B. The external causes producing morbid changes in the nervous elements are: I. Abnormal conditions of the blood and lymph, by which the neurones are poisoned and their metabolism morbidly affected. II. Excess or deficiency of normal stimulation, or existence of abnormal stimulation. III. Injury or diseases of supporting, enclosing or vascular tissues.
I. Abnormal Conditions of the Blood and Lymph.—The immediate environment of all the cellular elements of the body is lymph, and in the central nervous system there is a special form of lymph, the cerebro-spinal fluid, which is secreted by the choroid plexus in the venticles of the brain. The neurones, like other cellular elements, are bathed in the lymph, and extract from it the materials necessary for their growth and vital activities, casting out the waste products incidental to the bio-chemical changes which are continually taking place. The lymph, therefore, serves as a medium of exchange between the blood and the tissues, consequently the essential causes of change in environment of the nervous elements (neurones) are: (1) Deficiency or absence of blood-supply to the nervous system in general (as after severe haemorrhage), or to some particular portion, owing to local vascular disturbance or occlusion. (2) Alterations in the normal condition of the blood, due to (a) deficiency or absence of certain essential constituents, (b) excess of certain normal constituents, (c) the presence of certain abnormal constituents produced within the body, or entering it from without.
(1) Quantity of Blood Supply.—Syncope or fainting occurs when the blood supply suddenly fails to reach the higher centres of the brain; this usually arises from sudden reflex arrest of the heart’s action. If a portion of the central nervous system is cut off from its arterial blood supply by embolic plugging or by clotting of the blood in a vessel with diseased walls, the portion of the brain substance thus deprived of blood undergoes softening, the nervous elements are destroyed, and the systems of nerve fibres, which have had their trophic and genetic centres in the area destroyed, undergo secondary degeneration. Clotting of the blood in the veins may also give rise to destructive softening of the brain, and similar secondary degeneration.
(2) Quality of Blood Supply.—(a) Insufficiency of oxygen, due to poverty of the colouring matter or of the number of the red corpuscles, which constitutes the various forms of anaemia, leads to functional depression, lassitude and mental fatigue. Impoverishment of the blood in women by frequent pregnancies and excessive lactation causes neuralgia, nervous exhaustion and, in the neuropath, hysteria, neurasthenia, melancholia and mania. The mental depression, and the tendency that the various neuroses and psychoses have to occur and recur at the time of the menstrual and climacteric periods in women, suggests the possibility of an alteration in the composition of the blood, either in the nature of an auto-intoxication or “sub-minimal deficiency,” as the probable contributory factor of the mental disturbance. It may be remarked that eclampsia, puerperal and lactational mania are relatively common forms of insanity in women; although sometimes of septic origin, they more frequently are occasioned by some morbid metabolism as yet little understood. The most striking examples we have, however, of the effect of absence or “sub-minimal” deficiency of a normal constituent of the blood upon the development and functions of the nervous system are afforded by cretinous idiots, who are born without thyroid glands, and whose brains never develop in consequence; and by those people who suffer from the disease known as myxoedema, occasioned by the absence of iodothyrin, a product of the internal secretion of the thyroid gland. The proof of this is shown by the disappearance of the nervous phenomena, slowness of thought, slowness of speech, &c., after a preparation of the gland has been continuously administered by the mouth. Even cretinous idiots when subjected in early life to thyroid treatment improve considerably. The removal of the testicles in the male may produce a profound effect upon the nervous temperament; for probably there is an internal secretion of this gland in the male, as of the ovary in the female, which has some subtle influence upon the functional activity of the nervous system. The seminal fluid contains a large amount of complex phosphorus-containing substances, which, lost to the body by sexual excess or onanism, have to be replaced by the blood; the nervous system, which also needs these complex organic phosphorus compounds, is thereby robbed, and neurasthenia ensues. Brown-Séquard’s testicular injection treatment for many nervous complaints, based upon this idea, has not, however, met with much success.
(b) Excess of certain Normal Constituents in the Blood.—Excess of carbonic acid causes drowsiness, and probably in asphyxia is one of the causes of the convulsions. All the series of the nitrogenous waste products—the most highly oxidized, most soluble and least harmful of which is urea—are normal constituents of the blood; but should the oxidation process be incomplete, owing to functional or organic disease of the liver, or should these substances accumulate in the blood, owing to inadequate function of the kidneys, a toxic condition, called uraemia, may supervene, the nervous manifestations of which are headache, drowsiness, unconsciousness or coma, epileptiform convulsions and sometimes symptoms of polyneuritis. Again, in Graves’s disease, nervous phenomena, in the form of exophthalmos, fine tremors, palpitation and mental excitement, have by some authorities been explained by the excess of thyroid internal secretion, due to the enlargement and increased functional activity of the gland. The successful treatment of Graves’s disease by the administration of the blood serum and milk of animals (goats), which had the thyroid glands removed, supports this theory.
