on the ocean-bottom, except in very shallow water. Since it is degradational agencies which produce the most notable secondary features of land-surfaces, and sin^,e these agencies are little operative in the sea, the sea-bottom, in general, is without the hills, the ridges, the mountain-peaks due to erosion, the valleys, valley-plains and terraces which abound on the land. These physiographic forms are sometimes found on the sea-bottom, and the areas where they occur are thought to have been land at one time, though now submerged. On the other hand, the shallow sea-bottom is affected by bars, reefs etc., built by waves and shore-currents, and by coral polyps. Reefs are not formed on land, and only occur there, when an area of sea-bottom is elevated to the estate of land.


Davis' Physical Geography; Geikie's Earth Sculpture; Physiography of the United States; Jukes-Brown's Physical Geology, much of dynamic geology in any textbook on geology ; those portions of textbooks on physical geography which deal with the land and with the configuration of the ocean-beds; all treatises on rivers, lakes, glaciers, as Russell's Volcanoes; Judd's Physiographic Atlases of the U. S. Geological Survey; etc. R. D. Salisbury.

Phys'iol'ogy. Anatomy shows us that animals and plants are wonderfully constructed. But after we understand their architecture and even their minute structure, the questions remain: What are all the organs and tissues for, and what takes place within the parts that are actually alive? Physiology attempts to answer questions of this nature. It therefore stands in contrast with anatomy and is supplementary to it. The activities of the body are varied and depend on life for their manifestation — they may be called vital activities. Physiology embraces a study of them all. This subject began to attract the attention of the ancient medical men, who wished to fathom the activities of the body in order to heal it in disease. But it is so difficult a thing to begin to comprehend the activities of life, that even the simpler relationships were imperfectly understood, and they resorted to mystical explanations. They spoke of spirits and humors in the body as causing the various changes. The arteries were supposed to carry air, the veins only blood, and nothing was known of the circulation. In these early days, also, anatomy, physiology and medicine were united into a poorly digested mass of facts and fancies. This state of affairs lasted till the 16th century, and then the awakening came through the efforts of gifted men endued with the spirit of independent investigation. The advances made depended upon the work or leadership of these men, "and certain

periods of especial importance should be pointed out.

First is the period of Harvey (1578-1657). In his time the old idea of spirits and humors was giving way, but there was much vagueness about the relationships and activities of the body. He helped to illuminate the subject by showing a connection between arteries and veins and demonstrating the circulation of the blood. Harvey (. v.) did not see the blood passing through the capillaries from arteries to veins, but his reasoning was unassailable that such a connection must exist and that the blood makes a complete circulation. He gave this conclusion in his medical lectures as early as 1619, but did not publish his views until 1628. It was reserved for Malpighi, in 1661, actually to see the circulation through capillaries under the microscope, and for Leeuwenhoek, in 1669 and later years, greatly to extend the observations. The next great period was marked by the work of Haller (1708-77), who made physiology an independent subject. It had previously been united with anatomy and medicine; he made it a subject to be studied for its own sake. The period that marks the beginning of modern physiology came next, and was due to the genius and force of Johannes Miiller (q. v.). He studied physiology so broadly that he made it comparative. He used every means at his command — experiment, observations on simpler animals, the microsope, the discoveries in physics, chemistry and psychology. He (1801-58) made physiology systematic and broadly comparative. Not only did he do important work himself, but as professor of physiology at Berlin he trained many talented young men, among whom were Ludwig (1816-95), Du Bois-Raymond (1818-96) and Helmholtz (q v.). Thus his influence reached to the present time and affected recent physiology. With these distinguished German physiologists should be mentioned Claud Bernard (1813-78) of France and Sir Michael Foster of England (1836-).

Physiology has broadened and deepened until it includes the vital activities of the entire animal and vegetable worlds. Every action or function dependent upon life is embraced by it. These are so varied that they must be reduced to order and system, and, when that has been done, we observe that all the functions may be grouped under three great headings: Those concerned with nutrition; those connected with relation; and those pertaining to reproduction. Nutrition embraces every activity concerned in nourishing the body. It must include a discussion of the blood, its structure, circulation and changes; the heart and the influences which affect it; the digestive system, the kinds of food, the nature of digestion, the absorption of the food into