OF DIGESTION. 17 



fasting, this fluid not being present, these vessels are colour- 

 less. 



Absorption by the veins may also be demonstrated in the 

 same way, that is, by experiments on living animals. 



37. Circumstances influencing Absorption. The first 

 condition essential to absorption is the permeability of the 

 tissue interposed between the substance to be absorbed, and 

 the liquids which form the means of transport to its destina- 

 tion ; so that, cceteris paribus, absorption is rapid in the direct 

 ratio of the sponginess and softness of the tissue. It may 

 also be laid down as a principle, that absorption is rapid in 

 a direct ratio to the vascularity of the tissue. As absorp- 

 tion is mostly effected by the veins, the abundance of these 

 influences necessarily the function. Thus, anatomically, may 

 almost be predicated the enormous differences in the rapidity 

 with which various substances are absorbed by different 

 tissues. Of the lungs, for example, pre-eminently so spongy 

 and vascular, it might be predicted that absorption would in 

 them be most rapid ; and this is in fact the case. 



The cellular tissue, forming the basis and connecting 

 medium of most of the organs, is also, by its soft and spongy 

 nature, the seat of rapid absorption, but less so than the lungs 

 as being much less vascular. 



The skin, on the other hand, being but little vascular, and 

 being at the same time covered by the almost impermeable 

 epidermis or scarf-skin, explains why we can handle dan- 

 gerous poisons with safety, so long as the epidermis is 

 unbroken. 



A state of plethora (from 7r\r)da>, I fill) exercises an in- 

 fluence over the rapidity of absorption. The quantity of 

 liquids an animal body may contain is limited, and desiccation 

 also has its limits. The nearer the animal may be to the 

 point of plethora or saturation, the less will it absorb. 



Thus, poison administered to two living dogs will influence 

 the one in a state of plethora much more slowly than the 

 other which has been previously reduced by a copious bleed- 

 ing ; and finally, cceteris paribus, absorption will be less rapid 

 as the liquids to be absorbed are less liquid and less fitted to 

 moisten the tissues. 



OF DIGESTION. 



38. One of the principal means by which the absorp- 

 tion of the matters necessary for the nutrition of animals 

 c 



