166. In alcoholic preparations of mammalian muscle, the crossstriation is clearly seen, and is intensified by staining with hematoxylin. This stain colors everything anisotropic in the muscle, but does not affect the remaining structures. Similar results may be obtained with other stains, such as basic anilin dyes, but not with the same precision as with hematoxylin.

167. A certain species of beetle (Hydrophilus) is admirably adapted for the study of the finer details of striation. The beetle is first wiped dry and then immersed alive in 93% alcohol. On examining in dilute glycerin after from twenty-four to forty-eight hours, the substance of its muscles will show disintegration into Bowman's discs (vid. p. 128). The latter swell up in acids and are finally dissolved, as may be seen, by adding a drop of formic acid to a specimen prepared as above (Rollet, 85).

168. In order to study the relation of muscle to tendon, small muscles with their tendons are put into a 35% potassium hydrate solution for a quarter of an hour, after which the specimen is placed upon a slide and teased at the line of junction of the two tissues. This will separate the muscle-fibers from their respective tendon-fibrils (Weismann).

169. Similar results may be obtained by immersing a frog in water at a temperature of 55° C., in which the animal soon dies with muscles perfectly rigid. As soon as the water begins to cool (one-quarter hour) the frog is removed and a small piece of its muscle cut out and teased in water on a slide (Ranvier).

170. Cardiac muscle-cells are isolated by maceration for twenty-four hours in a 20% solution of fuming nitric acid (potassium hydrate with a specific gravity of 1.3 will do the same in one-half or one hour). The margins of the cells may be brought more clearly into view by placing pieces of heart muscle for twenty-four hours in a 0.5% aqueous solution of silver nitrate and then cutting into sections.

171. Isolated fibers of Purkinje are obtained by immersing pieces of endocardium (0.5 mm. in size) in 33% alcohol and then teasing them on a slide. The sheep's heart is especially well adapted for this purpose.

172. Nonstriated muscle-fibers are isolated in the same way as heart muscle. In thin cross-sections (under 5 μ in thickness) of intestinal muscle, preferably of a cat, fixed in osmic acid, the intercellular bridges may be seen here and there between the fibers.

D. THE NERVOUS TISSUES.

The entire nervous system, peripheral as well as central, is composed of cells possessing one or many processes. These cells develop early in embryonic life from certain ectodermal cells (neuroblasts) of the neural canal, which is formed by a dorsal invagination of the ectoderm. The neuroblasts soon develop processes,-many of them in loco, others only after wandering from the neural canal. The processes of the nerve-cells are of two kinds: (1) unbranched processes having a nearly uniform diameter throughout,

with lateral offshoots known as collateral branches; these, as we shall see, generally form the central part of a nerve-fiber, and are known as neuraxes (Deiters' processes, axis-cylinder processes, neurites, neuraxones or axones); and (2) processes which branch soon after leaving the cell-body and break up into many smaller branches; these are the dendrites, or protoplasmic branches. In the spinal ganglia and the homologous cranial ganglia these morphologic differences in the processes are not observed, the neuraxis and the dendrites of each presenting essentially the same structure.

To the entire nerve-cell, cell-body and processes the term neurone (Waldeyer, 91) has been applied; neura (Rauber), or neurodendron (Kölliker, 93).

The neuraxes of many neurones attain great length. Those of some of the neurones, the cell-bodies of which are situated in the lower part of the spinal cord, extend to the foot. In other regions neuraxes nearly as long are to be found, and in the majority of neurones the neuraxes terminate some distance from the cell-body. It is therefore manifestly impossible in the majority of cases to see a neurone in its entirety. Usually, only a portion of one can be studied. in any one preparation. Consequently, the more detailed description which follows will deal with the neurone in this fragmentary manner. The cell-bodies of the neurones, to which the term "nerve-cells" or "ganglion cells" is usually restricted, the dendrites and neuraxes, often forming parts of nerve-fibers, and their mode of terminating, will receive separate consideration.

NERVE-CELLS, OR GANGLION CELLS; THE CELL-BODIES OF NEURONES.

The cell-bodies of nerve-cells are usually large. The bodies of the motor neurones of the human spinal cord measure 75 to 150 μ, their nuclei 45 μ, and their nucleoli 15 . The smallest nerve-cells, the neurones of the granular layer of the cerebellum, are 4 to 9 u in diameter. The protoplasm of nerve-cells shows a distinct fibrillar structure and the fibrils may be followed into the processes. (Fig. 102.) Their nuclei are also large, with very little chromatin, but as a rule are supplied with a large nucleolus.

