:HAP. viii.] 



TERMINATION OF NERVES. 



221 



Fig. 60. 



Termination of Nerves. The connexion which the terminal fila- 

 ments of nerves form with the proximate constituents of the striped 

 muscle has been already described (p. 168). From that descrip- 

 tion it will appear that the nervous fibres do not come into imme- 

 diate communication with the sarcous substance, unless we have 

 recourse to the supposition that some minute elements proceed from 

 those fibres, and penetrate the sarcolemma. Such a supposition 

 has no foundation in anatomy, so far as our present knowledge 

 extends. It is a curious subject of investigation to determine 

 what becomes of the nerve-tubes, which, after the formation of 

 the loops which cross the muscular fibres, take a retrograde 

 course towards the nervous centre. Do they, for instance, return 

 to the spinal cord? and can it be their office to form a second 

 connexion with the vesicular matter of the cerebro-spinal centre, the 

 descending fibre coming from the brain, and the returning one 

 being implanted in the gray matter of the cord ? 



In the skin the arrangement is plexiform ; but this is reducible 

 to loopings, as will be explained in the chapter on Touch. 



The arrangement of the primitive 

 fibres in loops has been also seen by 

 Henle on some parts of mucous mem- 

 brane; in the membrana nictitans of 

 the frog, for example, and in the 

 mucous membrane of the throat in the 

 same animal. A similar disposition has 

 been described and delineated by Va- 

 lentin in the pulps of the teeth (fig. 60), 

 and we have seen it in the papillae of 

 the tongue. 



The so-called nerves of pure sense, 

 the olfactory, optic, and auditory 

 nerves, may more properly be regard- 

 ed as portions of the brain itself than 

 as mere nerves, for they possess most 

 of the anatomical characters of nervous 

 centres. The intra-cranial portion of 

 the first is as distinctly compounded of 

 vesicular and fibrous matter as a con- 



i ,. n ,1 i -r i Terminal nerves on the sac of the se- 



VOIUtlOn OI the brain. In the peripheral co d molar tooth of the lower jaw in the 



r -L- sheep, shewing the arrangement in loops. 



expansion of the optic nerve, the retina, (After Valentin). 



it will be hereafter shown that the vesicular elements of a nervous 



centre are as unequivocally present as in the olfactory bulb. As 



