Ganglion cell

“ These ganglion cells extend each of them an axone process which attains the organ (muscular cell or gland cell) , which it is the office of the sympathetic path to reach and influence. The axone-process of the first nerve cell is a myelinated nerve-fibre extending from the spinal cord to the ganglion; it constitutes the pre-ganglionic fibre of the conduction chain. The axone-process of the second nervecell, that is the neurone whose cell-body lies in the ganglion, is usually non-myelinate and constitutes the post-ganglionic fibre of the chain. ”

“ In short we may (with Enriques) lay it down as a general law that among animals, whether large or small, the ganglion-cells vary in size within narrow limits; and that, amidst all the great variety of structural type of ganglion observed in different classes of animals, it is always found that the smaller species have simpler ganglia than the larger, that is to say ganglia containing a smaller number of cellular elements63. The bearing of such simple facts as this upon the cell-theory in general is not to be disregarded ”

“ The most various individual motor apparatuses are connected with the most various individual motor ganglion cells. Sensations are certainly collected in definite ganglion cells. Still, among them, too, we do not find any single ganglion cell which can be in any way designated the centre of all sensation, but we again meet with a great number of very minute centres. All the operations which have their source in the nervous system, and there certainly are a very great number of them, do not allow us to recognise a unity anywhere else than in our own consciousness. ”

“ I I, pr) consists, as before, of nearly circular cells, each containing a nucleus, very large in proportion to the size of the cell. The cells have a diameter of about 1/3000 of an inch. This mass forms not only the junction between the ganglion and the spinal canal, but is also continued into a layer investing the outer side of the ganglion and continuous with the nerve beyond the ganglion.
The cells which compose the ganglion (fig. I I, sp.g) are easily distinguished from those of the root. Each cell is elongated with an oval nucleus, large in proportion to the cell ”

“ A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The evolution of the ganglion-cells, is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells ”

“ The meaning of a ganglion is, that it is a group of nerve cells dispersed among, or in continuation with, nerve fibres: it may be a centre of reflection, or it may not; and in the latter case its physiological office is at present undetermined. A ganglion is no more a centre in virtue of its cell-group than a muscle is a limb. All function depends on connection, and central function demands a connection of afferent and efferent parts. ”

“ In the first (1) we have the epithelium of the nerve (scn in Fig. 47) in surface view, the cells appearing polygonal in outline, with here and there a mucous cell. In (2) we find a very slight layer of ganglion cells and fibres having a transverse direction (gc and fp in Fig. 47) . These are continuous with the plexus of fibres and ganglion cells which lie above the muscle layer all over the subumbrella, and which represent the motor part of the nervous system. This connection with the nerve shows how co-ordination is effected. ”

“ Considerable differentiation has already set in in the constitution of the ganglia themselves, which are composed of an outer mass of ganglion cells enclosing a kernel of nerve fibres, which lie on the inner side and connect the successive ganglia. There are still distinct thoracic and abdominal ganglia for each segment, and there is also a pair of separate ganglion for the cheliceræ, which assists, however, in forming the œsophageal commissures. ”

“ An accurate radial section of an arm shows that there is, corresponding to the interspaces between the two vertebrae, a ganglionic swelling of the nerve-cord. As in Asteroids, there are not only ectodermic ganglion-cells on the under surface of the cord abutting on the epineural canal, but also coelomic ganglion-cells derived from the floor of the radial perihaemal canal. Both these categories of cells are largely increased in number in the ganglion. From the dorsal-cells arise a pair of large nerves which pass directly up and supply the great intervertebral muscles. ”

“ Again, while it is true that no ganglia are known to exist in the margins of the Scyphomedusæ outside of the ganglia in the marginal bodies, yet, ganglion cells and nerve fibers are found in the subumbral part of the margin as well as in the rest of the umbrella. And as I know no reason why scattered ganglion cells may not function as ganglia, it is possible that the contractions in question were spontaneous. ”

“ These ganglion cells lie chiefly in the sub-pericardial tissue in the posterior wall of the auricles between and around the orifices of the venae cavae and pulmonary veins and between the aorta and pulmonary artery. The minute structure of these ganglia and the terminations of the nerves have been studied particularly by Dogiel. The inhibitory fibres arise from a centre in the spinal bulb which is in tonic action and constantly bridles the heart's action. When the vagi are divided the frequency of the heart increases and the blood pressure rises. ”

“ The conclusion was strengthened by experiments of the students of motor localization, which showed that the veritable centres of their discovery lie, demonstrably, in the gray cortex of the brain, not in the white matter. But the full proof came from pathology. At the hands of a multitude of observers it was shown that in certain well-known diseases of the spinal cord, with resulting paralysis, it is the ganglion cells themselves that are found to be destroyed. ”

“ This strip, becoming gradually smaller behind, reaches nearly, though not quite, the posterior limit of the junction of the ganglia. Behind it there is, however, a region where [Pg 891] the whole dorsal surface of the ganglia is without any covering of nerve-cells.
This tongue of ganglion cells sends in, slightly behind the level of the eyes, a transverse vertical prolongation inwards into the white matter of the brain, which is shewn in the series of transverse sections in fig. ”

“ Each organ has the shape of a biconvex lens, sunk to about half its thickness in the skin. The inner half is formed of radially-arranged gland tubes filled with small granular cells, and converging towards the centre of the organ. Into the connective-tissue walls of the tubes extend blood-vessels and nerves. External to the gland tubes there is a layer of long slender cells arranged perpendicularly to the surface, and more externally still a layer of ganglion cells. There is evidence that these organs multiply by division. ”

