[Sidenote: 2. What second set of powers has the animal? What functions are mentioned? The advantage they give?] 2. The animal, in addition to these vegetative functions, has another set of powers, by the use of which he becomes conscious of a world external to himself, and brings {149} himself into active relations with it. By means of the vegetative processes, his life and species are maintained; while, by means of certain animal functions, he feels, acts, and thinks. These functions, among which are sensation, motion, and volition, not only distinguish the animal from the plant, but, in proportion to their development, elevate one creature above another; and it is by virtue of his pre-eminent endowment, in these respects, that man holds his position at the head of the animal creation. [Sidenote: 3. Animals whose structure is simple? As we approach man? Dependence of the animal functions of man?] 3. Among animals whose structure is very simple, the hydra, or fresh-water polyp, being an example, no special organs are empowered to perform separate functions; but every part is endowed alike, so that if the animal be cut into pieces, each portion has all the properties of the entire original; and, if the circumstances be favorable, each of the pieces will soon become a complete hydra. As we approach man, in the scale of beings, we find that the organs multiply, and the functions become more complete. The function of motion, the instruments of which--the muscles and bones--have been considered in former chapters, and all the other animal functions of man, depend upon the set of organs known as the nervous system. [Sidenote: 4. The nervous tissues, of what composed? When examined by the aid of the microscope? The white substance? The gray substance?] 4. THE NERVOUS SYSTEM.--The intimate structure of this system differs from any tissue which we have before examined. It is composed of a soft, pulpy substance, which, early in life, is almost fluid, but which gradually hardens with the growth of the body. When examined under the microscope, it is found to be composed of two distinct elements:--(1) the white substance, composing the larger proportion of the nervous organs of the body, which is formed of delicate cylindrical filaments, about 1/6000 of an inch in diameter, termed the nerve-fibres; and (2) the gray substance, composed of grayish-red, or {150} ashen-colored cells, of various sizes, generally possessing one or more off-shoots, which are continuous with the nerve-fibres just mentioned. [Sidenote: 5. Nervous centres and ganglia? Nerves? What do they serve? Cerebro-spinal system?]