TISSUES OF THE BODY. 319 



the latter. In such cases the connective-tissue envelope is perineurium 

 and neurilemraa at the same time. These facts, however, are variously 

 viewed, some regarding this simplified perineurium as a thick primitive 

 sheath. 



The trunks and branches of the sympathetic system are essentially the 

 same in structure as those just described, except that in them Remak's 

 fibres (already mentioned at 176) make their appearance in great 

 numbers. 



REMARKS. This name was first applied by Robin to the envelopes composed of 

 simple connective-tissue of the finest twigs. 



181. 



From time immemorial anatomists and physiologists have been occu- 

 pied with the question, how the nervous fibres terminate peripherally. 

 In older times, of course, before the introduction of microscopic analysis, 

 conjecture alone could be formed on this point. It was supposed that 

 the nervous twigs broke up into finer and finer fibrils, which became 

 fused finally with the tissue of the organ they supplied. 



With the aid of the microscope, about the thirtieth year of this 

 century, it was already a matter of no difficulty to follow the progressive 

 sub-division of the smaller branches down to their finest twigs, and to 

 recognise here and there the course of the latter through the tissue, as 

 well as the formation of the plexuses and minutest anastomoses already 

 mentioned ( 180). 



At that time many observers maintained that they had found a 

 looped termination, and, moreover, in the most different organs. They 

 supposed that two neighbouring fibres were always united at the peri- 

 phery in the form of a wider or narrower loop, or, what is but another 

 mode of expressing the same thing, that each nerve tube, on arriving at 

 the periphery, became doubled on itself, and took its course back again to 

 the nervous centre, from whence it came. This theory of terminal loops, 

 which held good for motor and sensitive fibres alike, led however to great 

 physiological difficulties. 



We now know, from a series of newer and more accurate investigations, 

 that these loops are of frequent occurrence among the peripheral ramifica- 

 tions of nerves, but that they possess no terminal significance, since the 

 nerve fibre in this curved course has not yet arrived at its final destina- 

 tion. This theory of the looped termination of nerves has, therefore, 

 disappeared from histology. 



As far as we know at present (though our knowledge on the subject 

 is still in a most unsatisfactory condition), nervous fibres terminate without 

 any medullary substance first of all in the form of simple or ramifying 

 axis cylinders, and, in the next place, in the form of primitive fibrillce. 

 The final disappearance of the fibre, moreover, is frequently seen to take 

 place in special "terminal structures" or "end corpuscles." These are 

 either complex or single-celled. 



182. 



It was for some time supposed that an insight of the mode of termina- 

 tion of the motor nerves in striped muscle (fig. 309) had been gained with 

 tolerable correctness through the labours of R. Wagner and Reichert (1). 

 It was believed that the motor nerve tube ceased on the striped fibre after 

 repeated sub-division in the form of pale terminal filaments. On account 



