The nervous system (Fig. 8) represents one of the most important of the enclosed organs. It serves an important function, not only in regulating and cooerdinating all functions, but by means of the special senses which are a part of it, the relations of the organism as a whole with the environment are adjusted. It consists of a large central mass, the brain and spinal cord, which is formed in the embryo by an infolding of the external surface, much in the same way that a gland is formed; but the connection with the surface is lost in further development and it becomes completely enclosed. Connected with the central nervous mass, forming really a part of it and developing from it, are the nerves, which appear as white fibrous cords and after dividing and subdividing, are as extremely fine microscopic filaments distributed to all parts of the body. By means of the nerves all impressions are conveyed to the brain and spinal cord; all impulses from this, whether conscious or unconscious, are conveyed to the muscles and other parts. The brain is the sole organ of psychical life; by means of its activity the impressions of the external world conveyed to it through the sense organs are converted into consciousness. Whatever consciousness is, and on this much has been written, it proceeds from or is associated with the activity of the brain cells just as truly as the secretion of gastric juice is due to the activity of the cells of the stomach. The activity of the nervous system is essential for extra-uterine life; life ceases by the cessation of circulation and respiration when either the whole or certain small areas of its tissue are destroyed. In intra-uterine life, with the narrow and unchanging environment of the fluid within the uterine cavity which encloses the foetus, life is compatible with the absence or rudimentary development of the nervous system. The foetus in this condition may be otherwise well developed, and it would be not a misuse of words to say that it was healthy, since it is adjusted to and in harmony with its narrow environment, but it would not be normal. The intra-uterine life of the unborn child, it must be remembered, is carried out by the transmission of energy from the mother to the foetus by means of the close relation between the maternal and foetal circulation. It is only when the free existence demands activities not necessary in intra-uterine life that existence without a central nervous system becomes impossible.