42 THE ELEMENTS OF STRUCTURE. 



nerves are usually surrounded by an enveloping nucleated layer called 

 Schwann's sheath, or else by neuroglia. 



A nerve fibre consists of numerous fibrils like those seen within a 

 ganglion cell. These are regarded by some as the essential elements in 

 conducting stimuli, while others maintain that the essential part is the 

 less compact, sometimes well-nigh fluid stuff between the fibrils, or that 

 the fibrils are but the walls of tubes within which the essentially nervous 

 stuff lies. 



According to some authorities, the nerve fibres arise as extensive pro- 

 longations of the ganglion cells ; accoiding to others, the neuroglia or 

 other ensheathing elements contribute to the extension of the nerve 

 fibres, or rather special neuroblast cells make both sheath and fibre. 



IV. Cells. — In discussing tissues, it was necessary to 

 refer to the component cells. Let us now consider the 

 chief characteristics of these elements. 



A cell is a unit mass of living matter. Most of the 

 simplest animals and plants (Protozoa and Protophyta) are 

 single cells ; eggs and male elements are single cells ; in 

 multicellular organisms the cells are combined into tissues 

 and organs. 



Most cells are too small to be distinguished except 

 through lenses ; many Protozoa, e.g. large Amcebas, are 

 just visible to our unaided eyes ; the chalk-forming Fora- 

 minifera are single cells, whose shells are often as large as 

 pin-heads, and some of the extinct kinds were as big as 

 half-crowns (see Fig. 17) ; the bast cells of plants may extend 

 for several inches ; the largest animal cells are eggs distended 

 with yolk. 



The typical and primitive form of cell is a sphere, — a 

 shape naturally assumed by a complex coherent substance 

 situated in a medium different from itself. Most egg cells 

 and many Protozoa retain this primitive form, but the 

 internal and external conditions of life (such as nutrition 

 and pressure) often evolve other shapes, — oval, rectangular, 

 flattened, thread-like, stellate, and so on. 



As to the structure of a cell, we may distinguish — 



(a) The general cell substance or cytoplasm, which con- 

 sists partly of genuinely living stuff or protoplasm, and 

 partly of complex materials not really living ; 



(b) A specialised kernel or nucleus, with a complex 

 structure, and important, but hardly, as yet, definable 

 functions ; 



(c) One or more specialised bodies called central 



