Fig. 248. Longitudinal section through the brain of Scyllium canicula at an advanced stage of development.
cer. cerebral hemisphere; pn. pineal gland; op.th. optic thalamus, connected with its fellow by a commissure (the middle commissure). In front of it is seen a fold of the roof of the fore-brain, which is connected with the choroid plexus of the third ventricle; op. optic chiasma; pt. pituitary body; in. infundibulum; cb. cerebellum; au.v. passage leading from the auditory vesicle to the exterior; mel. medulla oblongata; c.in. internal carotid artery.
The general nature of the changes which take place will perhaps best be understood by a comparison of [figs. 247] and [248] representing longitudinal sections at two stages through the brain of an embryo Elasmobranch. The actual cranial flexure, i.e. flexure of the floor of the brain, is obviously greater in the older of the two brains, though viewed from the exterior the axis of this brain appears to be quite straight. In the younger stage, [fig. 247], the mid-brain (mb) forms the end of the long axis of the body, while in the older one the cerebral hemispheres (cer) have grown very greatly, especially forwards and dorsalwards. They have thus come to lie in front of the mid-brain, and to form the end of the long axis of the body, and have at the same time compressed the originally large thalamencephalon against the mid-brain. The same general features may be seen in [fig. 250] representing a longitudinal section of the brain of an embryo fowl, and [fig. 255] representing a longitudinal section of the brain of a Mammal.
The infundibulum or perhaps rather the point of origin of the optic nerves is to be regarded as the anterior termination of the axis of the base of the brain.
The cranial flexure is least marked in Cyclostomata ([fig. 253]), Teleostei, Ganoidei, and Amphibia, while it is very pronounced in Elasmobranchii, Reptilia, Aves, and Mammalia. In Teleostei, and still more in Cyclostomata, it permanently remains slight, owing to the small development of the cerebral hemispheres.
In addition to the cranial flexures, two other flexures make their appearance in the base of the brain. A posterior at the junction of the brain and spinal cord, and an anterior at the boundary between the cerebellum and medulla oblongata, just at the point where the pons Varolii is formed in Mammalia. The anterior of these is the most marked and constant; it is shewn in [fig. 250]. It arises considerably later than the main cranial flexure, and since it is turned the opposite way it assists to a considerable extent in causing the apparent straightening of the cranial axis.
Histogenetic changes[161]. The walls of the brain are at first very thin and, like those of the spinal cord, are formed of a number of ranges of spindle-shaped cells. The processes of each of these cells are stated to be continued through the whole thickness of the wall. In the floor of the hind- and mid-brain a superficial layer of delicate nerve-fibres is formed at an early period. This layer appears in the first instance on the floor and sides of the hind-brain, and very slightly, if at all, later on the floor and the sides of the mid-brain. The cells internal to the nerve-fibres become differentiated into an innermost epithelial layer lining the cavities of the ventricles, and an outer layer of grey matter.
The similarity of the primitive arrangement and histological character of the parts of the brain behind the cerebral hemispheres to that of the spinal cord is very conclusively shewn by the examination of any good series of sections. In both brain and spinal cord the white matter forms a cap on the ventral and lateral parts considerably before it extends to the dorsal surface. In the medulla the white matter does not eventually extend to the roof owing to the peculiar degeneration which that part undergoes.
In the case of the fore-brain the earliest histological changes, except possibly in Mammals, take place on the same general plan as those of the remainder of the central nervous system[162]; but though the general plan is the same, yet the early histological distinction between the fore-brain, and the mid- and hind-brain is more marked than the distinction between the latter and the spinal cord.
On the floor and sides of the thalamencephalon, and apparently the whole of the hemispheres of the lower types, there is formed, somewhat later than in the remainder of the brain, a very delicate layer of white matter. The inner part of the wall, which still remains comparatively thin, is not at first clearly divided into an epithelial and nervous layer. This distinction soon however becomes more or less apparent, though it is not so marked as in most other parts of the brain; and it appears that in the subsequent growth the greater part of the original epithelial layer becomes converted into nervous tissue.
In Mammals the same plan of differentiation would seem to be followed, though somewhat less obviously than in the lower types. The walls of the hemispheres become first divided (Kölliker) into a superficial thinner layer of rounded elements, and a deeper and thicker epithelial layer, and between these the fibres of the crura cerebri soon interpose themselves. At a slightly later period a thin superficial layer of white matter, homologous with that of the remainder of the brain, becomes established.