404 THE POPULAR SCIENCE MONTHLY. 



cording to the degree of functional exaltation. The cutis vera being 

 a superficial vascular tissue, the excessive accumulation of red corpus- 

 cles in its capillaries is readily perceived by the consequent Acridity of 

 surface. Such sensible reaction to direct irritation implies the concur- 

 rence 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-ele- 

 ments 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 retro- 

 grade. 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 pre- 

 vent 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 stimu- 

 lation. 



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 efliorescence of surface, deeper at the 

 center and shading off gradually toward the circumference. This red- 

 ness 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 circu- 

 lation. By increasing the heat or prolonging its action the color be- 

 comes 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 spu- 

 rious 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 



