Outlines of zoology / by J. Arthur Thomson.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of zoology / by J. Arthur Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    59/748 (page 37)
    and how they gradually become less superficial, we shall see in our systematic studies. Structure.—Let us consider first the ganglionic cells which receive stimuli and shunt them, which regulate the whole life of the organism, and are the physical conditions of spontaneous activity and in- telligence. The simplest are prolonged at one pole into an outgrowth which branches into an afferent and efferent nerve-fibre. Most, how- ever, give off outgrowths from two poles or on all sides. Internally they exhibit a kernel or nucleus, and they consist in great part of a network or coil of fine fibrils. Within ganglia the ganglionic cells usually lie embedded in a fibrous cellular substance called neuroglia, which most histologists regard as an ensheathing and supporting material. In all but a few of the simplest multicellular animals, the nerve-fibres are surrounded by a sheath called the neurilemma, which is said to be formed by adjacent connective tissue. Several nerve-fibres may combine to form a nerve, but each still remains ensheathed in its neurilemma. In vertebrate animals, each nerve-fibre usually consists of an internal axis cylinder, the important part, and an external unessential medullar)' sheath whose texture suggests fat. But even in the higher Vertebrates, non-medullated or simply-contoured nerve-fibres are found in the sympathetic and olfactory nerves, and this sim]5ler type alone occurs in hagfish, lamprey, and lancelet, as well as in all the Invertebrates with distinct nerves. Furthermore, it should be noted that nerves are usually surrounded by an enveloping nucleated layer called Schwann's sheath, or else by neuroglia. Careful preparation of a nerve-fibre shows that it consists of numerous fibrils like those seen within a ganglion-cell. These are usually regarded as the essential elements in conducting impressions, but some maintain, whether rightly or wrongly I am not able to judge, that the essential part is the less compact, sometimes well-nigh fluid stuff Isetween the fibrils, or that the fibrils are but the walls of tubes within which the essentially nervous stuff lies. As in other cases, the microscopic morphologists discover intricacies in regard to the import of which physiological conclusions are hardly possible. But you may reasonably ask what these nerve-fibres are. I do not think that any one can at present give a decisive answer. According to most authorities, they are extensive prolongations of the ganglion-cells, and there is no doubt that the nerves of Vertebrates grow from the central system outwards. But to others it seems ]ilausii5le that the neuroglia or other ensheathing elements contrilnite to the extension of the nerve-fibres, or rather that special cells make both sheath and fibre. It is possible that both theories are right.