Page:Popular Science Monthly Volume 18.djvu/418

404 cording to the degree of functional exaltation. The cutis vera being a superficial vascular tissue, the excessive accumulation of red corpuscles in its capillaries is readily perceived by the consequent floridity of surface. Such sensible reaction to direct irritation implies the concurrence of several determinate acts in the structures directly involved, as well as the cooperation of more remote parts. Thus the tissue-elements must possess a responsive power to become exalted in function, and to solicit a surplus of blood-ingredients they must also retain a continuity with the presiding nerve-center, whereby the peripheral impression may be centripetally transmitted along the afferent nerve to this point, thence reflected along the vaso-motor nerve, causing relaxation of the arteriolar muscles, enlargement of caliber, and a freer flow of blood into the part. Cardiac contractions are also necessary to propel the corpuscles into the capillaries, as the attraction of the tissue-elements for these minute bodies can act only at insensible distances.

Man's structure conceals the changes which occur within the minute blood-vessels, but some animals admit the examination of the interior processes which accompany and conduce to the external manifestations of capillary congestion. Observing the circulation in the web of the frog's foot under the microscope, fluctuations in its current are noticed independent of the heart's action. The corpuscles, perhaps flowing uniformly at first, may slacken their speed, then oscillate or even retrograde. Apply an irritant to the part, the flow soon increases, and a greater number of red corpuscles pass through in a given time; they also show a tendency to cohere as well as to adhere to the walls of the vessels, which may proceed so far as to choke up their caliber and prevent the transmission of blood. As the effect passes off, the corpuscles gradually separate, move on, and at length the circulation resumes its normal state. Such investigation explains the nature of the changes which occur in the capillaries of the human skin under artificial stimulation.

Heat, which is the most potent and available form of irritant, when applied to the skin so as to considerably elevate its temperature above the normal point, causes first an efflorescence of surface, deeper at the center and shading off gradually toward the circumference. This redness can be temporarily displaced, leaving a white impression, which disappears on removal of the pressure, the part resuming its floridity with a rapidity commensurate with the activity of the capillary circulation. By increasing the heat or prolonging its action the color becomes more distinct, till at the point of greatest intensity the cuticle becomes detached from its subjacent cutis by the gradual exudation and accumulation of a fluid which thus forms a true vesicle. A spurious vesicle may be similarly produced on the dead subject, but such is a purely physical and local effect, entirely different from the more comprehensive action and characters of the physiological process.

In post-mortem vesication the contents are generally gaseous from