The Origin of Vertebrates

by Walter Holbrook Gaskell

Cr., membranous cranium; I, olfactory nerves; l.v., lateral ventricles; gl., glandular tissue which fills up the cranial cavity.

Here, again, it is seen how simple is the explanation of a peculiarity which has always puzzled anatomists—why should there be this seam in the infra-infundibular portion of the brain and not in the supra-infundibular or in the spinal cord? The corresponding compression in the upper brain-region forms the lateral ventricles, as is seen in the accompanying figure of the brain of Ammocœtes (Fig. [22]).

Fig. 23.—Section through Rhomboidal Sinus of Bird.

In yet another instance it is seen how markedly the nervous masses are arranged in the same position with respect to the central tube as are the nerve ganglia with respect to the intestinal tube in the case of the invertebrate. Thus in birds a portion of the spinal cord in the lumbo-sacral region presents a very different appearance from the rest of the cord; it is known as the rhomboidal sinus, and a section of the cord of an adult pigeon across this region is given in Fig. [23]. As is seen, the nervous portions are entirely confined to two masses connected together by the white anterior commissures which are situated laterally and ventrally to a median gelatinous mass; the small central canal is visible and the whole dorsal area of the cord is taken up by a peculiar non-nervous wedge-shaped mass of tissue. At its first formation this portion of the cord is formed exactly in the same manner as the rest of the cord; instead, however, of the nervous material invading the dorsal part of the tube to form the posterior fissure, it has been from some cause unable to do so, the walls of the original non-nervous tube have become thickened dorsally, been transformed into this peculiar tissue, and so caused the peculiar appearance of the cord here. The nervous parts have not suffered in their development; the mechanism for walking in the bird is as well developed as in any other animal; their position only is different, for they still retain the original ventro-lateral position, but the non-nervous tube, the remains of the old intestine, has undergone a peculiar gelatinous degeneration just where it has remained free from invasion by the nervous tissue.

Throughout the whole of that part of the nervous system which gives origin to the cranial and spinal segmental nerves, the evidence is absolutely uniform that the nervous material was originally arranged bilaterally and ventrally on each side of the central tube, exactly in the same way as the nerve-masses of the infra-œsophageal and ventral chain of ganglia are arranged with respect to the cephalic stomach and straight intestine of the arthropod. But, in addition, we find in the vertebrate nervous masses, the cerebral hemispheres, the corpora quadrigemina and the cerebellum situated on the dorsal side of the central tube in the brain-region; this nervous material is, however, of a different character to that which gives origin to the spinal and cranial segmental nerves. How is the presence of these dorsal masses to be explained on the supposition that the dilated anterior part of the nerve-tube was originally the cephalic stomach of the arthropod ancestor? The cerebral hemispheres are simple enough, for they represent the supra-œsophageal ganglia, which of necessity, as they increased in size, would grow round the anterior end of the cephalic stomach and become more and more dorsal in position.

The difficulty lies rather in the position of the cerebellum and corpora quadrigemina, and the solution is as simple as it is conclusive.

Let us again turn to embryology and see what help it gives. In all vertebrates the dilated anterior portion of the nerve-tube does not, as it grows, increase in size uniformly, but a constriction appears on its dorsal surface at one particular place, so as to divide it into an anterior and posterior vesicle; then the latter becomes divided into two portions by a second constriction. In this way three cerebral vesicles are formed; these three primary cerebral vesicles indicate the region of the fore-brain, mid-brain, and hind-brain respectively. Subsequently the first cerebral vesicle becomes divided into two to form the prosencephalon and thalamencephalon, while the third cerebral vesicle is also divided into two to form the region of the cerebellum and medulla oblongata.

These constrictions are in the position of commissural bands of nervous matter; of these the limiting nervous strands between the thalamencephalon and mesencephalon and between the mesencephalon and the hind-brain are of primary importance. The first of these commissural bands is in the position of the posterior commissure connecting the two optic thalami. In close connection with this are found, on the mid-dorsal region, the two pineal eyes with their optic ganglia, the so-called ganglia habenulæ. From these ganglia a peculiar tract of fibre, known as Meynert's bundle, passes on each side to the ventral infra-infundibular portion of the brain. In other words, the first constriction of the dilated tube is due to the presence and growth of nervous material in connection with the median pineal eyes. Here in precisely the same spot, as will be fully explained in the next chapter, there existed in the arthropod ancestor a pair of median eyes situated dorsally to the cephalic stomach, the pre-existence of which explains the reason for the first constriction.

The second primary constriction separating the mid-brain from the hind-brain is still more interesting, for it is coincident with the position of the trochlear or fourth cranial nerve. In all vertebrates without exception this nerve takes an extraordinary course; all other nerves, whether cranial or spinal, pass ventralwards to reach their destination. This nerve passes dorsalwards, crosses its fellow mid-dorsally in the valve of Vieussens, where the roof of the brain is thin, and then passes out to supply the superior oblique muscle of the eye of the opposite side. The two nerves form an arch constricting the dilated tube at this place. In the lowest vertebrate (Ammocœtes) the constriction formed by this nerve-pair is evident not only in the embryonic condition as in other vertebrates, but during the whole larval stage. As Fig. [20], A and B, shows, the whole of the dorsal region of the brain up to the region of the pineal eye and ganglion habenulæ is one large membranous bag, except for the single constriction where the fourth nerve on each side crosses over. The explanation of this peculiarity is given in Chapter VII., and follows simply from the facts of the arrangement of that musculature in the scorpion-group which gave rise to the eye-muscles of the vertebrate.