638 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



connected with the pupil and the nictitating membrane ; from the second 

 thoracic nerve, which is mainly associated with the cells controlling the 

 blood-vessels of the ear and in a less measure the nictitating membrane ; from 

 the third thoracic, which connects with a few cells which control the pupil ; 

 from the fourth thoracic, which connects mainly with cells controlling the 

 erection of the hairs on the face and neck ; from the fifth thoracic, which con- 

 nects with the cells controlling the vessels of the ear, and also the hairs of the 

 face and neck ; and from the sixth and seventh thoracic, supplying hairs only. 

 When the pre-ganglionic fibres were cut, therefore, and allowed to regenerate, 

 various things might happen. The newly-formed fibres might grow past the 

 ganglion, or they might form novel connections with the cells there contained, 

 or finally, they might repeat the original connections. As a matter of fact, 

 the last arrangement is the one accomplished, and in the case of the cat used 

 in this experiment, stimulation of the nerve-roots above mentioned gave after 

 regeneration the reactions characteristic for the several roots. It would ap- 

 pear, therefore, that in some way each group of the regenerating pre-gangli- 

 onic fibres had selected those cells which they had originally controlled. 



The regeneration which has thus far been described has been that of the 

 non-nucleated neuron by that portion of a nerve-cell which was nucleated. 

 The regenerated portion always lies in the peripheral nervous system. Con- 

 cerning the regeneration of the dendrons there are no observations. 



The possibility of the formation of entirely new cell-elements in the pro- 

 cess of repair remains to be mentioned. When the central system is injured 

 it sometimes happens that mature nerve-cells there present show in their 

 nuclei those changes which are characteristic of nuclei about to divide, but 

 division does not take place 1 either in the nuclei or in the cell-bodies. In 

 mammals there is no convincing record of the formation of new nerve-cells 

 in the central nervous system of the mature animal. In some lower verte- 

 brates (lizards) regeneration of the spinal cord has been reported, and in the 

 newt such regeneration has been obtained in the retina, but the result in both 

 cases appears to be due rather to the enlargement of embryonic cells still 

 remaining in these regions than to an exhibition in the mature cells of powers 

 absent from the corresponding cells of the mammalia. At various times and 

 in several places the idea has been advanced that in the peripheral nervous 

 system at least there was in progress a continuous process of degeneration and 

 regeneration, as though this portion of the system was being continually reno- 

 vated. What is known of the fixity of the central system and of the relation 

 between the central system and that of the periphery, very strongly supports 

 the idea that change in one would necessitate change in the other, and for 

 central changes of this sort the evidence has never been advanced. To be 

 sure, slow growth-changes occur in the central system until after the thirtieth 

 year, but the additions which are thus made result from the enlargement of 

 nerve-cells there present as structural units from a very early age, and such 



1 Sanarelli : " I process! riparativi nel Cervello e nel Cervelleto," R. Accademia del Lincei,, 

 1891. 



