2. Motor impulses travel from the upper to the lower segments of the cord in the white fibres around the anterior gray columns.

Diagram illustrating the course taken by the fibres in the spinal cord. (After Fick.) A, B and C represent oblique transverse sections of the cord, the tissue between being supposed to be transparent. At the lowest section (C), sensory nerve fibres (a) enter by the posterior root, and are connected with ganglion cells of the gray matter, and, through the posterior white column, with the brain (b).

Impulses arriving by the same posterior root may, to reach other parts, traverse the finer fibrils of the gray matter-shown by the fine lines.

When an impulse comes directly from the brain (voluntary centres) it adopts the direct routes (c or e), passing through the pyramidal tracts, to excite the motor ganglion cells of the cord to coördinated activity.

From many parts of the gray matter ganglion cells despatch impulses by the motor root (d).

Some white fibres only communicate between the cells of the various segments of the gray matter ().

3. Various afferent impulses cross at once on entering the cord to the posterior gray columns of the other side, and then ascend

by the neighboring white fibres of the posterior root zones, the direct cerebellar tract, the posterior median tract of Goll, and probably also by some of the white channels of the lateral column.

4. Besides their numerous thin protoplasmic connections in the various segments of the gray matter, all the cells of the cord are in communication with their more distant neighbors by means of the white fibres of the root zones.

SPINAL CORD AS A COLLECTION OF NERVE CENTRES.

In the gray substance there is still greater difficulty in tracing the course taken by the various kinds of impulses, and little is known on the subject beyond what is surmised from the proximity of the different parts to the anterior and posterior roots and to the white channels the function of which is known.

Though the attempt to localize the different functions to any anatomical region has not met with success, histology has taught us of the existence of certain groups of cells which, when viewed longitudinally, may be called vesicular columns. Of these, four may be named as distinctively marked. (Fig. 247.)

1. The anterior (motor) cellular columns occupy the gray matter seen in sections of the cord as the anterior cornua. They extend throughout the entire length of the cord, the cells being specially numerous where the large motor roots come off. The cells are characterized by their great size and the number of their branches, one of which forms the axis cylinder of one of the motor fibres passing to the roots of the spinal nerves.

2. The posterior cellular columns situated in the gray matter of the posterior cornua are much less obvious than the anterior. The cells are few, small and mostly spindle-shaped. Their pro

cesses are not readily traced to the roots of the spinal nerves.

3. The postero-median cellular column (Clarke) lies on the median side of the posterior gray column, so that it forms the inner part of the posterior cornua near its base. The cells are numerous, but much smaller than those of the anterior vesicular column. Clarke's column is best developed at the junction of the lower dorsal and upper lumbar nerves. It tapers off above and

below, and the cells cease to form a continuous column opposite the seventh cervical nerve. But scattered groups of cells in a corresponding position are found throughout all the cervical cord, and seem to link this spinal column with the vagus nucleus in the medulla.

4. The intermedio-lateral cellular columns lie in the lateral concavities seen on section between the anterior and posterior gray cornua. They thus occupy a position between the lateral white column and the central part of the gray matter. They are best marked in the dorsal region, as they seem fused with the cells of the anterior cornua in the lumbar and cervical enlargements.

The facts that cells functionally related are grouped in masses at the points where the spinal nerves arise, and that the various regions of the cord can respond to stimulus when severed from the rest, seem to indicate that a strict homology exists between the spinal centres of vertebrate and the central nervous system in many of the lower animals, which consists of a double chain of ganglia, united together by conducting channels.

We may then suppose the gray matter of the spinal cord to be made up of a series of segments, corresponding in number to the vertebral development, fused together into one continuous organ. These segments may be supposed to receive the afferent impulses from corresponding parts of the body, and send efferent impulses to muscles capable of moving that part, just as the separate ganglia of the invertebrate chain preside over the functions of the corresponding somite of the animal's body.

