Neuropathology

(b) Excess of certain Normal Constituents in the Blood.—Ex cess of carbonic acid causes drowsiness, and probably in asphyxia is one of the causes of the convulsions. All the nitrogenous waste products are normal constituents of the blood; but should oxida tion be incomplete, from disease of the liver, or should these sub stances accumulate in the blood, owing to inadequate function of the kidneys, uraemia may supervene, the manifestations of which are headache, drowsiness, unconsciousness or coma, epilep tiform convulsions and sometimes symptoms of polyneuritis.

Again, in Graves's disease (hyperthyroidism), nervous phenom ena, 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.

(c). The presence of abnormal constituents in the blood is a most important cause of disease of the nervous elements. These are : Poisons produced within the body (a) by perverted function of organs or tissues, auto-intoxication; (I3 ) by the action of micro-organisms, protozoa and bacteria, upon the living fluids and tissues of the body; ( 7, ) poisons introduced into the body from without.

(a) Poisons resulting from perverted Function of the Organs. —Nervous symptoms follow auto-intoxication by products of dis ordered digestion, fatigue products (e.g., sarcolactic acid in pro longed muscular spasm), excess of vric acid, phosphates, oxalates, sugar, bile, hepatic products as in acute yellow atrophy. In per nicious and certain grave anaemias, the degenerative changes in the spinal cord found in some cases is due, chiefly, to some neuro-toxin, which probably arises from imperfect metabolism or absorption from the alimentary canal. In auto-intoxication, dis ease of one organ or tissue is apt to establish a vicious circle which is constantly enlarging ; therefore nervous symptoms mani festing themselves in the course of a disease add much to the gravity of the complaint.

(0). Poisons produced by infective Micro-organisms.—Some of these have a general devitalizing influence by altering the blood and producing fever. In acute infectious diseases, delirium is a frequent complication ; in severe cases, stupor and coma may occur, and in this extreme stage the nerve cells undergo an acute morbid bio-chemical change. These particular poisons may have

marked selective toxic action upon particular parts of the nervous system.

Poisons may have a

selective influence upon some part of the nervous system. The syphilitic organism and its poisons are most important factors in the production of two progressive degenera tions of the nervous system—one affecting especially the afferent conducting tracts of the spinal cord, namely, tabes dorsalis (locomotor ataxia), and the other affecting especially the frontal and central convolutions of the cerebral hemispheres, namely, general paresis (general paralysis of the insane). When syphilis attacks the supporting, enclosing and nutrient vascular tissues, a predilection to affect structures about the base of the brain occurs, and paralyses of the third nerve are frequent in this disease. Other examples occur in rabies, tetanus, diphtheria (q.v.).

Protozoa and Diseases of the Nervous System.

The rela tion of protozoa to the existence of widespread diseases affecting men and animals is becoming yearly of greater importance. Cer tain diseases in which the nervous system is profoundly affected are now explained by the invasion of the tissues of the body by these lowly organisms, for example, African sleeping sickness due to a special protozoan; Syphilis due to Spirochaeta or Tre ponema pallida; Malaria due to Haematozoon malariae.

The bacterial invasion of tissues is generally characterized by a migration of polymorpho-nuclear leucocytes, but protozoal in vasion is characterized by a formative hyperplasia of the fixed cell tissues, endothelial, epithelial and conjunctival, and there is a close similarity in the defensive reaction of the tissues to all forms of protozoal invasion.

When the nervous system is affected a local or general chronic meningo-encephalitis is set up, characterized by a meningeal and perivascular infiltration with lymphocytes and plasma cells, occasioned by a chronic irritative process, presumably caused in the case of sleeping sickness for example by the presence of try panosomes in the actual cerebrospinal fluid. The same perivascu lar and meningeal infiltration with plasma cells and lymphocytes is found in syphilitic diseases of the nervous system.