Physiologically, we may divide the fibers of the posterior roots of- the spinal nerves into four groups, viz.: i. Spinal reflex fibers, which end within the gray crescent on the same side, above and below the point of entrance, and excite simple reflexes. 2. Cerebellar reflex fibers; they terminate in relation with the neurones that form the dorsal and ventral spino-cere bellar tracts, and they excite coordinated reflexes in the cere bellar cortex. Both these groups (r and 2) carry non-sensory impulses. The two groups following (3 and 4) convey their impulses to consciousness; their impulses result in sensations. 3. Fibers that form the fasciculus gracilis and fasciculus cuneatus and terminate in the nuclei of those tracts. This group con ducts impulses of the muscle-sense, impulses leading to tactile discrimination and to recognition of size, shape, form in three dimensions, of vibration, of weight, etc. Such impulses come from muscles, tendons, joint surfaces, skin and mucous mem branes. 4. Fibers that end within the gray matter in relation with the origin of the spino-thalamic tract. Through the fibers of this group pass impulses of pain, heat, cold, and tactile localization, which arise ldrgely in the skin and mucous membranes but also in the deep structures.
Impulses due to light touch and to pressure travel a direct path up the posterior column and a crossed path up the lateral column.
Lesions.—The posterior roots of the spinal nerves and the spinal ganglia are affected in locomotor ataxia, and the lesion extends to the marginal tract (of Lissauer) and the posterior col umn of the cord. Excepting the fasciculus proprius, the whole posterior column becomes involved. The spinal ganglia are the seat of specific inflammation in herpes zoster.
As stated above, peripheral common sensory nerves conduct impulses in combinations, but every fiber is not capable of conducting all varieties of common sensory impulse. The work of Head, Rivers and Sherren, and of Sherrington (Brain, Vol. 28 and Vol. 29) show that common afferent nerves form four great systems which are physiologically distinct and probably are evolved at different phylogenetic periods. These systems are as follows: r. The proprio-ceptors, the mechanism of deep sensibility.
2. The protopathic extero-ceptors, and 3. The epicritic extero-ceptors.
The extero-ceptors are the nerves of cutaneous sensibility, etc.
4. The intero-ceptors, the afferent nerves of the internal organs— the alimentary tract, the respiratory tract, the genitourinary tract, etc., excepting the regions near the external orifices.
r. The system belonging to deep sensibility, the proprio-ceptors, com prises the nerves that supply muscles, tendons, ligaments and joint surfaces, and the vestibule and semicircular canals of the labyrinth. The proprio
ceptors respond to such stimuli as tension, pressure, posture and move ment; and to painful stimuli due to excessive pressure or to pathologic condition of muscle, tendon, joint, etc. They enable the individual to locate the point of stimulation, to determine the d rection of move ment and the posture of any part. The common proprio-ceptors con stitute the peripheral mechanism of the muscle-sense, plus a part of the mechanisms of tactile localization and pain. The vestibular nerve is a special proprio-ceptor. As all stimuli of the proprio-ceptors, with the single exception of pressure, originate within the organism, this system of nerves is called by Sherrington the "proprio-ceptors." The impulses conducted by this system set up important reflexes; but most of them fail to reach consciousness.
The are the nerves of the skin and the adjacent parts of mucous membranes; also the nerves of the special senses—taste, smell, sight and hearing (not equilibrium). These nerves are stimulated by environment, hence the nerves adapted to receive stimuli from without the organism are called the "extero-ceptors." The extero-ceptors may be divided into a distance and a contact subgroup. The nerves of sight and hearing are the distance as through them we appear to perceive things at a distance. But in reality they are contact receptors, because they respond only to the air and ether vibrations that reach their end-organs.
2. The protopathic constitute a primitive system of afferent nerves, probably of great phyfogenetic age. They are con nected with specific "pain spots," "cold spots" and "heat spots" of the skin and mucous membranes (near the external orifices). Con sequently, this system responds to painful cutaneous stimuli and to ex treme degrees of cold (below 26° C.) and heat (above C). The system possesses three specific end-organs—for pain, cold and heat— and each is incapable of responding to any other kind of stimulus. The sensations excited through the protopathic system are intense, diffuse and non-localized; the point stimulated cannot be determined and false reference to some other part is characteristic of it. Light touch, warmth and coolness are ineffective stimuli of this system, they excite no response. Protopathic impulses produce their appropriate sensation and also excite reflexes. The connection of the protopathic extero-ceptors with the spinal cord is distinctly segmental; this is another evidence of its primitive character.