RELATION BETWEEN STRUCTURE AND FUNCTION. 5 



from the spinal cord. These nerves are bundles of nerve- 

 fibres, while each one of them is a branch of a nerve-cell 

 lying within the spinal cord. The course of these fibres 

 gives important information as to the morphological 

 arrangement of the structure of the limb. 



In this way can be discovered exactly what parts of the 

 musculature of the limb are innervated by each segmental 

 nerve. Another plan of investigation is based on the law 

 of degeneration. When an animal cell — for instance, one 

 of the large Amoebae obtained in the Ked Sea — is divided 

 into several pieces, by tearing with a needle under a dis- 

 secting lens, each of the several pieces isolated draws 

 itself together, performs movements, and at first appears 

 to be capable of leading an independent existence. Con- 

 tinued observation, however, reveals the fact that the 

 isolated pieces after a few days die and disintegrate, with 

 the exception only of that portion in which the nucleus 

 remains ; that lives on, and later repairs its injured bulk. 

 It is the same with nerve-cells, and probably with all 

 other animal cells. In the case of nerve-cells the 

 occurrence of degeneration in the portion of a cell which 

 has been isolated from the nucleus-containing portion of 

 the cell is, perhaps, the most valuable of all methods for 

 tracing the nerve-fibres to their parent nerve-cells. 



The method of degeneration can be combined with that 

 of excitation by electricity. In a few days after nerve- 

 fibres have been severed from their parent nerve-cells, the 

 nerve-fibres, even before they exhibit visible signs of decay, 

 have lost their capacity to respond to stimuli and to con- 

 duct the waves of change which are called nervous impulses. 

 In this circumstance lies a means of escape from the em- 

 barrassing, for the investigator, accidents of "escape of 

 current." The escape of the electric current from that 

 particular nerve to which it is intended to solely apply, it 



