596 



we describe the peripheral connexion of nerves 

 with the various tissues and organs, and it is 

 much to be regretted that our knowledge in 

 reference to that connexion is scarcely more 

 complete or accurate than that of their origin. 

 The only instance in which we can speak 

 pretty confidently respecting the peripheral 

 connection of nerves, is with regard to mus- 

 cles. In the striped muscle, nerves appear to 

 form loops, the convexities of which are di- 

 rected across the fibres of the muscles. Each 

 nerve-fibre passes at first parallel to the direc- 

 tion of the muscular fibres, and then crosses 

 them in an arched form to pass back into the 

 bundle from which it had emerged, or to be 

 mingled with the fibres of some neighbouring 

 bundle, passing back in it to the centre, pro- 

 bably to some part of it different from the 

 place of origin of the nerve. As far as present 

 means of observation enable us to judge, there 

 does not appear to be any other connexion be- 

 tween the nerve-tubes and the muscular fibres 

 beyond the simple contact of the tubular mem- 

 brane of the former with the sarcolemma of the 

 latter. \\ e have no evidence of any mingling 

 of the true nerve-substance with the sarcous 

 elements, and, therefore, we are forced to con- 

 clude that whatever be the nature of the in- 

 fluence which nerve exerts upon muscle to pro- 

 voke it to contraction, that influence is exer- 

 cised through the two layers of homogeneous 

 membrane which form the investments of the 

 nervous and sarcous elements respectively. 



The best mode of observing the disposition 

 of nerve in muscle is to examine under the 

 microscope very thin and transparent muscles 

 of some small animals. The abdominal mus- 

 cles of the frog first afforded to Hales, and long 

 after him to Prevost and Dumas, this opportu- 

 nity ; the muscles of the eyeball in small birds 

 were used by Valentin; Burdach examined the 

 muscles of the frog's tongue; I have found the 

 intercostal muscles of the mouse very suitable 

 for the purpose. 



Peripheral expansion of nerves on sentient 

 surfaces. With regard to the disposition of 

 nerves on sentient surfaces (the skin, for ex- 

 ample) the most probable view appears to be 

 that they are disposed in a plexiform manner. 

 The nervous trunks pass toward the surface 

 dividing and subdividing, the ramifications pass- 

 ing backwards to the centre in conjunction with 

 neighbouring bundles; so that, whilst a very 

 intricate plexus is formed, the looped arrange- 

 ment, similar to that described in muscle, pre- 

 vails, the convexities of the loops being di- 

 rected towards the deep surface of the integu- 

 ments. Gerber states, that in those parts of 

 the skin which are provided with papilla;, the 

 nerve-loops pass into the bases of the papilla 

 and form an element of their composition; and 

 he adds, that in some instances the nerve-tube 

 which forms the loop exhibits tortuosities or 

 convolutions similar to those which are seen 

 upon bloodvessels. According to the same 

 author, in parts of the skin where the tactile 

 sensibility is acute, the meshes of the nervous 

 plexus are extremely small, whilst they are of 

 large size where the skin is not highly sensitive. 



NERVOUS SYSTEM. (NERVE.) 



The arrangement of the primitive fibres in 

 loops has been seen by Henle on some parts 

 of mucous membrane, in the membrana nicti- 

 tans of the frog for example, and in the mu- 

 cous membrane of the throat in the same ani- 

 mal. A similar disposition has been described 

 and delineated by Valentin on the pulps of the 

 teeth. (Fig. 333.) 



Fig. 333. 



Terminal newes on the sac of the second molar tooth 

 of the lower jaw in the sheep, showing the arrange- 

 ment in loops. {From Valentin.) 



Retina and optic nerve. The examination 

 of the peripheral connexions of the nerves 

 of pure sense has not thrown light on the 

 general question. The peripheral expansion 

 of the optic nerve or the retina presents all 

 the elements of a nervous centre; the grey 

 matter is present in it in considerable quan- 

 tity, and certain fibres continuous with the pri- 

 mitive nerve-tubes are likewise expanded in it. 

 But the connexion of these fibres with the grey 

 matter has not been detected here any more than 

 in the centres themselves, nor has any arrange- 

 ment of looping or of plexuses been demon- 

 strated. Mr. Bowman has been led, by recent 

 examinations, to the opinion that these fibres 

 are the central parts of the nerve-fibre (the bands 

 of Remak) which have been deprived of the 

 tubular membrane and of the white substance 

 of Schwann. It is worthy of notice that the so 

 called optic nerve itself presents certain peculiar 

 characters, which entitle it more to be regarded 

 as a prolongation of the nervous centre rather 

 than as a distinct nerve. The nerve-tubes which 

 are met with in it are for the most part of very 

 minute size ; they admit of separation with 

 great difficulty, owing to their not being dis- 

 posed in fascicles like those of other cerebro- 

 s[.tiual nerves: they appear to be surrounded 

 by and deposited in an abundant granular 

 blastema, in which there seems to be some 

 scattered elements of grey matter. These 



