286 THE MICROSCOPE AND ITS REVELATIONS. 



composed of a simple outer capsule of connective tissue, filled with clear 

 soft matter, in the midst of which the nerve-fibre, after losing its dark 

 border, ends in a knob. The ' Pacinian corpuscles,' which are best seen 

 in the mesentery of the Cat, and are from l-15th to 1-10 of an inch long, 

 seem to be more developed forms of these ' end-bulbs.' 



683. 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 histo- 

 logical 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 ( 202 a), 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 

 whose works have been already referred to. 



684. Circulation of the Blood. One of the most interesting spectacles 

 that the Microscopist can enjoy, is that which is furnished by the Circu- 

 lation of the Blood in the capillary blood-vessels which distribute the 

 fluid through the tissues it nourishes. This, of course, can only be 

 observed in such parts of Animal bodies as are sufficiently thin and 

 transparent to allow of the transmission of light through them, without 

 any disturbance of their ordinary structure; and the number of these is 

 very limited. The web of the Frog's foot is perhaps the most suitable 

 for ordinary purposes, more especially since this animal is to be easily 

 obtained in almost every locality; and the following is the simple 



