Section of pipe or
moor grass.
Cross-section of bulrush,
Scirpus lacustris.
In the theodolite and sextant, in the geometric lathe of the bank-note engraver, are ball-and-socket joints allowing motion in any plane. Equally free in their movements are the shoulder and hip joints, while their surfaces are lubricated by a delicate synovial fluid supplied just as it is wanted. When pumps first received valves to direct their flow in one direction, their inventor was no doubt gratified at his skill. In the heart within his own breast, in his veins and arteries, were simple valves engaged in a similar task as they directed the currents of his blood. In pumps such as are common in farm-yards, the action is jerky, the stream flowing and ebbing from moment to moment as the arm rises and falls. The tide of human blood would have the same uneven pulse were it not for the elasticity of its arterial walls. Their elasticity serves to equalize the flow, much as the air does in large chambers on pumps for mines or waterworks.
Human hip joint in section. From “The Human Body,” by H. N. Martin. Copyright, Henry Holt & Co., New York, 1884. Reproduced by their permission.
Valves of veins.
C, a capillary; H, the heart end of the vessel. From “The Human Body,” by H. N. Martin. Copyright, 1884, Henry Holt & Co., New York, and reproduced by their permission.
Built-up gun.