DEVELOPMENT OF TISSUES FROM CELLS. 41 



Other, but each runs a separate and distinct course. Those fibres 

 which originate on the periphery and run towards the brain and 

 spinal cord, also form loops in these centres. 



Wherever the vesicular matter is found, it is looked upon as a 

 generator or originator of nervous influence ; whilst the white or 

 tubular^ is the carrier of thai influence to the various parts of the 

 system. The former, having the higher set of functions, receives by 

 far the larger quantity of blood. 



Plexuses are formed by the free interchange of fibres from several 

 neighbouring nerves. Four or five nerves, for instance, proceed 

 from the spinal cord and are plaited up together like the strands of a 

 coach whip. From the plexus thus formed, certain nerves emerge 

 which are composed of fibres from several of the original trunks. 

 The advantage of this arrangement is, that not only are nerves of 

 different endowments joined together, but the injurious effects which 

 would otherwise result from lesion of the spinal cord, are obviated ; 

 for the nerves which come off* below the injured part, all receiving 

 filaments from those which are above, the nervous influence is thus 

 transmitted unimpaired to all those parts supplied by filaments from 

 below the point of lesion. 



OF THE DEVELOPMENT OF TISSUES FROM CELLS. 



It has been seen that there is reason to doubt whether cells are 

 concerned in the development of all the tissues, further than in the 

 part which they take in elaborating the fluid from which the tissues 

 are derived, some of the structures seeming to be produced by a 

 consolidation of the plastic fluid which has been elaborated by their 

 agency. Many of the component structures, however, owe their 

 development to the agency of cells, and of these we shall now speak. 



Cells are formed in two ways, either in a previously existing, 

 structureless fluid called a blastema^ or within the interior of pre- 

 viously existing cells. In the first method, when a plastic fluid is 

 in contact with a living structure it is seen to become opalescent ; this 

 change in colour is owing to the deposition within it of a number of 

 small granules called nucleoli ; several of these aggregate themselves 

 together and form what is called the nucleus^ within the interior of 

 which the nucleolus can still be seen. This nucleus is also called 

 the cytohlast, (from xuto?, a vesicle, and /3Xa^of, a germ,) or cell-gerra. 

 From the side of this nucleus a thin transparent membrane is next 

 seen to project in the manner of a watch-crystal from the dial; this 

 gradually enlarges till at last the nucleus is seen only as a spot on its 

 wall. The whole is then called a nucleated cell ^ or germinal cell. 

 The fluid in which the granules are first deposited is called the 

 cytohlastema. 



In the second method, or the endogenous development as it is 

 called, the nucleus seems to perform an important office. Each 

 granule of which it has been shown to consist, has the power of 



4# 



