Page:Popular Science Monthly Volume 2.djvu/708

688 at work, have a highly-developed arterial system. The blood is renewed in the glands as in all the tissues, and the dilatable walls of the vessels admit it in different proportions, according to the state of rest or activity of the organ. In this case, as in all others, particular facts are merely the expression of a more general law. The flow of blood increases when a stimulant exerts its action. When a glandular element acts with energy, it produces a high degree of fulness of blood in all the neighboring parts; and physiologists have thoroughly proved this fact in the case of the salivary glands of animals. In a state of rest, the congestion of these glands is slight, the blood dark in the veins issuing from them; the organ is then gaining growth. When the animal taken for experiment emits saliva under the influence of artificial stimulus, the glands, on the contrary, fill with blood, the vessels grow turgid, and take a high vermilion color. Thus variations in the supply of blood always correspond with degrees in secreting activity, and secretion ceases when the blood no longer comes to the glands. If the vessels of the liver are obliterated, bile ceases to be formed; if the arteries of the kidneys are compressed, secretion by those organs stops. The statement of the conditions of the problem suffices to suggest its solution; the blood is a medium whence the glands draw the principles of their growth and functions; the circulation of the sanguine fluid within the glandular tissue is a true transfusion kept up incessantly by the heart, and which only artificial transfusion can be a substitute for.

Nutrition and secretion by their constant work keep up the organized state of vegetable and animal matter. Plants possess only these two functions, and we may almost say the same thing as to animals while asleep; but these are not the only functions assigned to the waking animal, which comes into relations with the external world, through motion, sensibility, and intelligence. The muscular fibre, the special organ of motion, has its activity, independent of the nervous system; local transfusion confirms this scientific view, which now rests on manifold proofs. Like the secreting element, the muscular fibre is distinct in some lower animals; the microscope detects it in that state in the transparent body of the infusoria called vorticelli. In the higher degrees of the animal series this fibre is found in connection with nerves and vessels, and, though it enjoys an excitability peculiar to itself, it receives an impulse to movement from the motor nerve. The contact of the muscular fibre with blood-vessels is very close; but the chemical composition of the blood that moistens it varies according to the quantity of work yielded; thus it is indispensable that new fluid should be constantly transfused into the network of veins in the muscle. If the motor fibre is at rest, the blood passing through it is scarcely modified. If it is in a state of half contraction, the oxygen decreases in the blood, and the carbonic acid increases. If contraction is evident and powerful, combustion and production of