On the other hand, a destructive lesion may be so placed in the spinal cord or brain as to allow centripetal impressions to reach healthy spinal gray matter in the normal way, but preventing their passage frontad (upward) to be recognized by consciousness. In such a case we observe normal, or more commonly exaggerated, reflex action in parts which are insensible in the ordinary sense of the term. Indeed, in many cases the disconnected caudal portion of the spinal cord is in a state of vastly exaggerated reflex activity, as shown by the tetanoid and convulsive involuntary and reflex movements which take place in completely paralyzed and anæsthetic limbs (paraplegia from transverse myelitis). In general terms, it may be stated that when anæsthesia is due to lesions of peripheral nervous endings, of nerve-trunks, and of the posterior root system of the spinal cord, reflexes are diminished or lost.
It is often stated that anæsthesia causes ataxia of movement. This, from the results of experiments on animals and from clinical study, we believe to be a gross error. In animals and in man loss of sensibility gives rise to awkwardness or uncertainty in movement (increased if the eyes be closed) which is properly to be classed as a special variety of inco-ordination; but it is not from ataxia, in which irregular, jerky, oscillating motions occur when a volitional act is attempted, these movements resulting from want of harmony in the action of antagonistic muscles which in the normal educated state automatically act together to produce the desired result. Besides, we occasionally observe cases of typical spinal ataxia in which no impairment of sensibility can be observed.
THE TOPOGRAPHICAL DISTRIBUTION of alterations of sensibility requires careful determination in practice, as from it we obtain most valuable aid in diagnosis. The following are the principal types observed:
(a) Alterations of sensibility in one lateral (vertical) half of the body and head. We thus have hemihyperæsthesia, hemiparæsthesia, or hemianæsthesia, and the special senses on one side are frequently involved. This clearly hemi-distribution indicates that the lesion or functional disorder is in the cerebral hemisphere of the opposite side, more especially in the caudal segment of the internal capsule or in its areas of cortical distribution (occipital, temporal, and parietal lobes). The distribution of hemianæsthesia, etc. from organic disease in these parts is identical with that observed in some functional (hysterical) cases; we can make the diagnosis only by the help of other data.
If the sensory disorder does not affect the head, but is limited to one lateral half of the body, it is, if due to organic disease, quite certainly of spinal origin.
(b) Two homonymous extremities or the two lateral halves of the body may exhibit opposite states of sensibility—anæsthesia on one side and hyperæsthesia on the other. This rare condition is witnessed in hysteria and in some forms of injury to the spine (lesion of one lateral half of the cord at a certain level). In the latter case paralysis is usually present on the hyperæsthetic side: the symptoms constituting, with some others, Brown-Séquard's spinal hemiplegia or hemiparaplegia.
(c) Alterations of sensibility in one caudal (horizontal) half of the body are said to have a paraplegic distribution, and are usually due to lesions of the spinal cord. The upper level of the symptom may be at any point between the neck and the toes; and the frontal (upper) level indicates, due regard being had to the origin and oblique distribution of the spinal nerves, the highest limit of disease in the nervous centres. Very often, in organic disease especially, this is also indicated by the presence of a cincture feeling (paræsthesia) at the frontal (upper) limit of the anæsthesia, etc.
(d) Disorders of sensibility may be limited to one extremity. This very rarely depends upon cerebral disease, and in such a case the anæsthesia, etc. is evenly distributed throughout the member, being most intense at its extremity, and being without sharp, distinct limits near the trunk. When due to diseases of the spinal cord, the cerebral (upper) limit of the symptom is usually clearly defined in accordance with the distribution of sensory nerves from the other (healthy) parts of the cord: a constriction band often marks the limit. Sometimes the peripheral anæsthesia, etc. is more or less in the territory of certain nerve-trunks. When we find the distribution of the sensory symptoms to coincide exactly in the areas supplied by the large nerves of the limb, without cincture feeling, it is certain that the lesions affect one or more of these nerve-trunks or the plexus above. In not a few cases the symptoms are due to hysterical or dyscrasic conditions, and the seat of the lesions (dynamic or molecular lesion) is uncertain. In judging of the distribution of anæsthesia, etc. in a limb due regard must be paid to variations in nerve-branching and to collateral nerve-supply.
(e) Alterations of sensibility occurring in well-defined areas of the hand, trunk, or extremities, corresponding to the known distribution of nerves, almost always indicates disease of the nerve itself, much more rarely disease in the spinal cord at the origin of the nerve. The reflexes are then diminished or lost. In judging such cases Van der Kolk's law of the distribution of the motor and sensory filaments of nerve-trunks should be remembered: it is, that of the two sets of fibres in a mixed nerve, the sensory fibres are distributed to parts which are moved by muscles which receive the motor fibres of the same nerve. Thus, in nerve lesions the chief sensory symptoms are always peripheral or distal from the chief motor symptoms.
(f) Disorders of sensibility sometimes appear in patches or irregular areas whose nervous connections are indefinite. Such patches of anæsthesia, hyperæsthesia, or paræsthesia sometimes indicate foci of disease in the spinal cord (and brain?); as, for example, the pains, etc. of posterior spinal sclerosis. These patches may also occur in consequence of interference with local circulation of peripheral parts; and we meet with them in such conditions as hysteria, neurasthenia, alcoholism, etc.