22 STEUCTUEAL COMPOSITION OF HUMAN BODY. 



belongs to a cell which constitutes part of a vegetable 

 structure, or even a cell which by itself forms an indepen- 

 dent 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 

 apparatus, 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 immea- 

 surable minuteness to the 10,000th of an inch in diameter; 

 of various and generally uncertain composition, but usually 

 so affecting light transmitted through them, that at dif- 

 ferent focal distances their centre, or margin, or whole 

 substance, appears black. From this character, as well as 

 from their low specific gravity (for in microscopic examina- 

 tions they always appear lighter than water), and from 

 their solubility in ether when they can be favourably 



