These are the real actors in the nerve operations, while the fibres are merely their means of communicating with one another. One set of terminals is placed on the surface of the body and is adapted to the reception of the various external influences which are brought to bear on it from without by its surroundings. These receivers of extrinsic stimuli are necessarily much varied, so as to be capable of appreciating all the different kinds of stimulation presented to them. They are either distributed over the entire surface so as to meet with general mechanical and thermic changes, or they are further specialized for the reception of luminous, sonorous, odorous or gustatory impulses. In the latter cases the special terminals are collected into one part, and form complex organs, which will be described presently in the chapters on the special senses.
Another set of terminals is placed in the deeper textures, where they act as local distributing agents; such as the nerve plates on skeletal muscles, and the ganglionic networks in the wall of the intestine. In many instances, however, the exact mode of connection between the nerve and the protoplasm of the tissue elements, to which it bears impulses, has not been satisfactorily made out. In the remaining class of nerve terminals the cells are grouped together so as to form larger and smaller colonies, and more definitely deserve the name of nerve or ganglion cells. These are the central terminals, and are placed either in the cerebro-spinal axis, or in swellings of the nerves called sporadic ganglia.
Fig. 205. Tactile nerve endings, composed of small capsules, in which the black axis cylinder of the nerve (a), and (n) meets with many protoplasmic units.
Of these nerve cells there are many varieties, all of which have the following characteristics. The cells are of considerable size and have processes branching off from them, by means of which they communicate with the nerve fibres. These processes may be single or many, hence they are spoken of as uni-, bi-, or multi-polar cells, etc. The nucleus is commonly very distinct, and contains a well-marked nucleolus. The abundant protoplasm, which is usually contained in a delicate cell wall, is in direct connection with the axis cylinder of the nerve fibres, with which it communicates by means of thin strands of protoplasm that pass out from the cell by the processes. A delicate striation of the protoplasm may sometimes be recognized, indicating the course of the nerve fibrils as they run into the cells from the processes.
Fig.206. Multipolar cells from the anterior gray column of the spinal cord of the dog-fish (a) lying in a texture of fibrils; (b) prolongation from cells; (c) nerve fibres cut across. (Cadiat).