of which the lamprey (Petromyzon) is an example, the minute brain is much more complex, though it is still only a very slight enlargement of the anterior end of the cord. The single cavity seen in Amphioxus is here subdivided into three: an anterior or prosencephalon, a middle or mesencephalon, and a hinder or rhombencephalon. The rhombencephalon has a very slight transverse thickening in the fore-part of its roof, this is the rudimentary cerebellum (_Cer._); the rest of this part of the brain is taken up by the large medulla, the cavity of which is the _fossa rhomboidalis_ or fourth ventricle. This fossa is roofed over by the epithelium lining the cavity of the ventricle, by pia mater and blood-vessels constituting a choroid plexus (fig. 16, B). The fourth ventricle communicates with the parts in front by means of a passage known as the aqueduct of Sylvius.
The mesencephalon or mid-brain, when looked at from the dorsal surface, shows a pair of large hollow swellings, the optic lobes or _corpora bigemina_. Their cavities open out from the aqueduct of Sylvius, and from the nervous tissue in their walls the optic nerves derive their fibres. From the front of the prosencephalon or anterior vesicle the olfactory nerves come off, and at the base of each of these are two hollow swellings; the larger and more anterior is the olfactory bulb, the smaller and more posterior the cerebral hemisphere. Both these swellings must be regarded as lateral outgrowths from the blind front end of the original single vesicle of the brain as seen in Amphioxus, and from the anterior subdivision or prosencephalon in the lamprey. The anterior vesicle, however, is now again subdivided, and that part from which the cerebral hemispheres bud out, and the hemispheres themselves, is called the telencephalon, while the posterior part of the original prosencephalon is known as the thalamencephalon, or more rarely the diencephalon. On the dorsal surface of the thalamencephalon are two nervous masses called the ganglia habenulae; the right is much larger than the left, and from it a stalk runs forward and upward to end in the vestigial pineal body (or epiphysis), which contains rudiments of a pigmented retina and of a lens, and which is usually regarded as the remains of one of a pair of median eyes, though it has been suggested that it may be an organ for the appreciation of temperature. From the small left ganglion habenulae a still more rudimentary pineal stalk projects, and there are signs of a third outgrowth (paraphysis) in front of these. On the floor of the thalamencephalon the blind pouch-like infundibulum is in contact with the pituitary body, an outgrowth from the combined pituitary and olfactory pouch, which in the adult opens on to the top of the head just in front of the pineal area. The anterior closed end of the nerve-tube, in front of the foramina of Munro or openings from which the hemispheres have grown out, is known as the _lamina terminalis_, and in this is seen a little white commissure, connecting the hemispheres of opposite sides and belonging entirely to the telencephalon, known as the anterior commissure. The roof of the telencephalon is mainly epithelial, and contains no traces of cortical structure. In the posterior part of the roof of the thalamencephalon is the small posterior commissure (Ahlborn, _Zeits. wiss. Zool._ Bd. xxxix., 1883, p. 191). In the Elasmobranch Fish, such as the sharks and rays, the cerebellum (_Cer._ fig. 17) is very large and contains the layers found in all the higher vertebrates. In the mesencephalon fibres corresponding with those of the fillet of higher vertebrates can be seen, and there is a nucleus in the hinder part of the _corpora bigemina_ foreshadowing the separation into corpora quadrigemina. There is only one pineal stalk in the roof of the thalamencephalon, and the ganglia habenulae--very constant structures in the vertebrate brain--are not so marked as in Petromyzon, but are, as usual, connected with the olfactory parts of the cerebrum, with the surface of the optic lobes (_tectum opticum_), and with the _corpus interpedunculare_ (Meynert's bundle). They are united across the middle line by a small _superior_ or _habenular commissure_. In the floor of the thalamencephalon are two masses of ganglionic tissue, the optic thalami. The infundibulum dilates into two rounded bodies, the _lobi inferiores_, while the pituitary body or _hypophysis cerebri_ has two lateral diverticula known as _sacci vasculosi_. Ganglia geniculata are found for the first time in connexion with the optic tracts in the lower part of the thalamus. The olfactory lobes (fig. 17, _Olf. Bulb_) are very large and often separated by long stalks from the cerebral hemispheres, which are comparatively much larger than those of the Cyclostomata; their roof or pallium is nervous, but devoid of cortical structure, while in the floor in some species large anterior basal ganglia or _corpora striata_ are found (Miklucho-Maclay, _Beitrage z. vergl. Neurol._, 1870; Edinger, _Arch. mikr. Anat._ Bd. lviii., 1901, p. 661, "Cerebellum"). The Teleostean Fish are chiefly remarkable for the great development of the optic lobes and suppression of the olfactory apparatus. The pallium is non-nervous, and the optic tracts merely cross one another instead of forming a commissure. A process of the cerebellum called _valvula cerebelli_ projects into the cavity of each optic lobe (Rabl. Ruckhard, _Arch. Anat. u. Phys_., 1898, p. 345 [Pallium]; Haller, _Morph. Jahrb._ Bd. xxvi., 1898, p. 632 [Histology and Bibliography]). The brain of the Dipnoi, or mud fish, shows no very important developments, except that the anterior pineal organ or paraphysis is large (Saunders, _Ann. and Mag. Nat. Hist._ ser. 6, vol. iii., 1889, p. 157; Burkhardt, _Centralnervensystem v. Protopterus_, Berlin, 1892).