“ One of them consists of a series of paired bodies, situated on the branches of the dorsal aorta, segmentally arranged, and forming a chain extending from close behind the heart to the hinder end of the body cavity. Each body is formed of a series of lobes, and exhibits a well-marked distinction into a cortical layer of columnar cells, and a medullary substance formed of irregular polygonal cells. As first shewn by Leydig, they are [Pg 665] closely connected with the sympathetic ganglia, and usually contain numerous ganglion cells distributed amongst the proper cells of the body. ”

“ They apparently must lie imbedded in some clear, watery fluid that does not show in the preserved material. The processes of the epithelial cells give the fibres the appearance of lying in alveoli, or being divided into strands, and one of these strands (ax) is always discernible among the others by reason of its more numerous or finer or more compactly massed fibres. This is the “axis” of Claus. Here and there in its course appear ganglion cells having their long axis in the longitudinal direction of the nerve. ”

“ In other words, the ganglion-cells of this Medusa are able to originate two very different kinds of impulse, according as they liberate their energy spontaneously or in answer to direct stimulation, and the muscular tissues respond with a totally different kind of contraction in the two cases. Possibly, indeed, direct stimulation of the ganglia is followed by a spasm of the muscular tissue only because a greater amount of ordinary ganglion influence is thus liberated than in the case of a merely spontaneous discharge. ”

“ Besides the eyes and the ampulla already mentioned, a concretion fills the lowermost part of the club, and a group of large cells, having a network-like structure and called network cells by Conant, fill the uppermost part of the club between the proximal complex eye and the attachment of the club to its peduncle (Plate II, Fig. 13) . What is evidently nerve tissue, fibers and ganglion cells, fills the rest of the club, with two groups of large ganglion cells disposed laterally from the network cells. A sensory (flagellate) epithelium covers the club. ”

“ In Bdelloida a second ganglion is present below the pharynx, and is connected with the former by lateral cords which contain ganglion cells. From the brain, nerves are given off to the disc, to the muscles, and to the integument of the body, as well as to the sense organs. The largest nerves are two given off from the sides of the brain, each of which divides into a lateral and a ventral trunk, which run nearly the whole length of the animal. ”

“ Fibres from these ganglia go to the skin, and bring to the nerve cells information from the outside world, which they duly pass on to the cells of the central column. The cells of the central column, when set in motion by the ganglion cells, send out impulses to the muscles, whose contraction is necessary to perform the movement which circumstances indicate. A movement brought about in this way is called reflex. ”

“ The most interesting features of my sections of the anterior bodies are the relations they bring to light between the sympathetic ganglia and the suprarenal bodies. In the case of one of the posterior suprarenal bodies, a small ganglion is generally found attached to both ends of the body, and invested in the same sheath; in addition to this a certain number of ganglion cells (very conspicuous by their size and other characters) are to be found scattered through the body. In the anterior suprarenal bodies the development of ganglion cells is very much greater. ”

“ If we follow the course of the motor nerves in a {103} direction away from the muscles to which they are distributed, they will be found, sooner or later, to terminate in ganglia (fig. 24 A, gl.c; fig. 25, gn. 1–13) . A ganglion is a body which is in great measure composed of nerve fibres; but, interspersed among these, or disposed around them, there are peculiar structures, which are termed ganglionic corpuscles, or nerve cells (fig. 24, B) . ”

“ In respect to the nervous system, there are no diagnostic characters; there are, however, not as many as two pairs of ganglia to a segment. The brain is well developed, sending a pair of slender nerves to the small eyes. The ganglia of the segment bearing the first pair of legs is fused with the subœsophageal ganglion. Grassi was unable to detect a true sympathetic system, but he suspects the existence of a very small frontal ganglion.
The slender dorsal vessel, provided with ostia and valvules, pulsates along the entire length of the trunk; an aorta passes into the head. ”

“ The nervous system is formed on the dorsal side of the embryo before the above projections make their appearance, but, though it seems probable that it originates in the same manner as in the more normal forms, its development has not been worked out. As soon as it is formed it consists of a nervous ganglion similar to that usually found in the adult. The history of the mass of mesoblast cells has been inadequately followed, but it continuously disappears as the heart, excretory organs, muscles, etc. become formed. ”

“ The notochord, nerve-tube, muscles, etc., are withdrawn into the body, where they break down and are absorbed by phagocytes. The posterior part of the nerve cord and its anterior end with the large sense-organs disappear, and the middle part or trunk-ganglion is reduced to form the relatively small ganglion of the adult, underneath which the hypophysial tube gives rise to the neural gland. While the locomotory, nervous and sensory organs are thus disappearing, or being reduced, the alimentary canal and reproductive viscera are growing largely. ”

“ The ganglia of Nucula are easily studied in section. The cerebral mass seems composed of one main mass, partly divided into four subdivisions, the two central most completely fused, and the lateral quite distinct in places. The central portion might represent the cerebral ganglia and the lateral, the pleural if we take that interpretation. The pedal ganglion is made of right and left parts quite completely fused except at the margins. The pedal mass is the smallest of the three chief ganglionic areas. The visceral ganglia are quite widely separated and a little larger than the pedal mass. ”