In a general way the function of glands is chemical. They filter out by osmosis, selecting the useful parts for secretion and the useless for excretion. In the chemical action that goes on considerable energy is given off, as is shown by the amount of pressure in the glands and by the fact that their temperature is higher than that of the blood. They all work in a reflex manner, being under the control of the central nervous system. Thus, what is eaten affects the nerve terminals in the mouth, the sensation passes to the nervous system, and an impulse is carried by the motor nerves to the salivary glands.
Most of the glands have ducts to convey away their secretion to other parts of the body or to send excretions out of the body, but there are also ductless glands, which, though they seem to have some important function in the process of metabolism, are not well understood. Most of them seem to manufacture some substance that is absorbed by the tissues and that plays an important part in the bodily metabolism, though nothing is secreted by them externally. They are said to have an internal secretion, whereas the glands with ducts have an external secretion. The liver has both forms of secretion, the bile which is sent out and the glycogen that is stored. The ductless glands are the thymus and thyroid glands, the suprarenal capsules, and the pituitary body in the brain.
Nervous Tissue.—Presiding over all the organs, muscles, and blood-vessels, as the source of all action and all sensation, are the nerves. Nervous tissue is of two kinds: 1. the gray or vesicular, which originates impulses and receives impressions, and 2. the white or fibrous, which conveys impressions. The gray matter consists of large granular cells of protoplasm containing nuclei, which give off many branches or dendrites. From the under surface there usually comes one main branch, the axis-cylinder process. These processes sometimes give off branches and sometimes not, but
