WITH THE MICROSCOPE. 337 



head, neck, and trunk, &c., of the frog, spherical, oval, and some- 

 times angular cells exist, which contrast remarkably in their structure 

 with the caudate nerve cells just described. Although from some of 

 these cells fibres had been traced, until recently, the opinion was very 

 generally entertained that the cells in question had but one fibre 

 connected with each of them, and Kolliker and others maintained that 

 some of the cells were entirely destitute of nerve fibres, that in short, 

 apolar, unipolar, and multipolar nerve cells existed in the nervous sys- 

 tem. But it is obvious that if the views advanced by me concerning 

 the fundamental arrangement of a nervous apparatus were correct, 

 all nerve cells must have at least two fibres proceeding from them 

 must be bipolar, and therefore that neither apolar nor unipolar 

 nerve cells anywhere existed. Thus a doubt was cast upon the cor- 

 rectness of the observation concerning apolar and unipolar nerve cells, 

 and it was necessary to re-investigate this matter with great care. 



My observations on this point were published in the Phil. Trans, for 

 1863, and render the existence of apolar and unipolar cells so very 

 doubtful, that some of those who had described them have since given 

 up the notion, although they by no means assent to the general proposi- 

 tion which I think has been established by my observations. I was 

 able to show that what appeared to be a single fibre proceeding from 

 a cell really consisted of two fibres which diverged from each other 

 and proceeded in opposite directions towards their destination, a fact 

 greatly in favour of my own general views of the structure of a 

 nervous apparatus, see p. 331, and fig. 223, pi. XXXIV. The fibres 

 could be readily traced to the body of the nerve cell, where the 

 straight fibre was seen to be continuous with the central portion, 

 while the spiral fibre passed into the matter forming the circumferen- 

 tial portion of the nerve cell. By carefully studying the development 

 of these nerve cells many points of great interest and importance, 

 both as regards the structure and action of nerve centres were demon- 

 strated. The reader may refer to the figs, in pis. LXVII and LXVIII, 

 which have been taken from the original memoir. Amongst the fully 

 formed cells are observed here and there some which are undergoing 

 the process of development. Such embryonic cells are seen in the 

 ganglia of full-grown as well as in those of young frogs. They are 

 to be found at all ages and are being constantly produced at all times 

 during the life of the animal, particularly during the spring, when the 

 nervous system is in its highest state of functional activity. Obser- 

 vation has shown that the same process goes on in the sympathetic- 

 ganglia of man and the higher animals as well as in those on the pos- 

 terior roots of the spinal nerves. See my paper on Apolar, Unipolar, 

 and Bipolar Nerve Cells, &c. These and some other facts lead me to 



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