TISSUES OF THE BODY. 



315 



point tliere are doubtless some few which are correct (always excluding 

 those based on optical illusions), but they are probably the exception (1). 



One discovery made by Beale some years ago, on the other hand, we do 

 look upon as correct, having satisfied ourselves further by personal observa- 

 tion. It applies to the ganglion cell of 

 the tree frog (fig. 306). Here may be 

 seen, namely, making its exit from the 

 interior of the pear-shaped or round 

 structure, at its pointed end, a straight 

 fibre, in which a nucleus (c, e) is not un- 

 frequently visible. This is surrounded 

 by the coils of one or more spiral fibres, 

 which also contain nuclei. These spring 

 from the surface of the cell-body in 

 close spiral turns (d, d), which increase 

 in girth as they proceed on their way 

 round the straight fibre, until they 

 eventually attain a straight course them- 

 selves, and pass on as spinal fibres with 

 their own special sheath (/). The first 

 mentioned fibre, which, as has been 

 already mentioned, springs from the in- 

 terior of the cell-body (whether from the 

 nucleus or no we are unable to say with 

 certainty), is of nervous structure. Beale 

 maintains also that the spiral have the 

 same character, though they appear to 

 us to be, as a rule, of elastic nature. 

 But we are far from denying the pos- 

 sibility that, when two fibres spring 

 from the one pole of a ganglion cell, 

 one of them may encircle the other 

 spirally. /. Arnold maintains that 

 these cells possess a much more highly 

 complicated structure. 



Deiters mentions having found a double mode of arrangement in the 

 ramifications of the cells of the nervous centres (fig. 307). The greater 

 number of the processes is formed, namely, by prolongations in various 

 directions of the protoplasm composing the body of the cell. These 

 "protoplasm processes" then undergo repeated subdivision, giving off 

 branches in all directions, until they finally dip into the sustentacular 

 matter in the form of the most extremely delicate fibrillaa. 



From these protoplasm ramifications we can distinguish at a glance one 

 peculiarly long process (a), which springs either from the cell itself or 

 from one of the first and broadest prolongations of its body. This pro- 

 cess never gives off branches, and becomes clothed later on with a 

 medullary sheath ; it is known as the " axis cylinder process" Of the 

 correctness of this we may easily convince ourselves. 



Beside these just mentioned, other very fine fibres may be recognised 

 (b, 5), leaving the protoplasm ramifications at right angles. In these 

 Deiters supposes (without, however, giving any reason for his opinion), 

 that we have before us a second system of extremely fine axis 

 cylinders. 



Fig. 306. Ganglion cells from the sympa- 

 thetic of the tree frog (from Beale). a, 

 cell-body; 6, envelope; c, straight nervous 

 fibre; d, spiral fibres; e, continuation of 

 the former; and /, of the latter. 