(c) The presence of abnormal constituents in the blood is a most important cause of disease of the nervous elements. We may consider the subject under the following headings: Poisons produced within the body (α) by perverted function of organs or tissues, auto-intoxication; (β) by the action of micro-organisms, protozoa and bacteria, upon the living fluids and tissues of the body; (γ) poisons introduced into the body from without, in the food and drink, or by inhalation.
(α) Poisons resulting from perverted Function of the Organs.—In the process of digestion a number of poisonous substances, e.g. albumoses, &c., are produced, which, although absorbed in the alimentary canal, are prevented by the living epithelium, and possibly by the liver, from entering the systemic circulation. Fatigue products, e.g. sarcolactic acid in prolonged muscular spasms, may lead to auto-intoxication. Excess of uric acid in the blood is associated with high arterial pressure, deposits of lithates in the urine, headache and nervous irritability; it is an indication of imperfect metabolism and auto-intoxication, as shown by the fact that marked improvement occurs by suitable diet and treatment. Phosphoruria, oxaluria and glycosuria, tokens of deranged metabolism, may be associated with various nervous phenomena. Bile in the blood, cholaemia, resulting from obstructive jaundice, may be attended by stupor and psychical depression; and the term melancholia, signifying “black bile,” indicates the importance which has long been attached to the liver as an organ the derangement of which causes nervous depression. The rapidly fatal results attending acute yellow atrophy of the liver, namely, the profound changes in the urine, the jaundice and the nervous phenomena of delirium, motor irritation, delusions, stupor and coma, demonstrate the important part this organ plays in preserving the normal quality of the blood. The delirium and coma which sometimes supervene in diabetes, heralded by acetonaemia, is another instance of auto-intoxication. The coma is very possibly due to the saturation of the sodium salts of the blood by aceto-acetic and oxybutyric acids, products of imperfect proteid metabolism. The effect of this would be an interference with the elimination of carbonic acid in the processes of tissue and pulmonary respiration. Again, in pernicious and certain grave anaemias, the degenerative changes in the spinal cord found in some cases is due, not so much to the defect in the red corpuscles, as to some neuro-toxin, which probably arises from imperfect metabolism or absorption from the alimentary canal. In this question of auto-intoxication, it must be remarked that all the tissues of the body are mutually interdependent. If one suffers, all suffer, and a disease of one organ or tissue is thereby apt to establish a vicious circle which is constantly enlarging; therefore nervous symptoms manifesting themselves in the course of a disease add much to the gravity of the complaint.
(β) Poisons produced by Infective Micro-organisms.—Some of these poisons have a general devitalizing influence, by an alteration of the blood and the production of fever. In the course of the acute infectious diseases, typhoid, typhus, smallpox, scarlet fever, measles, influenza, also tuberculosis and septicaemia, delirium is a frequent complication; it may be the result of high fever or prolonged fever, or directly due to the poison, or the two combined. In severe cases stupor and coma may occur, and it has been shown that in this extreme stage the nerve cells undergo an acute morbid bio-chemical change. These particular poisons have no selective toxic action upon a particular part of the nervous system, and symptoms not only during, but after, the acute illness are liable to supervene, especially in a neuropathic individual. Thus many cases of neurasthenia, insanity, neurosis, also neuritis, date their origin from an acute specific fever. In cerebro-spinal meningitis, tubercular meningitis, acute delirious mania and leprous neuritis, the inflammation of the membranes of the brain and spinal cord is due to the growth of the specific organism in the lymph and interstitial tissue elements.
Poisons may have a selective influence upon some part of the nervous system. The syphilitic poison is the most important factor in the production of two progressive degenerations of the nervous system—one affecting especially the afferent conducting tracts of the spinal cord, namely, locomotor ataxy, and the other affecting especially the frontal and central convolutions of the cerebral hemispheres, namely, general paralysis of the insane. A striking instance of the selective action of the syphilitic poison is shown in the fact that only in persons affected with acquired or inherited syphilis is a symptom known as Argyll-Robertson pupil found; this is the absence of the pupil reflex contraction to light, while that for accommodation persists. Seeing that this is the most common objective phenomenon in the two diseases mentioned, it strengthens the presumption,
