STRUCTURELESS MEMBRANE. 3 1 



only subserves a mechanical function, would be altered very 

 soon by any defect in the more essential parts concerned in 

 circulation, respiration, &c. But if we take simply the life- 

 history of one of the small cells which constitute the epider- 

 mis, we find that it absorbs nourishment from the parts 

 around, grows, and develops in a manner exactly analogous 

 to that which belongs to one of the cells which constitute 

 the outer covering of a tree, or a cell which by itself forms 

 an independent being. 



Remembering, however, the invariable presence of a 

 living albuminous matter or protoplasm of apparently 

 identical composition in all living tissues, animal and 

 vegetable, we must not forget that its relations to the 

 parts with which it is incorporated are still very doubtfully 

 known ; and all theories concerning it must be considered 

 only tentative and of uncertain stability. 



Among the anatomical elements of the human body, 

 some appear, even with the help of the best microscopic ap- 

 paratus, perfectly uniform and simple : they show no trace 

 of structure, i.e., of being composed of definitely arranged 

 dissimilar parts. These are named simple, structureless, or 

 amorphous substances. Such is the simple membrane which 

 forms the walls of most primary cells, of the finest gland- 

 ducts, and of the sarcolemma of muscular fibre ; and such 

 is the membrane enveloping the vitreous humour of the 

 eye. Such also, having a dimly granular appearance, 

 but no really granular structure, is the intercellular sub- 

 stance of the so-called hyaline cartilage. 



In the parts which present determinate structure, 

 certain primary forms may be distinguished, which, by 

 their various modifications and modes of combination make 

 up the tissues and organs of the body. Such are, i . Gra- 

 nules or molecules, the simplest and minutest of the primary 

 forms. They are particles of various sizes, from immeasur- 

 able minuteness to the io,ooothof an inch in diameter; of 

 various and generally uncertain composition, but usually 



