DEVELOPMENT OF NERVOUS TISSUE. 991 



they increase in number. In other words, the individual sarcous elements 

 retain their size, but they are accumulated in a greater number of longitudinal 

 rows. In the enlargement of the muscles which takes place from exercise, in 

 all probability the fibres do not increase in number, but in size, and contain 

 either more or larger fibrillse. In the opposite condition of the wasting of a 

 muscle, the fibres remain the same in number, but become smaller, owing to 

 a diminution of their contents ; the fibrillse also decrease in number, grow 

 indistinct, or even disappear altogether. In a wholly paralyzed, unused, or 

 diseased muscle, fatty matter is substituted for the characteristic sarcous 

 elements. 



It is obvious that such striped muscular fibres as, like those of the heart, 

 are but indistinctly striated, may be regarded as less perfectly developed fibres. 

 Certain of the smooth fibres, in which the sarcous elements are very distinctly 

 granular, or dotted, also approach in character and development to the higher 

 or striated form of fibre. The fusiform fibre cells, and last of all, the elongated, 

 spindle-shaped, oval, or round contractile cells of the heart of the embryo, are 

 the lowest form of all. There is thus a gradual transition from the simplest 

 to the most complex form of muscle cells. 



In the case of the ramified form of the striated muscular fibre, noticed in the 

 tongue, lips, and face, of the reticular form observed in the walls of the adult 

 heart, the primitive nucleated cells out of which they are developed, either 

 simply ramify, or ramify and coalesce with the branches of other cells, and 

 then acquire their sarcous contents. 



Nervous Tissue. 



The ganglionic cells, Fig. 122, 6, are derived from metamorphosed embryo- 

 cells, and from the direct descendants of those cells, by ordinary modes of 

 multiplication. The rounded ganglionic cells are formed by a simple enlarge- 

 ment, and a gradual alteration of their contents ; the nuclei persist, and are 

 very distinct ; the branched cells are formed by the outgrowth of one or more 

 of the peculiar processes with which they are provided. The envelope is pro- 

 longed on to the processes, and becomes connected with the homogeneous 

 tubules of the nerve fibres ; the processes contain nervous substance. The very 

 small rounded cells, and the free nuclei, found in some parts of the gray sub- 

 stance of the nervous centres, may be early stages of future ganglionic cells. 



The gray or gelatinous fibres, found chiefly in the sympathetic system, whether 

 they be regarded as true nervous elements, or as a peculiar form of connective 

 tissue cells, appear to be produced by the coalescence of elongating fusiform 

 nucleated cells, the contents of which, as the cells enlarge, become soft and 

 finely granular, whilst the nuclei appear wider and wider apart. Even in the 

 most highly developed of these fibres there is but little evidence of a tubular 

 character or wall. The medullary sheath or fatty layer is absent. They have 

 been compared, by some writers, *to the non-medullated portions of the white 

 nerve fibres, or to the axis cylinder or central band only of those fibres, which, 

 however, have a tubular sheath. The most perfect gray fibres certainly re- 

 semble a transitory condition of the fibres out of which the white or tubular 

 nerve fibres are developed. 



The white dark-bordered, or double contoured tubular fibres, are themselves 

 derived from fusiform nucleated cells, which are embryo cells, or their descend- 

 ants. By coalescing, they first form gray granular fibres, with elongated 

 nuclei at intervals, and, in that stage, resemble the gray or gelatinous fibres 

 of the sympathetic system. But the grayish contents of these fibres soon be- 

 come opaque and white, and resolve themselves into the central albuminoid 

 band or axis cylinder, and then acquire the surrounding fatty layer or medul- 

 lary sheath ; whilst the walls of the coalesced cells are said to unite, to form 

 the outer tubular membranous sheath of the perfect fibre. Instead of imagin- 

 ing many cells to coalesce, a single cell may be supposed to go on dividing, to 

 form a nerve fibre. 



The branched terminations of the nerves, according to what has been seen 

 in the tadpole, originate in the junction of ramified formative cells, which keep 

 on joining those already further developed. 



