TO ELEMENTS^OF NERVOUS TISSUE. 



Development. Nerve fibres are developed, like the other tissues, 

 from embryonic cells. These cells unite by their 

 extremities to form lubes, whilst their contents are 

 being transformed into nervous marrow and axis- 

 cylinder. We learn from the investigations of Kol- 

 liker, made upon the tail of the tadpole, that tl>e 

 peripheral extremities of nerve fibres take their 

 origin independently of branching or star-shaped 

 cells, and thus we have a clue to the mode of forma- 

 tion of their terminal divisions and loops. As to the 

 development of nerve cells, it is effected by a simple 

 change of shape and* volume of primordial cells. 



Nerve fibres, when divided, are reproduced, but 

 the exact mode in which the process is accomplished 

 is not well ascertained. It is asserted by some that 

 the peripheral extremity of the injured fibre disap- 

 pears, and is replaced by a newly formed fibre ; 

 others again suppose that the medulla alone is the 

 seat of change, and that the regeneration of the fibre 

 is effected by the formation of a new medulla in the 

 original tube. Farther research is obviously required 

 for the elucidation of this point of histogenesis. 



and bnt a few nerve fibres y^-^th to y^ths of a line in diameter ; and, 

 for the most part, a large quantity of sandy particles. 



The pituitary body contains, in its anterior reddish lobe, no nervous 

 elements, but rather, according to Ecker, the elements of a vascular 

 gland. The posterior smaller lobe consists of a finely granular substance, 

 with nuclei and blood-vessels, and possesses, also, fine varicose nerve- 

 tubes, which, like the vessels, descend to it from the infundibulum. K61- 

 liker, last edition, p. 233. (Ed.} 



