1892.] MICROSCOPICAL JOURNAL. 27 



to be seen witli the naked eye, but not very small as cell structures go ; 

 these threads are the ultimate nerve fibres. They are very long threads 

 of protoplasm which are irritable ; any change on any part of the 

 thread disturbs the molecular arrangement, which disturbance sub- 

 sides at the first spot, but before doing so disturbs the protoplasm next 

 it in every direction. The thread is surrounded by a fatty sheath and 

 bound in by a very delicate membrane, but we are to regard these lat- 

 ter as merely accessory, like, for example, the wrapping around a wire 

 for electric conduction which has nothing primarily to do with the 

 conductive work of the wire. I do not want any one to think that I 

 mean to sav that the nerve force is a form of electricity, for it is not 

 any more than muscle force or gland force, but my illustration means 

 only that the white sheath is not the active irritable protoplasm but an 

 inert cover for that substance. 



Biologists do not pretend to state positively the real nature of any of 

 the molecular processes taking place in protoplasm ; they do not even 

 know the precise structure of any one molecule of the various ones 

 which enter into it. But we know, from the study of blood going into 

 and from a nervous organ and in other ways, that they are extremely 

 unstable compounds and that decomposition accompanies the activity 

 of the nerve fibre, and hence we infer that the stimulus to the nerve at 

 any part causes a chemical disturbance and slight metabolic destruction, 

 which causes in turn a similar one in the next and next and next parts 

 which is a propagation of the eflect of the stimulus along the nerve. 

 We know that the chemical composition of nerve tissue is peculiar and 

 difterent from that of muscle, gland, etc., and we may perhaps con- 

 clude that nerve protoplasm is a peculiar substance the product of the 

 metabolic activity of a peculiar kind, and that by virtue of the preponder- 

 ance of this substance the power of irritability is heightened in such tissue. 

 No certain method of distinguishing this peculiar compound histologic- 

 ally is known, and hence we cannot trace the ev^olution of nerve quite as 

 satisfactorily as we can that of muscle, but areas of specially irritable 

 cells usually furnished with long processes are recognized in ccelenterates 

 and even sponges as nervous in fimction. The tubular form of a nerve 

 fibre is the form best adapted for the propagation or conduction of 

 stimuli and in very low animals, e. g.^ the annelids and even the medu- 

 sae nerve fibres exist. 



But the irritability of nerve fibres is not the only or the most special- 

 ized case of this function in the frog's body. The nerves travel to all 

 parts of the body, and place all organs in subjection to the will of the 

 animal, or at least, to the central nervous system. But between every 

 surface nerve and the organs affected by it, the spinal cord or the brain 

 intervene as a controlling centre. Here are located the irritable cells or 

 ganglion cells of which the nerve fibres are physiologically, and, per- 

 haps, also histologically extremely produced elongations. These cells 

 are always large bodies with a large spherical nucleus. The body of 

 the cell is globular, but drawn out into a few or many protoplasmic 

 processes, which are directly continued into the protoplasm of the nerve 

 fibre. The disturbance of the fibre reaches and stimulates the cell. 

 The cell is so much more highly irritable that a greater disturbance is 

 aroused in it. This, then, in turn, irritates all the outgoing fibres in 

 connection with the cell. It is thus that the muscle is reached" and 

 moved in the experiment referretl to above. 



