98 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



parts of the same neurone, the condition of activity which spreads through the 

 neurone and which we call the nerve impulse, has the same general character- 

 istics in all forms of nerves whether medullated or non-medullated, motor, 

 sensory, or secretory. The character of a movement or secretion depends on 

 the character of the organ excited, and not on the nature of the change trans- 

 mitted along the efferent nerve, and the specific character of a special sensa- 

 tion depends on the form of psychic activity developed in the central nervous 

 system, and not on the nature of the process of transmission in the afferent 

 neurone. This view that the nerve impulse is to be regarded merely as an 

 excitatory process, and that it has the same general characteristics in all 

 kinds of nerves, is strengthened by two sets of experiments which have been 

 reported lately. 



One of these sets of experiments was reported by Langley. 1 He found 

 that preganglionic sympathetic fibres — i.e. fibres between the ganglion and 

 the cord — if cut centrally from the ganglion, after a time regenerate and make 

 new connections with the nerve-cells of the ganglion. In some cases they 

 unite with cells of their own class, and sometimes with other cells; for 

 example, pupil lo-dilator fibres were found to have established connection 

 with pilo-motor neurones — i. e. with ganglion-cells which send their axones to 

 the erector muscles of the hairs. Further, by section of post-gangl ionic 

 fibres — i. e. fibres between the ganglion and the periphery — it was found, after 

 regeneration had occurred, that pilo-motor fibres can form nerve-endings in 

 the iris and become pupillo-dilator fibres. Evidently ganglion-cells and 

 muscle-fibres can be excited by nerve impulses developed in other nerves 

 than those normally connected with them. 



A still more remarkable result was obtained by Budgett and Green. 

 A description of this experiment is given on page 85. They succeeded in 

 causing sensory fibres of the pneumogastric to grow down a degenerated 

 motor trunk, the hypoglossal, and connect with the muscles of the tongue. 

 In this case excitation of the peripheral part of the afferent nerve caused 

 muscular contractions. If we should think of the nerve which was excited, 

 we would be inclined to say that a sensory impulse was generated ; if we 

 should think of the effect on the muscle, we would call it a motor impulse, 

 and the latter would be the proper term. Evidently the condition of 

 activity which can be aroused in a sensory nerve is capable under suitable 

 conditions of exciting muscles, and sensory nerves cannot be considered to 

 be the seat of specific forms of energy different from those generated in 

 motor nerves. 



D. Contractility. 



Contractility is the property of protoplasm by virtue of which the cell is 



able to change its form when subjected to certain external influences called 



irritants, or when excited by certain changes occurring within itself. The 



change of form does not involve a change of size. The contraction is the 



1 Langley : Journal of Physiology, 1897, xxii. p. 215. 



