SECT. III. 



APERTURES. 365 



the membrane composing them is often interrupted 

 by the occurrence of a number of apertures or 

 openings effected in its fabric, and affecting its 

 general aspect ; and which may be reduced to the 

 two following species : namely, Pores and Gaps. 



SUBSECTION I. 



Pores. Pores are small and minute openings of Descrip- 

 various shapes and dimensions, that seem to be 

 destined to the absorption, transmission, or ex- 

 halation of fluids. They are distinguishable into 

 the two following sorts : Perceptible Pores, and 

 Imperceptible Pores. 



ARTICLE 1. Perceptible Pores. The perceptible Found i 

 pores are either external or internal, and are the 

 apertures described by Hedwig as discoverable in 

 the net-work constituting the epidermis ; or by 

 Mirbel as perforating the membranes composing 

 the cells and tubes, and forming a communication 

 between them. Saussure, in his observations on. 

 the leaves and flowers of plants, says indeed that 

 he could discover no pores in the epidermis. And 

 yet it is plain that he describes under the appellation 

 of glands what Hedwig describes under the appella- 

 tion of pores, though they might not use the term 

 epidermis in the same extent. But Senebier, who 

 understood by the term the same as Hedwig, says 

 he looked for the pores in question in a great variety 



