On the Nervous System of Cassiopea Xamachana. 165 



nerve-fibers convey trophic impulses as an accessory function, which 

 would imply the necessity of qualitative differences in the impulses 

 which traverse any fiber. On the other hand, the work of Head 

 and Campbell, on Herpes zoster, supports the conception of a trophic 

 function of nerve-fibers. In this disease the presence of change in the 

 dorsal root ganglia causes abnormal impulses to be sent in an efferent 

 direction along the sensory fibers of the skin. On account of this 

 excitation, blistering of the skin takes place over the area of distribu- 

 tion of the fibers (axones) terminating in the affected ganglia. 



Jacobson (loc. cit.} states that "in the lower animals the main- 

 tenance of life processes are largely a phenomenon of chemical coordina- 

 tion," and that trophic influences ought to be greater hi the higher 

 than in the lower animal phyla. Her own experiments showed, hi her 

 opinion, that there was no difference in this respect between pigeons 

 and dogs, both animals giving negative results. 



Among the lower invertebrates, in Cassiopea (and in this respect it is 

 unlikely to differ markedly from other scyphomedusse) the general 

 metabolic activities, including regeneration (a form of growth), total 

 CO 2 production, and the using of body-substances for metabolism 

 when no food is being taken, are strongly influenced by the condition 

 of presence or absence of some sort of impulses from the sense-organs 

 (nerve-centers) . That there are neurones which are specific conductors 

 of trophic impulses as distinct from motor or sensory nerves is by no 

 means implied by this statement. In fact, in a nervous system such as 

 that of the medusse it is not always possible, on morphological grounds 

 alone, to separate those neurones which are sensory from those of a 

 motor function. In the absence of even this means of establishing the 

 identity of the character of a portion of the neurones it is useless to 

 attempt, in the present state of our knowledge of the anatomy of this 

 nervous system, to make any distinction between the character of the 

 function of any particular portion of it. 



In spite of this lack of completeness in our knowledge of the nature 

 of the nervous system of Cassiopea, it should be emphasized that, 

 through whatever morphological mechanism it may be exercised, the 

 trophic influence of the sense-organs is a constant factor in the normal 

 metabolic activity. Without these impulses, even while muscular 

 activity is far above normal, all the measurable metabolic activities 

 except that of the motor system are maintained at a considerably 

 lower level than when under the control of the sense-organs, by which 

 muscular activity is held at a much lower rate than that at which these 

 tissues are capable of maintaining a constant activity for long periods. 



