265.  Divisions of the Nervous System.  This system in man consists of two great divisions.  The first is the great nerve center of the body, the cerebro-spinal system, which rules the organs of animal life.  This includes the brain, the spinal cord, and the cerebro-spinal nerves.  Nerves are given off from the brain and the cord, and form the mediums of communication between the external parts of the body, the muscles or the sense organs, and the brain.

The second part is the sympathetic system, which regulates the organic life.  This consists of numerous small nerve centers arranged in oval masses varying greatly in size, called ganglia or knots.  These are either scattered irregularly through the body, or arranged in a double chain of knots lying on the front of the spine, within the chest and abdomen.  From this chain large numbers of nerves are given off, which end chiefly in the organs of digestion, circulation, and respiration.  The sympathetic system serves to bring all portions of the animal economy into direct sympathy with one another.

266.  The Brain as a Whole.  The brain is the seat of the intellect, the will, the affections, the emotions, the memory, and sensation.  It has also many other and complex functions.  In it are established many reflex, automatic, and coordinating centers, which are as independent of consciousness as are those of the spinal cord.

The brain is the largest and most complex mass of nerve tissue in the body, made up of an enormous collection of gray cells and nerve fibers.  This organ consists of a vast number of distinct ganglia, or separate masses of nerve matter, each capable of performing separate functions, but united through the cerebral action into a harmonious whole.

[Illustration:  Fig. 114.—­The Upper Surface of the Cerebrum. (Showing its division into two hemispheres, and also the convolutions)]

The average weight of the adult human brain is about 50 ounces for men and 45 ounces for women.  Other things being equal, the size and weight of the brain bear a general relation to the mental power of the individual.  As a rule, a large, healthy brain stands for a vigorous and superior intellect.  The brains of many eminent men have been found to be 8 to 12 ounces above the average weight, but there are notable exceptions.  The brains of idiots are small; indeed, any weight under a certain size, about 30 ounces, seems to be invariably associated with an imbecile mind.

The human brain is absolutely heavier than that of any other animal, except the whale and elephant.  Comparing the size of these animals with that of man, it is instructive to notice how much larger in proportion to the body is man’s brain.  The average proportion of the weight of the brain to the weight of the body is greater in man than in most animals, being about 1 to 36.  In some small birds, in the smaller monkeys, and in some rodents, the proportional weight of the brain to that of the body is even greater than in man.