Each of these nervous masses contains nerve-cells as well as nerve-fibres, and is capable of generating nerve-force. Each, therefore, acts like a minute brain; and, in fact, the entire ganglionic system of nerves is analogous to the nervous system of certain among the lower animals--the crustacea and mollusks. These possess neither brain nor spinal cord; their nerve-centres, instead of being concentrated in a cranium and vertebral canal, are entirely disseminated through the cavities of the trunk, as are the visceral plexuses in vertebrated animals. In these, however, the addition of a brain and spinal cord to the original rudimentary nervous system, powerfully modifies and controls the action of the latter. The degree of control is variable, according to the relative predominance of the one or the other; and this predominance varies, not only according to different species of vertebrated animals, but also according to different individuals, in that which presents the most conspicuous capacity for individual variation--the human species. Up to a certain point, increased development of the cerebro-spinal system, attended by an increased development of the osseo-muscular framework of the body, is also accompanied by greater elaboration of the ganglionic nerves supplying the viscera, upon whose efficient action the nutrition of this frame depends. But beyond a certain point in the ascending scale, the exactness of this correlation ceases. The muscles and bones are smaller; yet the structure of the cerebro-spinal organs, especially the brain, becomes more elaborate; and hence the control exercised over the functions of the ganglionic system is more complete, although the relative size of the two systems is not much changed.