212 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



other hand, it seems equally certain that there are many nerve-tubes which 

 simply enter the ganglionic masses, pass round and amongst the cells, and 

 then emerge from them, without having undergone any distinct change, save 

 that they present a soft and varicose appearance, whilst threading their way 

 through the cells. And it is equally certain that there are many ganglionic 

 corpuscles, which never acquire the caudate prolongations, and which appear 

 specially destined to act upon this class of nerve fibres. Some observations 

 which have been made upon the nervous system of foetuses, in which the 

 brain and spinal cord were wanting, present a remarkable confirmation of this 

 view.* The nervous cords were for the most part developed ; and at their 

 (so called) origins or central extremities, they were found to hang as loose 

 threads in the cavities of the cranium and spine. On examining these threads, 

 it was found that the nerve-tubes, of which they consisted, formed distinct 

 loops ; each of which was composed of a fibre that entered the cavity, and 

 then returned from it. These loops were imbedded in granular matter, 

 resembling that interposed between the vesicles in the cortical substance of 

 the brain; and perhaps to be regarded as vesicular matter in an early stage 

 of its formation. All that is known of the laws regulating the formation of 

 such irregular productions, leads to the belief, that we may rightly consider 

 this arrangement of the nerve-tubes as one which exists in the nervous cen- 

 tres, when they are normally developed. But it may not be the only one; 

 for, as already pointed out, some of the nerve-fibres appear to originate from 

 the filamentous prolongations of certain ganglionic cells. Additional informa- 

 tion is much needed upon this point. 



248. The arrangement of the nerve-fibres, at their peripheral extremities, 

 seems to be essentially of the same character. It has been already shown 

 that the motor fibres, which are distributed to the muscles, have no proper 

 terminations ; a series of loops, returning into themselves or joining others, 

 being formed by the ultimate ramifications of the main trunks. The arrange- 

 ment of the sensory fibres seems to be usually of the same nature. The 

 principal trunks subdivide into numerous anastomosing branches, forming a 

 sort of plexus in the substance of the skin ; and from this, single filaments 

 detach themselves at intervals, rising up into the papillary elevations of its 

 surface, and then returning again into the plexus, after making a series of 



Fisr. 119. 



Distribution of the tactile nerves at the extremity of the. human thumb, as seen 

 in a tliin perpendicular section of the skin. 



loops, ill which a sort of varicose enlargement of the fibre may often be 

 noticed. Similar looped terminations have been traced in the nerves supply- 



* Dr. Lonsdalc, in Edinb. Mnl. ;iml Surg. Journal, No. CLTII.; and Mr. Fagot in Brit, and 

 For. Med. Rev., No. XLIII. p. 273. 



