226 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



389. The regeneration of Nervous tissue that has been destroyed, 

 takes place very readily in continuity with that which is left sound. 

 This may be more easily, proved by the return of the sensory and motor 

 endowments of the part, whose nerves have been separated, than by 

 microscopic examination of the reunited trunks themselves, which is 

 not always satisfactory. All our knowledge of the functions of the 

 nervous system leads to the belief, that perfect continuity of the nerve- 

 tubes is requisite for the conduction of an impression of any kind, 

 whether this be destined to produce motion or sensation ; and various 

 facts, well known to Surgeons, prove that such restoration may be com- 

 plete. In the various operations which are practised for the restoration 

 of lost parts, a portion of tissue removed from one spot is grafted, as it 

 were, upon another ; its original attachments are more or less completely 

 severed, frequently altogether destroyed, and new ones are formed. 

 Now in such a part, so long as its original connexions exist, and the 

 new ones are not completely formed, the, sensation is referred to the 

 spot from which it was taken ; thus when a new nose is made, by partly 

 detaching and bringing down a piece of skin from the forehead, the 

 patient at first feels, when anything touches the tip of his nose, as if the 

 contact were really with his forehead. After time has been given, 

 however, for the. establishment of new connexions with the parts into 

 whose neighbourhood it has been brought, the old connexions of the 

 grafted portion are completely severed ; and an interval then ensues, 

 during which it frequently loses all sensibility ; but after a time its 

 power of feeling is restored, and the sensations received through it are 

 referred to the right spot. A more familiar case is the regeneration of 

 Skin, containing sensory nerves, which takes place in the well-managed 

 healing of wounds involving loss of substance. Here there must ob- 

 viously be, not merely a prolongation of the nerve-tubes from the sub- 

 jacent and surrounding trunks, but also a formation of new sensory 

 papillae. A still more striking example of the regeneration of Nervous 

 tissue, however, is to be found in those cases (of which there are now 

 several on record), in which portions of the extremities, that have been 

 completely severed by accident, have been made to adhere to the stump ; 

 and have, in time, completely recovered their connexion with the Ner- 

 vous as with the other systems, as is indicated by the restoration of 

 their sensory and motor endowments. Of the degree in which the vesi- 

 cular substance of the Nervous system may be regenerated, we have no 

 certain knowledge ; but there can be little doubt, from the activity of 

 its usual nutritive changes, that a complete reproduction may be effected 

 in cases of loss of substance, where it can commence from a neighbour- 

 ing mass of the same tissue. 



390. We have now to inquire into the conditions, under which the 

 peculiar properties of the- Nervous System are manifested in an active 

 form ; and it will first be desirable to explain, somewhat more in detail, 

 the nature of the different operations to which it is subservient. These 

 operations present themselves in their most complex form, in Man and 

 the higher animals ; but they may often be most satisfactorily studied 

 in the lower. In the first place, when an impression is made upon any 

 part of the surface of the body by mechanical contact, by heat, elec- 



