574 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



vasomotor centers are excited at a rather late stage, but they continue active 

 at a time when the other centers are on the wane. 



Although this is not the place for a full discussion of the influence of dif- 

 ferent poisons upon the nervous system, attention ought to be directed to the 

 fact that certain observations have been made which tend to show that these 

 substances do not act alike on all nerve cells. Thus, according to Baglioni, 

 carbolic acid in weak solution has a heightening effect upon the irritability of 

 the motor mechanisms in the anterior horn, while the sensory mechanisms of 

 the posterior horn are not perceptibly affected. Strychnia on the other hand 

 increases the excitability of the sensory mechanisms of the posterior horn and 

 leaves the motor mechanisms of the anterior unaffected. Immediately after its 

 application nicotine stimulates the motor elements of the medulla and spinal 

 cord, also the cells of the sympathetic ganglia, but has no effect upon the cells 

 of the spinal ganglia (Langley). 



E. MORPHOLOGICAL CHANGES IN THE NERVE CELL. REPRODUCTION 



AND REGENERATION 



Within recent years the structure of the nerve cell has been made out in 

 great detail, thanks to the great progress in histological technique, and certain 

 differences' have been noted in the microscopical appearance of resting and of 

 active or fatigued cells. A full account of this difference will be found in text- 

 books of histology. 



Various attempts have been made also to demonstrate that nerve cells or their 

 dendrites at least are capable of ameboid movements, and far-reaching physio- 

 logical and psychological hypotheses have been erected on the basis of such 

 assumptions. But these have with justice been very vigorously contested, and 

 if the newer discoveries concerning the neurofibrils prove to be true in all respects 

 (cf. page 560), ameboid movement will have lost its last vestige of support. 



It is a question of great fundamental importance, whether nerve cells can 

 be reproduced in post-embryonic life. Birge counted the motor cells in the spinal 

 cord and the nerve fibers in the anterior spinal roots in frogs of different age 

 and convinced himself that both either multiply from preexisting nerve elements 

 or develop from other elements throughout life, for he found an unmistakable 

 relation between the weight of the animal and the number of cells and fibers. 

 For example, a frog weighing 1| g. had 5,984 motor nerve fibers, one of 9J g. 

 6,481, one of 23 g. 7,048, up to a frog of 11 g. with 11,468 fibers. On the average 

 for each 1 g. increase in weight 52 motor fibers had been added. 



How long after birth this new formation may take place we do not know. 

 In certain inflammatory processes in the brain mitotic figures have been seen 

 in the vicinity of nerve cells, but these facts teach us nothing with regard to 

 the normal multiplication of nerve cells in the adult body. . 



Most authors deny the regeneration of nerve tissues after extensive destruc- 

 tion of them in the higher animals. But we have two observations recorded in 

 the literature supporting such regenerations : One by Voit concerns regeneration 

 in both hemispheres of the pigeon, the other by Vitzou regeneration of the 

 occipital lobes in the monkey. These are extremely important observations and 

 urgently demand confirmation. 



It has been established by a great many observations that peripheral 

 efferent fibers regenerate if the nerve cells to which they belong remain 

 uninjured. 