The various groups of cells in the spinal cord are in more or less direct union with the roots of the nerves and the conducting fibrils of the cord itself, so that they participate in the transmission of the impulses to and from the centres situated in the brain. In the transmission of these impulses the cells seem to have a certain directing and controlling influence which deserves special attention, as it gives us the key to the more complex mechanisms of the higher centres. Although the various powers exerted by the cells of the spinal cord are so intimately associated together as to be practically inseparable, it is found convenient to consider their functions under distinct headings.

REFLEX ACTION.

When an afferent impulse arrives at the cells of the posterior column, it is communicated to the cells in the same segment, and reaching motor cells it gives rise to a movement of the muscles of the neighborhood from which the impulse first started. At the same time impulses travel to the brain, and there give rise to a consciousness of the various events taking place, i. e., a local stimulation and a local movement. The action of the cells of the cord takes place without the aid of the will, and occurs before the mind is conscious of it. These movements, being a turning back of the impulse, are called reflex acts.

Reflex action forms the most ordinary function of the cells of the spinal cord. Even the gentlest stimulation may give rise to a complex movement, the execution of which requires many muscles to act together, as it were, with a common object. An unexpected touch to the finger causes a person to withdraw the hand quickly. If greater or more prolonged stimulus be applied, more extensive movements occur; by the well-arranged coöperation of many muscles, a forcible, definite and familiar action is performed. For example, if the burning head of a match adhere under the thumb nail, more than a mere withdrawal of the hand takes place. The entire arm is violently shaken, before the will has time to come into operation. We have here a complex form of purposeful muscular movement, the immediate result of an impulse coming from a single point of the skin, owing to the spreading of the impulse to the cells of the segments in the vicinity. The movement is regular, performed with a definite purpose, as if it were the result of thought, but since there is no consciousness, it cannot be mental.

If the degree of stimulation be carefully regulated, it will be found that the results obtained by peripheral stimulation depend on (a) the strength of the stimulus, and the length of time for which it is applied; (b) the degree of excitability of the cells, and the readiness with which the impulses pass along the thin, conducting channels to the gray matter, and (c) the functional, activity of the muscles which act as indicators of the reflex effects.

All these points may be easily studied on a frog decapitated about an hour beforehand. If the animal be suspended by the lower jaw and the toe touched, the foot is gently withdrawn. If the toe be smartly pinched, the entire limb is forcibly raised; with intense or prolonged stimulus both legs are violently moved. If a fragment of blotting paper, moistened in weak acid, be placed on the belly, in a position not easily reached by the foot, a complex series of movements follows. The muscular action is both elaborate and purposeful, and the movements of the headless animal might almost be called ingenious.

Strength and Duration of Stimulus.—By graduating the strength of the acid used to moisten a square millimetre of blotting paper, the following results are obtained: When very weak acid is employed only slight local and unilateral movement is caused. Stronger acid produces a series of reflex movements, spreading to several muscles on both sides of the body. If further strengthened, the movements become violent and more extended until the whole body is thrown into convulsion. The movements spread from the nerve cells to their neighbors, and then to those governing the corresponding muscles of the other side, in which, however, they are less marked than in those of the side stimulated.

Slight stimulation, when of short duration, not sufficient to produce immediate response, may, after a time, give rise to definite reflex action, as if the weak impulses arriving at the nerve cells in the cord were stored up until their sum sufficed to produce a definite reflex movement. This may be seen in animals whose nerve centres are intact, for the cells of more remote parts exercise a kind of checking influence on those in the region. receiving the stimulus, and thus the accumulative action (summation) comes more effectively into play. In the human subject, where slight visceral stimulations exist for a long time, this summation may be observed. In some of these cases, even without sensory appreciation of the local excitation, an amount of energy may be accumulated in the gray tracts of the cord, that will bring on the most extensive forms of reflex muscular moveThese movements differ often from the regular coördi

ment.