STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 49 



by the second cell to the third. And there we have what is called 

 a reflex act. 



The essential part of a nerve fibre is a kind of protoplasmic 

 band, in which the finest fibrilla or thread-like marking can be 

 made out with the aid of reagents and a powerful microscope. 

 This is called the axis cylinder. In some nerve fibres (mostly in 

 the brain and spinal chord) the axis cylinder is naked, and even 

 a single fibrilla may so pass from one cell to another in the brain 

 matter. In other parts the axis cylinder is generally covered by 

 a thin membrane, called the primitive sheath, or with a soft, oil-like 



FIG. 19. 



Multipolar cells from the anterior gray column of the spinal chord of the dog-fish 

 (a) lying in a texture of fibrils ; (b) prolongation from cells ; (c) nerve fibres cut across 

 (Cadiat.) 



substance, called the medullary sheath, or, as is commonly the case 

 in all peripheral nerves, by both. The primitive sheath incloses 

 the medullary sheath, which surrounds the axis cylinder. 



These fibres are made of peculiarly modified cells, which are, 

 however, so elongated as not to be very easily recognized as such 

 in adult tissue. 



The nerve or ganglion cells vary extremely in general form and 

 size. The commonest in the nerve centres are large bodies with 

 a clear, well-defined single nucleus, and distinct nucleolus ; they 

 5 



