Every additional burden thrown upon the heart increases the evil. In women the additional burden of pregnancy may suffice to overcome a compensation which has been perfect, and the same may result from an acute attack of disease. Age, diminishing as it does the capacity for work in all organs, diminishes the compensation capacity of the heart, and a heart which at the age of forty acts perfectly may break down at the age of fifty. Compensation may be gained in other ways, as by reducing the demand made upon the heart by changing the mode of life, by leading an inactive rather than an active life, by avoiding excitement or any condition which entails work of the heart. Social conditions are of great importance; it makes a great difference whether the unfortunate possessor of such a heart be a stevedore whose capital lies in the strength of his muscles, or a more fortunately placed member of society for whom the stevedore works and whose occupation or lack of occupation does not interfere with the adjustment of his external relations to the condition of his heart.

Disease of the nervous system does not differ from disease elsewhere. The system is complex in structure and in function. It consists in nerves which are composed of very fine fibrils distributed in all parts of the body and serve the purpose of conduction, and a central body composed of the brain and spinal cord which is largely cellular in character; it receives impressions by means of the nerves and sends out impulses which produce or affect action in all parts. By means of the organs of special sense, the brain receives impressions from the outer world which it transforms into the concepts of consciousness. Many of the impressions which the central nervous system receives from nerves other than those of special sense and even many of the impressions from these and the impulses which it sends out do not affect consciousness. The memory faculty is seated in the brain and all parts of the brain are closely connected by means of small nerve fibres. The nervous system plays an important part in the internal regulation and coordination of all parts of the body, and it is by means of this that the general adjustment of man with his environment is effected.

Malformations of the brain, except very gross conditions which are incompatible with extra-uterine existence, are not very common. At birth those parts of the brain which are the seat of memory and what are understood as the higher faculties are very imperfectly developed. Variations in structure are extremely common, there are differences in different individuals in the nerves and in the number, size, form and arrangement of the nerve cells, and so complex is the structure that considerable variation can exist without detection. The tissue of the central nervous system has a considerable degree of resistance to the action of bacteria, but is, however, very susceptible to injury by means of poisons. Serious injury or destruction of tissue of the brain and spinal cord is never regenerated or repaired, but adjustment to such conditions may be effected by reciprocity of function, other cells taking up the functions of those which were destroyed.

Certain parts of the brain are associated with definite functions; thus, there are areas which influence or control speech and motion of parts as the arm or leg, and there are large areas known as the silent areas whose function we do not know. All activity of the central nervous system, however expressed, is due to cell activity and is associated with consumption of cell material which is renewed in periods of repose and sleep. Fig. 13 shows a nerve cell of a sparrow at the end of a day's activity and the same after the repose of a night.

Diseases of the nervous system have a special interest in that they so often interfere with man in his relations with his fellows. In diseases of other organs the disturbances set up concern the individual only. Thus, others need not be disturbed save by the demands made on their sympathies by an individual with a cold in the head or a cancer of the stomach. Disease of the nervous system is another affair, instead of those reactions and expressions of activity to which we are accustomed and to which society is adjusted, the reactions and activities are unusual and the individual in consequence does not fit into the social state and is said to be anti-social. There are all possible grades of this, from mere unpleasantness in the social relations with such an individual, to states in which he is dangerous to society and must be isolated from it. Insanity is an extreme case. There is no disease signified in the expression, but it is merely a legal term to designate those individuals whose actions are opposed to the social state and who are not responsible for them. In insanity there is falsity in impressions, in conceptions, in judgment, a defective power of will and an uncontrollable violence of emotion. The individual is prevented from thinking the thoughts or feeling the feelings and doing the duties of the social body in the community in which he lives. The insane are out of harmony with their social environment, but not necessarily in opposition to it.

There is no very sharp line between insanity and criminality. The criminal is in direct antagonism to the laws of social life. An insane person may cause the same injury to society as a criminal, but his actions are not voluntary, whereas the criminal is one who can control his actions, but does not. Mentally degenerated persons, however, can be both insane and criminal. Whatever the state of society, this reprobates the actions of one opposed to it; in a society in which it were usual to appropriate the possessions of others or to devour unpleasant or useless relatives, virtue and lack of appetite would be reprobated as unsocial.

The symptoms of insanity or the manner in which the defective action of the brain expresses itself and the various underlying pathological changes vary, and by combining these it has been possible to subdivide insanity into a number of distinct forms. There are both intrinsic and extrinsic causes of insanity. The intrinsic are the structural differences in the brain as compared with the normal or usual, whether these are due to imperfection in development or to defective heredity or to the injury of disease; the extrinsic causes are those which come from without and bring the intrinsic into activity. Syphilis is a frequent cause of insanity, and probably the only cause of the condition known as general paralysis of the insane, acting by means of the injury which it produces in the cortex of the brain. The abuse of alcohol is another fertile cause, but the changes produced in this are not so obvious as in the case of syphilis. Tumors of the brain are not infrequently a cause, and the same is true of infections, even those not located in the brain. How susceptible the brain is to the effects of the toxines of the infectious diseases is shown in the frequency of delirium in these diseases. There is an interesting relation between this and alcoholism. Alcohol abuse may produce injury, but not sufficient to manifest itself under ordinary conditions; when, however, the action of toxic substance is superadded to the effect of the alcohol the delirium of fever is more marked.

Probably of greater importance than the acquired pathological conditions of the brain in producing insanity is a congenital condition in which the nervous system is defective. The most fertile cause of insanity lies in the inheritance; by this it must not be understood that insane parents produce insane offsprings, but that conditions inherited from immediate or remote ancestors appear in a diminished resistance of the nervous system which is sooner or later expressed as insanity. Given such a defective nervous system, extrinsic conditions which would have no effect on another individual or would be felt in different ways may produce insanity. In these cases occupation plays a great role. The excitement and privations of war especially in the tropics and the ennui of camps leads to insanity in soldiers; occupations such as that of the baker in which there is loss of sleep and the mental strain of students can all act in the same way. A woman who gives no sign of nervous defect may become insane under the strain of pregnancy.

Although insanity is determined by the social relations of man, that part of the social organization which is termed Society, and which has been developed by the idle as a diverting game, is a fertile source of nervous disease and even of insanity, affecting particularly females. The strenuosity of the life, the nervous excitement alternating with ennui, the lack and improper times of sleep, the lack of rest and particularly of restful occupation, the not infrequent use of alcohol in injurious amounts, are all factors calculated to make a defect operative. The so-called "coming out" of young girls is an important element in the game, and their headlong plunge into such a life at a period under any conditions full of danger to the nervous system is especially to be reprobated. If we consider the influence of the game in other respects as conducing to lack of moral sense, to alcoholic abuse (for without the seeming stimulation, but which is really the blunting of impressions which alcohol brings, the game would not be possible), to discontent, to mental enfeeblement, it is all bad. Curiously enough the game is one which in all periods has been played by the idle, but its evil influence is greater now than before when it was the game of royalty chiefly, because there are now more people living from the work of others.

The unusual mental action of the insane not infrequently expresses itself by suicide. The analysis of three hundred deaths from suicide showed pathological changes in the brain in forty-three per cent, and when we think that mental disturbances are very often without recognizable anatomical changes after death, the percentage is very large. In another analysis of one hundred and twenty-four suicides forty-four of these were mentally affected to various degrees. Five of the men and seven women were epileptics, in ten of the families there was hysteria, twenty-four of the men and four of the women were chronic alcoholics.