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.