NEUROPATHOLOGY, the general name for the science concerned with diseases of the nervous system. For the anatomy and physiology, see NERVE, NERVOUS SYSTEM, BRAIN, SPINAL CORD and SYMPATHETIC SYSTEM. The morbid processes affect ing the nervous system are usually clinically divided into two great groups of (I) organic disease, (2) functional disturbance, depend ing on whether or not symptoms observed during life can be asso ciated with recognizable changes after death. Knowledge of the first group is much more advanced than of the latter, for, given certain symptoms during life, we can, as a rule, predict not only the nature of the morbid process, but its particular locality.
The histological elements which make up the nervous system may also be divided into two groups: (I) the nervous units or neurones, (2) the supporting, protecting and nutrient tissues. Ner vous diseases may start primarily in the neurones and cause their degeneration; such may bring about "diseases," or "syndromes" within the nervous system. The nervous units, however, may be affected secondarily by disease processes starting in the supporting, protecting and nutrient tissues within the nervous system ; such include changes of the blood-vessels, lymphatics, membranes and the special nervous connective tissue, neuroglia (a residue of the embryonal structure from which the nervous system was devel oped). Tumours and new growths must also be included.
The causes of pathological processes occurring in the nervous units (neurones) may be divided into internal and external; in all cases except direct injury the two groups are generally more or less combined.
Internal Causes.—Of the factors involved in nervous disease hereditary predisposition may first be accented. In 70% of 15o cases of idiocy or imbecility in the London county asylums, Dr. Tredgold found a family history of insanity in some form or another. This predisposition may be convergent, paternal, ma ternal; from grandparents or even more remote ancestors. More over, no study of heredity is complete that does not take into consideration collaterals. Especially does this apply to functional neuroses, e.g., epilepsy, migraine, hysteria and neurasthenia; and to psychoses, e.g., delusional insanity, mania and melancholia, manic-depressive, recurrent or periodic insanity and dementia praecox or adolescent insanity. Strictly speaking, it is the tend ency to nervous disease rather than the disease itself that is in herited, and this is frequently spoken of as a neuropathic or psychopathic taint.
The external causes producing morbid changes in the nervous elements are : I. Abnormal conditions of the blood and lymph.
II. Excess or deficiency of normal stimulation, or existence of abnormal stimulation. III. Injury or diseases of supporting, enclosing or vascular tissues.
Abnormal Conditions of the Blood and Lymph.—The essential causes of change in environment of the nervous elements (neurones) are: (I) 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 cer tain abnormal constituents produced within the body, or entering it from without. All these act through the cerebro-spinal fluid, a special lymph secreted by the choroid plexus in the ventricles of the brain by which the neurones are bathed.
(I) Quantity of Blood Supply.—Syncope or fainting occurs when the blood supply suddenly fails to reach the higher centres of the brain ; such may arise from sudden reflex arrest of the heart's action, or from localized spasms of the cerebral vessels. The effects of embolism and thrombosis are considered later.
(2) Quality of Blood Supply.—(a) Insufficiency of oxygen, as in anaemia, leads to functional depression, lassitude and mental fatigue. Impoverishment of the blood in women by frequent pregnancies and excessive lactation causes neuralgia, nervous ex haustion and may aid in the development of neurasthenic or hysterical reactions. The tendency of psychoneuroses and psy choses to occur and recur at menstrual and climacteric periods in women, indicates that these factors themselves are of periodic significance. These are connected with the reproduction function, rather than with the blood however. The most striking examples 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 conse quence; and by those people who suffer from myxoedema (q.v.) occasioned by the absence of thyroxin or other products 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, etc., after thyroxin or a preparation of the gland has been continuously administered. This is an ex cellent example of a reversible process.