[Illustration: From _Cat. R.C.S. England_.
FIG. 17.--Section of the Brain of Porbeagle Shark (_Lamna_).]
In the Amphibia the brain is of a low type, the most marked advances on that of the fish being that the anterior commissure is divided into a dorsal and ventral part, of which the ventral is the true anterior commissure of higher vertebrates, while the dorsal is a hippocampal commissure and coincides in its appearance with the presence of a small mass of cells in the outer layer of the median wall of the pallium, which is probably the first indication of a hippocampal cortex or cortex of any kind (Osborn, _Journ. Morph._ vol. ii., 1889, p. 51).
[Illustration: From _Cat. R.C.S. England_.
Fig. 18.--Section of Brain of Turtle (_Chelone_).]
In the Reptilia the medulla has a marked flexure with a ventral convexity, and an undoubted cerebral cortex for the first time makes its appearance. The mesial wall of the cerebral hemisphere is divided into a large dorsal hippocampal area (fig. 18, _Hip._) and a smaller ventral olfactory tubercle. Between these two a narrow area of ganglionic matter runs forward from the side of the _lamina terminalis_ and is known as the paraterminal or precommissural area (Elliot Smith, _Journ. Anat. and Phys._ vol. xxxii. p. 411). To the upper lateral part of the hemisphere Elliot Smith has given the name of _neopallium_, while the lower lateral part, imperfectly separated from it, is called the _pyriform lobe_. In the Lacertilia the pineal eye, if it be an eye, is better developed than in any existing vertebrate, though even in them there is no evidence of its being used for sight. Behind the so-called pineal eye and its stalk is the _epiphysis_ or pineal body, and sometimes there is a dorsal sac between them (see fig. 18).[1] The middle or soft commissure appears in certain reptiles (_Crocodilia_ and _Chelonia_), as does also the _corpus mammillare_ (Edinger, Senckenberg, _Naturf. Gesell._ Bd. xix., 1896, and Bd. xxii., 1899; Haller, _Morph. Jahrb._ Bd. xxviii., 1900, p. 252). Among the birds there is great unity of type, the cerebellum is large and, by its forward projection, presses the optic lobes down toward the ventro-lateral part of the brain. The cerebral hemispheres are also large, owing chiefly to the great size of the _corpora striata_, which already show a differentiation into caudate nucleus, putamen and globus pallidus. The pallium is reptilian in character, though its cortical area is more extensive. The geniculate bodies are very large (Bumm, _Zeits. wiss. Zool._ Bd. xxxviii., 1883, p. 430; Brandis, _Arch. mikr. Anat._ Bd. xli., 1893, p. 623, and xliii., 1894, p. 96, and xliv., 1895, p. 534; Boyce and Warrington, _Phil. Trans._ vol. cxci., 1899, p. 293).
Among the Mammalia the Monotremata have a cerebellum which shows, in addition to the central lobe of the lower vertebrates, a flocculus on each side, and the two halves of the cerebellum are united by a ventral commissure, the _pons varolii_. The pallium is reptilian in its arrangement, but that part of it which Elliot Smith has named the neopallium is very large, both in the Ornithorynchus and Echidna, a fact very difficult to account for. In the latter animal the cortical area is so extensive as to be thrown into many and deep sulci, and yet the Echidna is one of the lowliest of mammals in other respects. A well-marked rhinal fissure separates the pyriform lobe from the neopallium, while, on the mesial surface, the hippocampal fissure separates the neopallium from the hippocampal area. Just below the hippocampal fissure a specially coloured tract indicates the first appearance of the fascia dentata (see fig. 20). The anterior commissure is divided, as in reptiles, into dorsal and ventral parts, of which the latter is the larger (fig. 20, _Comm. V. and D_.), while just behind the dorsal part is the first appearance of the fimbria or fornix. In addition to the two fissures already named, there is, in the Echidna, one which in position and mode of formation corresponds with the Sylvian fissure of higher mammals. Elliot Smith, however, wisely refuses to homologize it absolutely with that fissure, and proposes the name of pseudosylvian for it. The pineal body is rudimentary, and the optic lobes are now, and throughout the Mammalia, subdivided into four _corpora quadrigemina_.
[Illustration: From _Cat. R.C.S. England_.
FIG. 19.--Ventral and Dorsal Views of the Brain of Ornithorynchus.]
Among the Marsupialia the Tasmanian devil (Sarcophilus) gives a very good idea of a generalized mammalian brain, and shows a large development of the parts concerned in the sense of smell. The most important advance on the monotreme brain is that the calcarine fissure has now appeared on the posterior part of the mesial surface and causes a bulging into the ventricle, called the _calcar avis_ or hippocampus minor, just as the hippocampal fissure causes the _hippocampus major_ (Gervais, _Nuov. Arch. Mus_. tom. v., 1869; Ziehen, _Jenaische Denkschr_. Bd. vi., 1897).
