314 HANDBOOK OF PHYSIOLOGY. 



less because of the greater solvent property of the secretion of the former 

 than of the latter. 



With regard to the degree of absorption by living blood-vessels, much 

 depends on the facility with which the substance to be absorbed can 

 penetrate the membrane or tissue which lies between it and the blood- 

 vessels. Thus, absorption will hardly take place through the epidermis, 

 but is quick when the epidermis is removed, and the same vessels are 

 covered with only the surface of the cutis, or with granulations. In 

 general; the absorption through membranes is in an inverse proportion 

 to the thickness o-f their epithelia^ so that the urinary bladder of a frog 

 is traversed in less than a second; and the absorption of poisons by the 

 stomach or lungs appears sometimes accomplished in an immeasurably 

 small time. \ 



Conditions for Absorption. 1. The substance to be absorbed must, 

 as a general rule, be in the liquid or gaseous state, or, if a solid, must be 

 soluble in the fluids ivith which it is brought into contact. Hence the 

 marks of tattooing, and the discoloration produced by silver nitrate 

 taken internally, remain. Mercury may be absorbed even in the metallic 

 state; and in that state may pass into and remain in the blood-vessels, 

 or be deposited from them; and such substances as exceedingly finely- 

 divided charcoal, when taken into the alimentary canal, have been found 

 in the mesenteric veins; the insoluble materials of ointments may also 

 be rubbed into the blood-vessels; but there are no facts to determine 

 how these various substances effect their passage. Oil, minutely divided, 

 in an emulsion, will pass slowly into blood-vessels, as it will through 

 a\ filter moistened with water ; and, without doubt, fatty matters find 

 their way into the blood-vessels as well as the lymph-vessels of the in- 

 testinal canal, although the latter seem to be specially intended for their 

 .absorption. 



2. The less dense the fluid to be absorbed, the more speedy, as a gen- 

 eral rule, is its absorption by the living blood-vessels. Hence the rapid 

 absorption of water from the stomach ; also of weak saline solutions ; 

 but with strong solutions, there appears less absorption into, than effu- 

 sion from, the blood-vessels. 



3. The absorption is the less rapid the fuller and tenser the blood- 

 vessels are ; and the tension may be so great as to hinder altogether the 

 entrance of more fluid. Thus, if water is injected into a dog's veins to 

 repletion, poison is absorbed very slowly ; but when the tension of the 

 vessels is diminished by bleeding, the poison acts quickly. So, when 

 cupping-glasses are placed over a poisoned wound, they retard the ab- 

 sorption of the poison not only by diminishing the velocity of the circu- 

 lation in the part, but by filling all its vessels too full to admit more. 



4. On the same ground, absorption is the quicker the more rapid the 



