14 SUPPOSED PORES ARE PITS. [BOOK i. 



opinion to the contrary of this has been held by some bota- 

 nists, 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 ill-constructed glasses, is now 

 generally abandoned. (See p. 16.) 



Different explanations have been given of the supposed 

 pores. Dutrochet asserted them to be grains of 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 trans- 

 parency, a property incompatible with a perforation. But 

 the so-called pores operated upon by this observer were 

 particles of some secreted matter sticking to the membrane. 

 Slack believed them to be, in other cases, thin spaces in the 

 sides of tissue, such as might be produced by the adhesion 

 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 Mohl, 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 miscroscope 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). To the latter cause is undoubtedly 

 owing the general appearance of dots, as has now been ascer- 

 tained 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 anywhere else. 

 (See Plate II. fig. 2.) They are therefore not dots, but pits- 



Should the young observer fail in seeing the pits in their 

 natural state, the application of tincture of iodine to the 

 subject under examination will enable him to discover them 



