THE REFLEX PATHS.

65

eral sensory neuron, the connecting branch by the reflex. collaterals which ramify among the cells of the anterior horn. It is probable that only a part of the cells in the anterior horn are concerned in this reflex. We distinguish a short and a long reflex arc.

(a) The short reflex are consists of a collateral which passes directly from the posterior column through the posterior horn to the cell in the anterior horn (plantar reflex, patellar reflex, spinal reflexes).

(b) The long reflex are is formed by the reflex collateral splitting up about a cell in the anterior horn. From this cell an ascending and a descending branch, with several collaterals, pass to one or more motor ganglion cells which may be situated at various levels of the anterior horn and brain-stem. This gives the possibility of reflex movements being transmitted to more remote muscle groups. The fibers of the posterior longitudinal bundle appear to be concerned in this function (reflex transmission of sensory impulses to the movements of the eye).

The localization of some of the reflexes belonging to this group is seen in Plate 13. (For more detailed description see Part IV, 3.)

2. The Complicated Reflex Arcs.-We have very little definite knowledge of the course of these reflexes. The pharyngeal, nasal, and bronchial reflex are is composed of sensory fibers from the trifacial, glossopharyngeal, and pneumogastric nerves and the corresponding motor nerves, the pneumogastric and spinal accessory.

The conjunctival reflex is composed of fibers from the trigeminal and facial nerves.

The pupillary reflex is composed of fibers from the optic and oculomotor nerves (corpora quadrigemina-oculomotor nucleus). The reflex collaterals evidently pass from the sensory nerves to the corresponding motor nuclei. It is supposed that the cortex is also included in some of these reflex ares. We are still without any sufficient data for the localization of other important reflexes connected

with the auditory, optic, and other nerves. (For a detailed description of these reflexes see Part IV, 4.)

The reflexes for the functions of the bladder and rec

tum are situated in the sacral cord. motor paths for these organs.)

(See sensory and

(b) The Volitional Pathways.—Superimposed above the reflex arc, which is composed of the two peripheral neurons, there is a second arc, formed by the central sensory and motor neurons and their connecting fibers in the cerebral cortex. This arc effects the transmission of a conscious sensory impulse to a volitional motor act.

At the same time the central motor neuron exercises an inhibitory, and the sensory central neuron a controlling, influence on reflex processes.

All conscious processes are enacted in the cerebral cortex, in which the sensory nerve tracts end and the motor nerve tracts begin. The cerebral cortex also contains the connecting fibers between the motor and sensory portions of the cortex. It is improbable, although conceivable, that the sensory neuron fibers terminate directly about the motor ganglion cells; it is more likely that one or more neurons of another kind (transcortical) are interposed. (See the schema on Plate 17.)

A study of the development of the fetal and childish psychic activity affords an approximate idea of the character of conscious processes in the cerebellar cortex. Although it does not actually begin until after birth, yet a series of sensory and reflex processes are enacted even during intra-uterine life.

The first movements are undoubtedly reflex in character, since, as has been already stated, the reflex ares early assume a medullary sheath. Since the fibers in the tegmentum (fillet) become medullated long before those in the central motor tracts, it is evident that there is a primary transmission of sensory impressions from the entire periphery of the body to the cerebral cortex, especially the posterior central convolution and the parietal lobe, forming

THE VOLITIONAL PATHWAYS-IDEATION.

67

the basis for the volitional activity that develops later. The neurons and neuron complexes situated in these areas possess the peculiar property of permanently preserving these sensory impressions as memory pictures, and reproducing them later under certain conditions. In this they are assisted by the primary association processes (connections with other cortical neurons, and corresponding chemic and physical processes in the structure of the neurons?).

After the child is born, another set of memory pictures, produced by the sound-waves that reach the upper temporal convolution through the lateral auditory fillet, are stored up. In like manner also are stored up the visual impressions which pass through the optic radiation and are deposited in the occipital lobe, especially in the cortex of the cuneus and in the gray matter surrounding the calcarine fissure. In addition there is a similar storing-up of olfactory and gustatory impressions.

