The Origin of Vertebrates

by Walter Holbrook Gaskell

One of the strangest facts known about the system of inhibitory nerves is their marked tendency to leave the central nervous system at a different level to the corresponding motor nerves, as is well known in the case of the heart, where the inhibitory nerve—the vagus—arises from the medulla oblongata, while the motor nerve—the augmentor or accelerator—leaves the spinal cord in the upper thoracic region. It is very difficult to obtain any idea of the origin of such a peculiarity; I know of only one suggestive fact, which concerns the innervation of the muscles which open and close the chela of the crayfish, lobster, etc. These muscles are antagonistic to each other, and both possess inhibitory as well as motor nerves. The central nervous system arrangements are of such a character that the contraction of the one muscle is accompanied by the inhibition of its opposer, and the nerves which inhibit the contraction of the one, leave the central nervous system with the nerves which cause the other to contract. Thus the inhibitory and motor nerves of either the abductor (opener) or adductor (closer) muscles of the crayfish claw do not leave the central nervous system together, but in separate nerves.

If now for some cause the one set of muscles either disappeared, or were so altered as no longer to present any appearance of antagonism, then there would be left a single set of muscles, the inhibitory and motor nerves of which would leave the central nervous system at different levels, and the older such systems might be, the greater would be the modification in the shape and arrangements of parts in the animal, so that the two sets of fibres might ultimately arise from very different levels.

As mentioned in the introductory chapter, the whole of this investigation into the origin of vertebrates arose from my work on the system of efferent nerves which innervate the vascular and visceral systems. One of the main points of that investigation was the proof that such nerves did not leave the central nervous system uniformly along the whole length of it, but in three great outflows, cranial, thoracico-lumbar, and sacral; there being two marked gaps separating the three outflows, caused by the interpolation of the plexuses for the innervation of the anterior and posterior limbs respectively. All these nerves are characterized by the presence of ganglion-cells in their course to the periphery, they are, therefore, distinguished from ordinary motor nerves to striated muscle in that their impulses pass through a ganglion-cell before they reach the muscle.

The ganglia of the large middle thoracico-lumbar outflow constitute the ganglia of the sympathetic system.

The functions of the nerves constituting these three outflows are very different, as I pointed out in my original papers. Since then a large amount of further information has been obtained by various observers, especially Langley and Anderson, which enable the following statements to be made:—

All the nerves which cause contraction of the unstriped muscles of the skin, whether pilomotor or not, all the nerves which cause secretion of sweat glands wherever situated, all the nerves which cause contraction or augmentation of the action of muscles belonging to the vascular system, all the nerves which are motor to the muscles belonging to all organs derived from the Wolffian and Müllerian ducts, e.g. the uterus, ureters, urethra, arise from the thoracico-lumbar outflow, never from the cranial or sacral outflows. It is essentially an efferent skin-system.

On the other hand, the latter two sets of nerves are concerned with the supply of motor nerves to the alimentary canal; they form essentially an efferent gut-system in contradistinction to the sympathetic or skin-system.

A marked distinction exists between these cranial and sacral nerves. The vagus never supplies the large intestine, the sacral nerves never supply the small intestine. Associated with the large intestine is the bladder, the whole system arising from the original cloacal region; the vagus never supplies the bladder, its motor nerves belong to the sacral outflow. The motor nerves to the ureters, to the urethra, and to the trigonal portion of the bladder between the ureters and the urethra, do not arise from the sacral outflow, but from the thoracico-lumbar. These muscles belong really to the muscles in connection with the Müllerian and Wolffian ducts and skin, not to the cloacal region.

The motor innervation then of the alimentary canal reveals this striking and suggestive state of affairs. The motor innervation of the whole of the small intestine arises from the cranial region, and is immediately followed by an innervation from the sacral region for the whole of the muscles of the cloaca. It thus indicates a head-region and a tail-region in close contiguity, the whole of the spinal cord region between these two extremes being apparently unrepresented. Not, however, quite unrepresented, for Elliott has shown recently that the ileo-colic valve at the junction of the small and large intestine is in reality an ileo-colic sphincter muscle, and that this muscle receives its motor nerves neither from the vagus nor from the sacral nerves, but from the thoracico-lumbar outflow or sympathetic system. This may mean one of two things, either that a band of fibres belonging to the skin-system has been added to the gut-musculature, for the purpose of forming a sphincter at this spot, or that the region between the vagus territory and the cloaca is represented by this small band of muscle. The second explanation seems to me the more probable of the two. Between the mesosomatic region represented by the vagus, and the cloacal region, there existed a small metasomatic region, represented by the pronephros, with its segmental duct, as already discussed in Chapter XII. That part of the new alimentary canal which belonged to this region is the short piece indicated by the ileo-colic sphincter, and innervated, therefore, from the same region as the organs derived from the segmental duct.

Such innervation seems to me to suggest that originally the vertebrate consisted, as far as its gut was concerned, of a prosomatic and mesosomatic (branchial) region, close behind which came the cloaca and anus. Between the two there was a short metasomatic region (possibly pronephric), so that the respiratory chamber did not open directly into the cloaca.