CHAP. I. OF THE ELEMENTARY ORGANS. 5 
readily permeable by fluids, it must necessarily be furnished 
with invisible passages. An opinion to the contrary of this 
has been held by some botanists, who have described the 
existence of holes or pores in the membrane of tissue, and 
have even thought they saw a distinct rim to them ; but this 
idea, which originated in imperfect observation with illcon- 
structed glasses, is now generally abandoned. Different ex- 
planations have been given of the nature of the supposed 
pores. Dutrochet asserted them to be grains of semi-trans- 
parent matter sticking to the membrane : he found that 
boiling them in hot nitric acid rendered them opaque, and 
that treating them with a solution of caustic potash restored 
their transparency, — a property incompatible with a perfo- 
ration. Slack believed them to be, in other cases, thin spaces 
in the sides of tissue, such as might be produced by the ad- 
hesion and separation at regular intervals of a thread developed 
spirally within a membranous sac [Trans. Soc. Arts, xlix.). 
A nearly similar opinion was previously offered by Mold, who 
considers the dots on the membrane of tissue to be thinner 
portions of it. He says it may be distinctly seen by the aid 
of a powerful microscope that the little circles which are 
visible on the surface of the tissue of Palm-trees are passages 
(meatus) in the thickness of the membrane, opening into the 
cavity of the cells, and closed externally by the membrane 
itself. He adds, that when dotted tissue is in contact, these 
passages are placed exactly opposite to each other. [Martins 
Palm. Anat. v. col. 2.) The latter is undoubtedly the general 
cause of the appearance of dots, as has now been ascertained 
by repeated observations. If a thin section of any vessel or 
cell, the sides of which appear to be dotted, is placed under a 
good microscope, it will be found to have the matter deposited 
on its sides, pierced with short passages, which give the 
appearance of dotting, because the sides of the membrane are 
thinner where they are stationed than any where else. (See 
Plate II. fig. 2.) They are therefore not dots, but pits. 
Should the observer fail in seeing the pits in their 
natural state, the application of tincture of iodine to the sub- 
ject under examination will enable him to discover them 
readily, with a magnifying power of 350 diameters. But it is 
B 3 
