FORMATION OF NERVE TISSUE. 99 



the lines in different cases may be accounted for by 

 the altered rate of performance of the operations 

 above referred to. 



144. Formation of nerve tissue. It has been 

 generally considered that the tissue of the nerve fibre 

 was peculiar, and that its function's was in some 

 manner determined by the peculiarity of its structure, 

 or by its chemical composition. Such a view is, how- 

 ever, not supported by facts. For when we come to 

 examine the axis cylinder or core of the nerve fibre, 

 which is undoubtedly the active and really essential 

 part, and that which is alone instrumental in trans- 

 mitting the nerve current, we find that this filament 

 possesses an exceedingly simple structure, and, at 

 least in some animals, looks very like ordinary fibrous 

 tissue. Indeed, if we were shown only a very small 

 piece of an axis cylinder of a frog's nerve fibre, and 

 some pieces of fibrous tissue of the same shape and 

 size, we should not be able with certainty to distin- 

 guish one from the other. 



145. Formation of fibrin. One of the simplest 

 forms of tissue which is most widely distributed 

 among other tissues of man, and the higher vertebrata, 

 appears under the microscope to consist entirely of 

 delicate fibres passing from one point to another. 

 Extremely delicate fibres may be formed by every 

 kind of bioplasm, and these result from its death. 

 The substance known as fibrin consists of fibres which 

 interlace in all directions, and which probably have 

 been formed from matter produced by the white blood 

 corpuscles. A white blood corpuscle, a mucus cor- 

 puscle, or other kind of bioplasm may move onwards, 

 leaving behind it a trail of newly formed lifeless ma- 

 terial consisting of a mucus-like mass of delicate fibres. 



Fibres of fibrin gradually acquire firmness by the 

 closer approximation of the material of which they 

 consist, and the gradual expression from its substance 

 of more and more of the fluid existing in relation 



H 2 