[Illustration: From _Cat. R.C.S. England_.
FIG. 20.--Mesial and Lateral Views of the Brain of Ornithorynchus.]
[Illustration: From _Cat. R.C.S. England_.
FIG. 2l.--Mesial and Lateral Views of the Brain of the Tasmanian Devil (_Sarcophilus_).]
In the Eutheria or mammals above the marsupials, the cerebellum gradually becomes more complex, owing to the appearance of lateral lobes between the flocculus and the vermis, as well as the paraflocculus on the outer side of the flocculus. The corpus callosum now first appears as a bridge between the neopallia, and its development leads to the stretching of the hippocampal formation, so that in the higher mammals the hippocampus is only found in the lower and back part of the ventricle, while the rudiments of the dorsal part remain as the _striae longitudinals_ on the corpus callosum. The dorsal part of the original anterior commissure becomes the fornix, and the paraterminal area is modified to form the septum lucidum. The first appearance of the fissure of Rolando is probably in some of the Carnivora, in which, as the _sulcus crucialis_, it forms the posterior boundary of the "ursine lozenge" described by Mivart (_Journ. Linn. Soc_. vol. xix., 1886) (see fig. 22, _Sulc. Cru_.). In the higher apes or Anthropoidea the human fissures and sulci are largely recognizable, so that a gibbon's brain, apart from all question of comparative anatomy, forms a useful means of demonstrating to a junior class the main gyri and sulci of Man in a simple and diagrammatic way. The main points of difference, apart from greater simplicity, are that the central lobe or island of Reil is exposed on the surface of the brain, as it is in the human foetus, and that the anterior part of the occipital lobe has a well-marked vertical sulcus, called the simian sulcus or _Affenspalte_; this often has a semilunar shape with its convexity forward, and is then called the _sulcus lunatus_. It is usually concealed in European brains by the overgrowth of the surrounding gyri, but it occasionally remains, though less frequently than in the brains of Egyptian fellaheen. Its relation to the _white stria of Gennari_ is especially interesting, and is recorded by Elliot Smith in the _Anatomischer Anzeiger_, Bd. xxiv., 1904, p. 436. The rhinal fissure, which is so characteristic a feature of the lower mammals, almost disappears in Man, and is only represented by the _incisura temporalis_ (see fig. 11, _i.t_). The hippocampal fissure persists with little modification all through the mammalian class. The calcarine fissure remains with many modifications from the marsupials to man, and in view of the famous controversy of 1864, in which Owen, Huxley and the then bishop of Oxford took part, it is interesting to note that its hippocampus minor can now be clearly demonstrated, even in the Marsupialia. Another very ancient and stable sulcus is the _orbital_, which is a simple antero-posterior line until Man is reached (see fig. 23, _Sulc. Orb._). The great point of importance, however, in the evolution of the mammalian brain is the gradual suppression of the olfactory region, and the development of the neopallium, a development which takes a sudden stride between the Anthropoid apes and Man. (For further particulars of this and other points in the comparative anatomy of the brain, see _Catalogue of the Physiological Series_ of the Museum of the Royal College of Surgeons of England, vol. ii. 2nd ed., by R.H. Burne and G. Elliot Smith, London, 1902.)
[Illustration: From _Cat. R.C.S. England_.
FIG. 22.--Dorsal and Lateral Views of the Brain of a Ratel (_Mellivora indica_).]
_Embryology._
The brain, like the rest of the nervous system, is developed from the ectoderm or outer layer of the embryo by the formation of a groove in the mid-dorsal line. The lips of this _medullary groove_ unite to form a canal beginning at the place where the neck of the embryo is to be. The part of the neural canal in front of the earliest union forms the brain and very early becomes constricted into three vesicles, to which the names of _prosencephalon_, _mesencephalon_ and _rhombencephalon_ are now usually given. The simple tubular brain we have seen as a permanent arrangement in Amphioxus, but the stage of the three vesicles is a transitory one, and is not found in the adult of any existing animal. From the sides of the prosencephalon, the optic vesicles grow out before the neural tube is completely closed, and eventually form the optic nerves and retinae, while, soon after this, the cerebral hemispheres bulge from the antero-dorsal part of the first primary vesicle, their points of evagination being the _foramina of Munro_. From the ventral parts of these cerebral hemispheres the olfactory lobes are constricted off, while just behind the openings of the foramina of Munro a constriction occurs which divides the prosencephalon into two secondary vesicles, the anterior of which, containing the foramina of Munro, is called the _telencephalon_, while the posterior is the _thalamencephalon_ or _diencephalon_. A constriction also occurs in the hind vesicle or _rhombencephalon_, dividing it into an anterior part, the _metencephalon_, from which the cerebellum is developed, and a posterior or _myelencephalon_, the primitive _medulla oblongata_. At this stage the general resemblance of the brain to that of the lamprey is striking.