656 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



passing out of the cord. Among the insane, too, the variations in this tonic 

 condition follow in a marked way the nutritive changes in the central system, 

 and both facial and bodily expression have a value as an index of the strength 

 and variability of those impulses on which the tone of the skeletal muscles 

 depends. Indeed, so wide in the insane is the variation thus brought about, 

 that when the expressions of the same individual at one time in a phase of 

 mental exaltation and at another in that of mental depression are com- 

 pared, it appears hardly possible that they can be those of the same person. 



This continuous outflow of impulses from the central system is indicated 

 also by the continuous changes within glands, and the variations in these 

 metabolic processes according to the activities of the central system. 



Rigor Mortis. Even in the very act of dying, the influence of these im- 

 pulses can be again traced. The death of the central nerve-tissues being ex- 

 pressed as a chemical change, causes impulses to pass down the efferent nerves, 

 and these impulses modify those chemical changes which, in the muscles of a 

 frog's leg for example, lead to rigor mortis. It thus happens that a frog sud- 

 denly killed and then left until the onset of rigor, will under ordinary condi- 

 tions show this at about the same time in both legs. If, however, the sciatic 

 nerve on one side be cut immediately after the death of the animal, the begin- 

 ning of rigor in that leg is much delayed ; thus showing that the nervous con- 

 nection is an important factor in modifying the time of this occurrence 

 (Hermann). 



Summary. In their most general form the activities of the nervous sys- 

 tem can therefore be pictured as follows : The peripheral termini of the sensory 

 or afferent nerves are isolated and there pass into the central system at least 

 as many distinct impulses as there are nerves that have been stimulated. The 

 point of entrance of these impulses is in each case the point at which the affer- 

 ent nerve connects with the cerebro-spinal system, and these points taken all 

 together form a corresponding projection of the sensory surfaces upon the cen- 

 tral system. Once entered into the central system and transmitted to the cen- 

 tral cells by the collaterals and terminals of the afferent fibre, such an incom- 

 ing impulse has open to it many pathways among the central cells, and by these 

 pathways it can reach any group of efferent cells. That all the pathways by 

 which it can travel are traversed by it, and that all the efferent cells are in 

 some measure affected, is very probable. Both the diffusion and the response 

 are, however, subject to wide modifications. 



The evident response which we commonly regard as the reaction to any 

 stimulus, arises from a more or less localized group of efferent cells and 

 emerges as a series of impulses which pass by the efferent nerves either to find 

 a comparatively limited expression in the contractions of the voluntary muscles 

 or enter into the series of ganglia and plexuses forming the sympathetic system 

 to be distributed in a diffuse manner to the unstriped muscles and the secret- 

 ing tissues. 



In brief, then, the impulses enter the cerebro-spinal system according to 

 the fixed anatomical relation of the afferent nerves. They leave this system 



