168 



LOCOMOTION. 



[CHAP. vn. 



inquiry to those extremely simple and minute muscles which have 

 been already alluded to as existing in the smallest of living crea- 

 tures, though we can no longer trace nerves, yet our inability to do 

 so more probably depends on a corresponding simplicity of the ner- 

 vous substance itself, so that it ceases to be anatomically recogniz- 

 able, than on its entire absence. So general is the connexion of 

 the two tissues, and so apparently indispensable for the subjection 

 of the muscles to the purposes of the organism of which they con- 

 stitute a part, that we may regard it as constant and necessary. 



The distribution of the nerves through muscular structures has 

 always been a subject of great interest with those who looked to 

 this line of inquiry for some cluetothe explanation, either of thatwon- 

 derful active connexion subsisting between them, or of the nature of 

 the contractile act itself. But though the anatomical results accruing 

 from this inquiry are of a highly satisfactory kind, considered in 

 themselves alone, yet they cannot be said to have hitherto con- 

 tributed in any great degree to the elucidation of these mysterious 

 questions. The best mode of inspecting the arrangement of the 

 ultimate nervous twigs is, to select a very thin muscle, as one of 

 the abdominal muscles of any small animal, or one of the muscles 



of the eye of a small bird, to steep it in 

 weak acetic acid, and then to thin it out 

 under the compressorium. The primi- 

 tive tubules of the nerve may then be 

 readily distinguished with a moderately 

 high power. They separate from one 

 another at first in sets, afterwards in 

 twos, threes, or fours ; and, if these 

 be followed, they will be found ulti- 

 mately separating from one another, 

 forming arches, and returning either 

 to the same bundle from which they 

 set out, or to some neighbouring one 

 (fig. 48). 



In this ^Op-like COUTS6 they aCCOm- 

 p an y ^ O SOme extent the minute blood- 



vessels, but do not accurately follow them to their last windings, 

 since their distribution is in a different figure. They pass among 

 the fibres of the muscle, and touch the sarcolemma as they pass ; 

 but, as far as present researches have informed us, they are entirely 

 precluded by this structure from all contact with the contractile 



Fig. 48. 



Loop-like termination of the nerves in 

 voluntary muscle.-After Burdach. 





