LESSON XVIII.

STRUCTURE OF NERVE-CELLS.

small piece of a spinal Notice the spheroidal Look for cells which

1. TEASE in Farrant's solution or in dilute glycerine a ganglion that has been preserved with osmic acid. ganglion-cells; their large nuclei and distinct nucleoli. still retain the axis-cylinder process and for T-shaped junctions of nervefibres with this.

2. Prepare a piece of sympathetic ganglion in the same way. Cells may be found with three or more axis-cylinder processes. If from a rabbit observe that the cells are bi-nucleated.

3. Mount stained sections of ganglia in Canada balsam. These will serve to show the arrangement of the cells and fibres in a ganglion and the nucleated sheaths around the nerve-cells.

4. Tease out a portion of the grey matter from a piece of spinal cord that has been a day or two in dilute chromic acid (per cent.). Before covering, look for the nerve-cells with a low power, and if possible get out one or two clear of the surrounding substance. Mount in water with a thick hair under the cover-glass. Notice the large branching cells some with a mass of pigment near the nucleus. Observe the fibrillation of the cell-processes. Notice also the reticular character of the tissue in which the cells are embedded. Many axis-cylinders will be seen in this preparation deprived wholly or partially of their medullary sheath, and their fibrillar structure can then also be well seen. Carefully sketch these appearances. To keep this preparation run very dilute logwood solution or osmic acid under the cover-glass, and when the cells are stained allow a drop of glycerine to pass in by diffusion. Measure two or three cells in each of the above preparations.

Nerve-cells only occur in the grey matter of the nerve-centres, and in little groups on the course of certain of the peripheral nerves, these groups often causing nodular enlargements of the nerves, which are known as ganglia. The most important ganglia are those which are found upon the posterior roots of the spinal nerves, upon the roots of some of the cranial nerves, and upon the trunk and principal branches of the sympathetic nerve. Minute ganglia are also found very numerously in connection with the nerves which are supplied to involuntary muscular tissue, as in the heart, alimentary canal, bladder, uterus, &c.

Nerve-cells vary much in size and shape; they are mostly large, some being amongst the largest cells met with in the body, but others are quite small. The nucleus is generally large, clear, and

spherical, with a single large and distinct nucleolus. The shape depends a good deal on the number of processes, and the manner in which they come off from the cell. If there is but one process, the cell is generally spherical. This is the case with the cells of the

FIG. 91.-CELL FROM A SPINAL GANGLION.

sh, nucleated sheath of the cell; n, n', the nerve-fibre which the single process of the cell, after a number of coils, joins.

spinal ganglia (fig. 91); in these the single process, after a short course, joins one of the nerve-fibres which is traversing the ganglion. When there are two processes, they often go off in opposite directions

from the cell, which is thus rendered somewhat spindle-shaped, but occasionally they emerge at the same part, and the cell, being tapered in their direction, becomes pyriform (fig. 92). In these cases one fibre often coils spirally round the other (fig. 92, b) before they separate to proceed in opposite directions as the axis-cylinders of nerve-fibres. When there are three or more processes, the cell becomes irregularly angular or stellate. Sometimes, as in the sympathetic ganglia (fig. 93), all the processes appear to become nerve-fibres, but in other instances, as in the large cells of the grey matter of the spinal cord, only one

FIG. 94.-NERVE-CELL FROM SPINAL CORD OF OX, ISOLATED AFTER MACERATION IN VERY DILUTE CHROMIC ACID. (Magnified 175 diameters.)

The cell has a well-defined, clear, round nucleus, and a bright nucleolus. The cell processes are seen to be finely fibrillated, the fibrils passing from one process into another through the body of the cell. a, axis-cylinder process broken a short distance from the cell.

process becomes the axis-cylinder of a nerve-fibre (process of Deiters), the others dividing and subdividing in a ramified manner until their further course can no longer be traced. Their ultimate branches appear to lose themselves in a network which pervades the whole of the grey matter.

According to the number of their processes, nerve-cells are termed uni-, bi-, or multi-polar.

Many nerve-cells, and notably those of the spinal cord, have a finely fibrillar structure. The fibrils can be traced into the branches of the cells and into the axis-cylinders of nerve-fibres which are connected with the cells (fig. 95). Otherwise the cells have a finely granular appearance; often with a clump of black, brown, or yellow pigmentgranules placed at one side of the nucleus.

In the ganglia the nerve-cells have a nucleated sheath (figs. 9193) which is continuous with the primitive sheath of the nerve-fibres with which they are connected. In the spinal ganglia, and in many of the ganglia at the roots of the cranial nerves, the cells are unipolar,

and the cell-process joins a traversing nerve-fibre by a T-shaped junction (fig. 91). In the sympathetic ganglia they are multipolar. The cells are disposed in aggregations of different size, separated by the bundles of nerve-fibres which are traversing the ganglion (fig. 96).

b

The ganglion if large is enclosed by an investing capsule of connective tissue which is continuous with the epi- and peri-neurium of the entering and issuing nerve-trunks.

The structure of the nerve-centres and the arrangement of the cells and fibres in them are given in Lessons XXXVI. to XXXIX.

FIG. 96.-LONGITUDINAL SECTION THROUGH THE MIDDLE OF A GANGLION ON THE POSTERIOR ROOT OF ONE OF THE SACRAL NERVES OF THE DOG, AS SEEN UNDER A LOW MAGNIFYING POWER.

a, nerve-root entering the ganglion; b, fibres leaving the ganglion to join the mixed spinal nerve; c, connective tissue coat of the ganglion; d, principal group of nerve-cells, with fibres passing down from amongst the cells, probably to unite with the longitudinally coursing nerve-fibres by T-shaped junctions.

Development. The cells and fibres of the nervous tissue are developed from cells derived from the ectoderm or epiblast of the embryo. The nerve-fibres are at first developed as pale fibres like the fibres of Remak; it is uncertain whether they are formed by the coalescence of a number of cells, or whether they grow out as the processes of nerve-cells. The medullary sheath is subsequently added.

When a nerve is cut, the fibres beyond the section as far as their terminations undergo a process of degeneration, the medullary sheath being broken up and the axis-cylinder interrupted and eventually absorbed. New nerve-fibres are at length produced by a growth of the axis-cylinders in the proximal end of the nerve.