ABSORPTION BY BLOOD-VESSELS. 377 



takes place more rapidly from the rectum than from the 

 stomach. Strychnia, for example, when in solution, pro- 

 duces its poisonous effects much more speedily when intro- 

 duced into the rectum than into the stomach. When 

 introduced in the solid form, however, it is absorbed more 

 rapidly from the stomach than from the rectum, doubtless 

 because of the greater solvent property of the secretion of 

 the former than of that of the latter. 



It is, probably, a general truth, that in parts which are 

 supplied with both blood-vessels and lymphatics, the lym- 

 phatics (or lacteals for the intestines) absorb only certain 

 materials for the replenishing of the blood, while the 

 blood-vessels absorb not only nutrient matters, but all 

 other soluble materials that are accidentally brought into 

 contact with them. But in parts which receive only blood- 

 vessels, these alone must perform the whole function of 

 absorption, as they do in invertebrate animals. 



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; for, 

 naturally, the blood-vessels are not bare to absorb. 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 of 

 their epithelia ; so Miiller found the urinary bladder of a 

 frog traversed in less than a second ; and the absorption 

 of poisons by the stomach or lungs appears sometimes 

 accomplished in an immeasurably small time. 



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 with which it is brought in contact. Hence 

 the marks of tattooing, and the discoloration produced by 

 nitrate of silver taken internally, remain. Mercury may 