These groups of memory pictures, which, as we have seen, are localized in different portions of the cortex, are connected by means of the secondary association processes with each other and with memory pictures in the hemispheres both of the same (association tracts in the stricter sense) and of the opposite side (commissural fibers). Thus each individual psychic process is associated with a distinct complex of specially trained neurons, belonging both to the projection and association fibrillation; these associated and cooperating neuron articulations may be designated as neuron elements.

It is by means of these association processes between the neuron elements that ideation takes place. Every idea or concept consists of a collection of associated memory pictures. The higher associations form the ideas, and the connection between these higher associations gives rise to logical mental processes. The reproduction of this association is called a concept. Only a part of all the associations is actively engaged at any definite time, the other part remaining dormant. The organ of the intellect is,

therefore, the association fibrillation. The sum of the active associations plus the newly arrived sensory impressions make up the greater part of the contents of consciousness. Hence consciousness is subject to continual change.

It is probable that a considerable portion of the association fibers are concerned in what is known as subconsciousness (unconscious associations). In addition to the above-named sensory impressions there is another set of impressions which reach the cerebral cortex and which are worked up into unconscious associations. These include the muscular and articular impressions which are conducted toward the center in the early reflex processes. They are deposited in the central convolutions, probably in the immediate vicinity of the motor centers, and form the so-called innervation feelings, produced by the muscular activity of the various motor acts (kinesthetic memory pictures).

By the reproduction of these memory pictures of innervation-feelings which are deposited in postfetal life and connected with each other by unconscious associationsconscious volitional muscular activity is in some way made possible and subjected to the necessary control. A volitional movement is, therefore, merely the external expression of certain concepts. The path by which the impressions are transmitted from the endings of the sensory central neurons to those of the central motor neurons, which must be situated in the cortex, is the true analogue of the reflex collaterals of the peripheral neurons.

Perhaps an example will serve to illustrate the foregoing discussion. On Plate 17, Figure 3, we see the reflex are: the sensory portion, sp (brown), the reflex portion, r, the motor portion, mp (blue). Above the reflex are we see the central tract for conscious actions: the sensory portion (green), the connecting neuron (black), and the motor portion (red). The memory pictures of the special sense impressions and muscular sensations are deposited around the sensory

en) fibrillations in the cortex-that is to say, in the

commissural and association neurons of the cortex (black) which go to, and come from, other areas in the cortex.

If the toe is irritated, there is a reflex twitching in the corresponding leg (reflex arc from the brown neuron to the blue, reflex transmission in the lumbar cord); at the same time the sensory irritation is transmitted by means of the green neuron to the cortex, where it becomes associated with various mental pictures and produces the well-known sensation of pain. As a second irritation is threatened (association with recent optic impressions), the painful sensation sends a message through the motor tracts (red-blue) necessary for the movement of the leg, by means of the corresponding innervation feelings (paracentral lobule), which convey the impulse to the muscles they supply.

Among the various cortical activities that are set in motion in this way we are chiefly interested in the act of speech. As the child learns to speak the sound pictures of the words that it hears are deposited in the superior temporal convolution of the left hemisphere (word-sound center, sensory speech center). In order to understand the meaning of the sound pictures (their interpretation) the associative activity by which they are encompassed is

necessary.

The sensory speech center is connected by means of the fasciculus uncinatus with the motor speech center, which is situated at the base of the inferior frontal convolution on the left side (Broca's convolution). In this area, or at least in its immediate neighborhood, the innervation feelings (kinesthetic memory pictures) of all the muscular movements necessary for the act of speech (tongue, mouth, palate, larynx) are deposited by constant exercise and imitation. The production of these movements in response to an impulse from the sensory speech center ultimately leads to the complicated muscular acts necessary for the production of letters, words, and sentences. If by any accident the hearing of the child is destroyed, the power of learning to speak is necessarily lost and the child