1054 VERTEBRATED ANIMALS 



an inch long, seem to be more developed forms of these ' end- 

 bulbs.' 



For the sake of obtaining a general acquaintance with the 

 microscopic characters of these principal forms of nerve-substance, 

 it is best to have recourse to minute nerves and ganglia. The small 

 nerves which are found between the skin and the muscles of the back 

 of the frog, and which become apparent when the former is being 

 stripped off, are extremely suitable for this purpose ; but they are best 

 seen in the Hyla or ' tree-frog,' which is recommended by Dr. Beale 

 as being much superior to the common frog for the general purposes 

 of minute histological investigation. If it be wished to examine the 

 natural appearance of the nerve-fibres, no other fluid should be used 

 than a little blood-serum ; but if they be treated with strong acetic 

 acid, a contraction of their tubes takes place, by which the axis- 

 cylinders are forced out from their cut extremities, so as to be made 

 more apparent than they can be in any other way. On the other 

 hand, by immersion of the tissue in a dilute solution of chromic acid 

 (about one part of the solid crystals to two hundred of water), the 

 nerve-fibres are rendered firmer and more distinct. Again, the axis- 

 cylinders are brought into distinct view by the staining process, 

 being dyed much more quickly than their envelopes; and they 

 may thus be readily made out by reflected light in transverse 

 sections of nerves that have been thus treated. The gelatinous 

 fibres are found in the greatest abundance in the sympathetic nerves ; 

 and their characters may be best studied in the smaller branches of 

 that system. So for the examination of the ganglionic cells, and of 

 their relation to the nerve-tubes, it is better to take some minute 

 ganglion as a whole (such as one of the sympathetic ganglia of the 

 frog, mouse, or other small animal) than to dissect the larger 

 ganglionic masses, whose structure can only be successfully studied 

 by such as are proficient in this kind of investigation. The nerves 

 of the orbit of the eyes of fishes, with the ophthalmic ganglion and 

 its branches, which may be very readily got at in the skate, and of 

 which the components may be separated without much difficulty, 

 form one of the most convenient objects for the demonstration of the 

 principal forms of nerve-tissue, and especially for the connection of 

 nerve-fibres and ganglion-cells. For minute inquiries, however, into 

 the ultimate distribution of the nerve-fibres in muscles and sense- 

 organs, certain special methods must be followed, and very high 

 magnifying powers must be employed. Those who desire to follow 

 out this inquiry should acquaint themselves with the methods which 

 have been found most successful in the hands of the able histologists 

 who have devoted themselves to it. 1 



Circulation of the Blood. One of the most interesting spectacles 

 that the microscopist can enjoy is that which is furnished by the 



1 For further information regarding the nervous system the memoir of F. Nansen 

 on 'The Structure and Combination of the Histological Elements of the Central 

 Nervous System ' in Bergen's Museums Aarsberetning for 1886 (1887), p. 29, should 

 be consulted. An excellent summary of the more valuable modern methods of 

 staining nerve-fibres and cells was given in 1892 to the Koyal Microscopical Society 

 by Dr. C. E. Beevor. See their Journal, 1892, p. 897, 



