STRUCTURAL, CHARACTERS OF ANIMAL ORGANISMS. 41 



motor or secretory apparatus. Where the effect of a stimulus can 

 be definitely traced from one nerve cell to another, and from thence 

 by a second fibre to a third cell, the impulse is said to be reflected 

 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. 



FIG. 19. 



Multipolar cells from the anterior gray column of the spinal cord of the 

 dog-fish (a) lying in a texture of fibrils ; (6) prolongation from cells ; (c) 

 nerve fibres cut across. (Cadiat.) 



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

 the brain and spinal cord) 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 

 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. 



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