After treatment by certain special methods, the protoplasm of the ganglion cells shows granules or groups of granules which show special affinity to certain stains, consequently known as chromatophile granules; these are densely grouped around the nucleus, so that the cell-body shows an inner darker and an outer lighter portion. These chromatophile granules, also spoken of as tigroid granules or as the tigroid substance, as a rule are not arranged in concentric layers, but lie mostly in groups, giving to the protoplasm a mottled or reticular appearance. In the cells of the anterior horns (man, ox, rabbit) the granules join to form flakes, which are also more numerous in the region of the nucleus. In all cases the

granules or flakes are continued into the dendrites of the cell. Here they change their shape into long pointed rods, with here and there nodules, which are probably the chief causes of the varicosities so often seen in dendrites (Golgi's method). The cell usually has a clear, nongranular peripheral border (not a membrane), and in the case of large cells there is a similar area around the nucleus, the inner border of which belongs to the nuclear membrane. H. Held has found that the chromatophile granules are brought out by treatment with alcohol and acid fixing fluids, but not in alkaline or neutral. They appear, according to the treatment, as fine or coarse granules. They can not be seen in fresh nerve-cells. He consequently regards them as artefacts-precipitations of the protoplasm due to reagents (vid. A. Fischer, T. 124). At its junction with the cell the neuraxis spreads out into a cone which is entirely free from granules, and apparently fitted into a depression in the granular substance of the cell (implantation cone).

The cellular substance between the chromatophile granules consists also of very fine, highly refractive granules, which appear to be arranged in a reticulum surrounding the chromatophile granules

Nucleus.

Nucleolus.

Fibrillar structure.
Medullary sheath.

Fig. 102.-Bipolar ganglion cell from the ganglion acusticum of a teleost (longitudinal section). The medullary sheath of the neuraxis and dendrite is continued over the ganglion cell; X800. Technic No. 175.

(vid. Nissl, 94, and v. Lenhossék, 95), and the recent observations of Apáthy and Bethe make it very probable that in the intergranular substance of the protoplasm of the nerve-cell there exist very fine fibrils which may be traced into the processes of the cell. It requires, however, further observation before more positive statements may be made concerning them.

Besides the granules above mentioned, and which are revealed by special methods, there are found in the protoplasm of many of the larger nerve-cells pigment granules of a yellow or brown color which stain black with osmic acid.

The dendrites are usually relatively thick at their origin, but gradually, as a result of repeated divisions, taper until their widely distributed arborescent endings appear as minute threads of widely different shapes. When treated by certain methods, they present uneven surfaces studded with varicosities and nodules, in contradistinction to the neuraxes, which are smooth and straight. Their terminal branches end either in points or in small terminal thickenings. The groups of terminal end-branches of a dendrite (also of a neuraxis) are known as telodendria (Rauber), or end-branches. The

branches of the dendrites form a dense feltwork, which, together with the cell-bodies of the neurones and with other elements to be described later, constitute the gray substance (gray matter) of the brain and spinal cord.

All neurones, with possibly a few exceptions, possess only a single neuraxis. Neurones without a neuraxis have never been found in vertebrates. The neuraxis usually arises from a coneshaped extension of the cell-body free from chromatophile granules, the implantation cone, more rarely from the base of one of its dendrites, or from a dendrite at some distance from the cell-body. Its most important characteristics are its smooth and regular contour and its uniform diameter. At some distance from the cellbody, usually near its termination, now and then in its course, a neuraxis may divide into two

equal parts. Golgi (94) called attention to the fact that the neuraxes of certain neurones (Purkinje's cells in the cerebellum, pyramidal cells of the cerebral cortex, and certain cells of the spinal cord) give off lateral processes, the collateral branches.

a b

Nucleus.

Implantation cone.

cone.

Fig. 104.-Nerve-cell from the anterior horn of the spinal cord of an ox, showing coarse chromatophile flakes.

Two types of cell are recognized according to the disposition of their neuraxes : In the first the neuraxis is continued as a nervefiber; in the second and rarer type it does not long preserve its independence, nor is it continued as a nerve-fiber, but soon breaks up into a complicated arborization, the neuropodia of Kölliker (93). The latter type of cell occurs in the cortex of the cerebrum and cerebellum and in the gray matter of the spinal cord. The cells of the two types can be simply described as having long (type I) or short, branched neuraxes (type II). The neuraxes of the cells of type I possess the collateral branches which end in small branching tufts.

In its simplest form, a neurone consists of a cell-body and a neuraxis with its telodendrion. In more complicated types one or several

dendrites may be present, as also collaterals from the neuraxis, and in rare cases even several neuraxes. According to the number of its processes, a ganglion cell is known as unipolar, bipolar, or multipolar.

Fig. 105.-Motor neurones from the anterior horn of the spinal cord of a new-born cat. Chrome-silver method.

Although neurones present a great variety of morphologic differences,-large and variously shaped cell-bodies or small ones scarcely larger than the nucleus; large and numerous dendrites or

Telodendrion.

Dendrite.

Cell-body.

Neuraxis.

Fig. 106.-A nerve-cell with branched dendrites (Purkinje's cell), from the cerebellar cortex of a rabbit; chrome-silver method; X 125.

few and less conspicuous ones, and although these various forms are widely distributed and intermingled in the different parts of the nervous system, yet in many regions there are found nerve-cells of fixed and characteristic morphologic appearance, which would