GENERAL DIVISION OF THE CONDUCTION PATHS.
The entire nervous system is built up of nervous units or neurones. With regard to their physiological tasks, the neurones may be divided into two chief groups, those which conduct impulses centrifugally and those which conduct centripetally.
• The centrifugal paths serve to convey impulses from the central nervous system to peripheral organs, especially to the organs of movements or the muscles. These may, in a general way, also be called motor paths. The centripetal paths, on the contrary, convey impulses coming from the periphery to the central nervous system. By means of them we receive information of what goes on in nature outside of our bodies (higher sense-nerves) ; they bring us, however, also information of the processes, which are taking place within all the organs of our own bodies, information of which we are in part conscious and in part unconscious, the latter impulses being continually active in regulating the most diverse functions of our bodies. The centripetal paths are also, in a general way, designated as sensory paths.
It is particularly to be noted, that, as a rule, more than a single neurone is con cerned in the constitution of the afferent and efferent paths, and that these are made up of two, three or several neurones in sequence. In this way, for example, the great cortico-muscular paths, by means of which voluntary movements of the musculature of the extremities are called forth, consist of two neurones. The first neurone extends from the motor cortical centre through the brain-stem to the spinal cord, where it ends within the gray substance of the anterior horn. The second neurone extends from the anterior horn of the cord to the. muscle. In like manner, the sensory path is composed of several neurones, which conduct impulses from the periphery, as for example the Integument of the leg, through the peripheral nerves, the spinal cord and the brain-stem to the sensory region. The first neurone conducts the impulse from the periphery to the spinal cord or to the posterior column nuclei, the second from the cord or the nuclei of the posterior column to the thalamus, and the third neurone arises in the thalamus and ends in the cerebral cortex. Owing to the insertion of further neurones, the entire make-up may become still more complicated, in this manner longer or indirect paths being formed in addition to the shorter direct ones. Since the motor and sensory paths
conduct impulses from the centre to the periphery and, conversely, from the periphery to the centre, that is similarly " project," these paths are also called projection tracts.
Two additional important connecting links, the association conduction and the reflex conduction, exist between the motor and sensory paths. They are established by means of intercentral tracts. Through the reflex conduction, a reflex movement, the reflex, is liberated without the accompaniment of psychic processes. This conduction is effected by the so-called reflex collaterals, although individual neurones, intercalated between the centripetal and centrifugal tracts, may also participate. Let us take as an example of a simple reflex, the patellar or the corneal reflex. The patellar reflex is manifested by a contraction of the quadriceps muscle and extension of the leg, in response to stimulation of the sensory nerves in the quadriceps tendon, as when, for example, the tendon is struck with the percussion hammer below the patella, while the leg is relaxed and dependent. This entire phenomenon is carried out by the following paths : the impulse is carried from the tendon of the muscle to the spinal cord, by way of the spinal ganglion, by the sensory or afferent nerves. On entering the spinal cord; the sensory fibre divides into an ascending and a descending branch, which branches subse quently end within the gray substance of the cord, or, as in the case of the ascending branch, first within the nuclei of the posterior column. Before dividing into these branches, however, the entering sensory fibre gives off a delicate collateral branch, a reflex collateral, which runs to the anterior horn and there ends. Similar reflex collaterals, moreover, are also given off from the ascending primary branch, as shown in Fig. 123, a. By means of these reflex collaterals, the impulse may be directly transferred to motor anterior horn-cells and thence conveyed by motor fibres to the muscle. In a similar manner the corneal reflex or tactile lid-reflex occurs. This reflex consists in con traction of the M. orbicularis oculi on touching the integument of the eyelid, the conjunctiva or the cornea. The afferent path is here the ophthalmic branch of the N.