The velum transversum is a transverse, inwardly-projecting fold of the roof of the primitive fore-brain in front of the dorsal sac. To those morphologists who regard the hemisphere region or telencephalon as a primitively unpaired structure the velum is an important landmark indicating the posterior limit of the telencephalon. Those who hold the view taken in this article that the hemispheres are to be regarded as paired outpushings of the side wall of the primitive fore-brain attribute less morphological importance to the velum. Physiologically the velum is frequently important from the plexus of blood-vessels which passes with it into the III. ventricle.
In Petromyzon and Chimaera the velum is not developed. In Dipnoans there are present in its place paired transverse folds which are probably merely extensions backwards of the lateral plexuses.
The Paraphysis is a projection from the roof of the primitive fore-brain near its anterior end. It is well seen in Dipnoans[42] (Lepidosiren and Protopterus) where in the larva (exactly as in the urodele larva) it forms a blindly ending tube sloping upwards and forwards between the two hemispheres. In the adult it becomes mixed with the two lateral plexuses and is liable to be confused with them. In the other groups—except the Teleosts where it is small (Anguilla) or absent (most Teleosts)—the paraphysis is by no means such a definite structure, but generally there is present a more or less branched and divided diverticulum of the brain wall, frequently glandular, which is homologized with the paraphysis. The morphological significance of the paraphysis is uncertain. It may represent the remains of an ancient sense organ, or it may simply represent the last connexion between the brain and the external ectoderm from which it was derived.
An important derivative of the primitive fore-brain is seen in the pair of cerebral hemispheres which in the higher vertebrates become of such relatively gigantic dimensions. The hemispheres appear to be primitively associated with the special sense of smell, and they are prolonged anteriorly into a pair of olfactory lobes which come into close relation with the olfactory organ. From a consideration of their adult relations and of their development—particularly in those groups where there is no disturbing factor in the shape of a large yolk sac—it seems probable that the hemispheres are primitively paired outpushings of the lateral wall of the primitive fore-brain[43]—in order to give increased space for the increased mass of nervous matter associated with the olfactory sense. They are most highly developed in the Dipneusti amongst fishes. They are there (cf. fig. 29, C) of relatively enormous size with thick nervous floor (corpus striatum) and side walls and roof (pallium) surrounding a central cavity (lateral ventricle) which opens into the third ventricle. At the posterior end of the hemisphere a small area of its wall remains thin and membranous, and this becomes pushed into the lateral ventricle by an ingrowth of blood-vessel to form the huge lateral plexus (= plexus hemisphaerium). In this great size of the hemispheres[44] and also in the presence of a rudimentary cortex in the Dipnoi we see, as in many other features in these fishes, a distinct foreshadowing of conditions occurring in the higher groups of vertebrates. The Cyclostomes possess a distinct though small pair of hemispheres. In the Selachians the relatively archaic Notidanidae[45] possess a pair of thick-walled hemispheres, but in the majority of the members of the group the paired condition is obscured (fig. 29, A).
In the Teleostomes the mass of nervous matter which in other groups forms the hemispheres does not undergo any pushing outwards except as regards the small olfactory lobes. On the contrary, it remains as a great thickening of the lateral wall of the thalamencephalon (the so-called basal ganglia), additional space for which, however, may be obtained by a considerable increase in length of the fore-brain region (cf. fig. 30, A) or by actual involution into the third ventricle (Polypterus).[46] The great nervous thickenings of the thalamencephalic wall bulge into its cavity and are covered over by the thin epithelial roof of the thalamencephalon which is as a consequence liable to be confused with the pallium or roof of the hemispheres with which it has nothing to do: the homologue of the pallium as of other parts of the hemisphere is contained within the lateral thickening of the thelamencephalic wall, not in its membranous roof.[47]
Associated with the parts of the fore-brain devoted to the sense of smell (especially the corpora striata) is the important system of bridging fibres forming the anterior commissure which lies near the anterior end of the floor, or in the front wall, of the primitive fore-brain. It is of great interest to note the appearance in the Dipnoans (Lepidosiren and Protopterus) of a corpus callosum (cf. fig. 30 B) lying dorsal to the anterior commissure and composed of fibres connected with the pallial region of the two hemispheres.
Sense Organs.—The olfactory organs are of special interest in the Selachians, where each remains through life as a widely-open, saccular involution of the ectoderm which may be prolonged backwards to the margin of the buccal cavity by an open oronasal groove, thus retaining a condition familiar in the embryo of the higher vertebrates. In Dipnoans the olfactory organ communicates with the roof of the buccal cavity by definite posterior nares as in the higher forms—the communicating passage being doubtless the morphological equivalent of the oronasal groove, although there is no direct embryological evidence for this. In the Teleostomes the olfactory organ varies from a condition of great complexity in the Crossopterygians down to a condition of almost complete atrophy in certain Teleosts (Plectognathi).[48]
The eyes are usually of large size. The lens is large and spherical and in the case of most Teleostomes accommodation for distant vision is effected by the lens being pulled bodily nearer the retina. This movement is brought about by the contraction of smooth muscle fibres contained in the processus falciformis, a projection from the choroid which terminates in contact with the lens in a swelling, the campanula Halleri. In Amia and in Teleosts a network of capillaries forming the so-called choroid gland surrounds the optic nerve just outside the retina. As a rule the eyes of fishes have a silvery, shining appearance due to the deposition of shining flakes of guanin in the outer layer of the choroid (Argentea) or, in the case of Selachians, in the inner layers (tapetum). Fishes which inhabit dark recesses, e.g. of caves or of the deep sea, show an enlargement, or, more frequently, a reduction, of the eyes. Certain deep-sea Teleosts possess remarkable telescopic eyes with a curious asymmetrical development of the retina.[49]
The otocyst or auditory organ agrees in its main features with that of other vertebrates. In Selachians the otocyst remains in the adult open to the exterior by the ductus endolymphaticus. In Squatina[50] this is unusually wide and correlated; with this the calcareous otoconia are replaced by sand-grains from the exterior. In Dipnoans (Lepidosiren and Protopterus) curious outgrowths arise from the ductus endolymphaticus and come to overlie the roof of the fourth ventricle, recalling the somewhat similar condition met with in certain Amphibians.
In various Teleosts the swimbladder enters into intimate relations with the otocyst. In the simplest condition these relations consist in the prolongation forwards of the swimbladder as a blindly ending tube on either side, the blind end coming into direct contact either with the wall of the otocyst itself or with the fluid surrounding it (perilymph) through a gap in the rigid periotic capsule. A wave of compression causing a slight inward movement of the swimbladder wall will bring about a greatly magnified movement of that part of the wall which is not in relation with the external medium, viz. the part in relation with the interior of the auditory capsule. In this way the perception of delicate sound waves may be rendered much more perfect. In the Ostariophysi (Sagemehl), including the Cyprinidae, the Siluridae, the Characinidae and the Gymnotidae, a physiologically similar connexion between swimbladder and otocyst is brought about by the intervention of a chain of auditory ossicles (Weberian ossicles) formed by modification of the anterior vertebrae.[51]