50 



MANUAL OF PHYSIOLOGY. 



commonly have two or more processes, which are connected by 

 nerve fibres to other cells, and to the axis cylinder of nerves. 

 This latter does not branch as the other processes sometimes do, 

 and when it can be traced, appears to enter the protoplasm, run- 

 ning toward the nucleus. 



The peripheral nerve cells are generally much modified, and 

 often small compared with those in the centres. Besides the cells 

 in the sporadic ganglia, which are large, rounded corpuscles with 

 but few processes there arc many other bodies connected with 

 the peripheral nerves which cannot be called ganglion corpuscles. 

 They are, however, nevertheless nerve cells. 



FIG. 20. 



FIG. 21. 



FIG. 20. Ganglion cells of frog, show ing straight and spiral fibres (After Beale and 

 Arnold.) 



FIG. 21. Cells from the sympathetic ganglion of a cut. The protoplasm is retracted 

 here and there from the cell wall. 



Muscles or Contractile Tissues. When changes take 

 place in protoplasm adapting it specially for contraction, it is 

 termed muscle tissue. The large masses of this tissue attached to 

 the skeleton so as to move its various parts, form the flesh of the 

 higher animals. Muscle tissue is, almost invariably, connected 

 with nerve tissue, and acts in response to stimuli communicated 

 from the nerves. In some of the lower animals the two tissues 

 are so intimately related that it is not easy to separate them, and 

 the development of both progresses equally as we ascend the scale 



