616 VERTEBRATED ANIMALS. 



the ganglionic vesicles, 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 profi- 

 cient in this kind of investigation. The nerves of the orbit of 

 the eye of Fish, 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 ganglionic corpuscles, l^o method of pre- 

 serving the nerve-tissue has yet been devised, which makes it 

 worth while to attempt to mount preparations for the sake of 

 displaying its minute characters ; but the general course of the 

 nerve-tubes, and the disposition of the ganglionic vesicles, may 

 be demonstrated in preparations preserved in weak spirit; and 

 when the skin has been injected, the passage of the nerve-fibres 

 to the papillae can sometimes be traced in vertical sections, 

 mounted as opaque objects, and viewed by reflected light. The 

 following method, recommended by Mr. J. Lockhart Clarke, for 

 the examination of the structure of the Spinal Cord,' would be 

 equally applicable to that of other large ganglionic masses: — A 

 perfectly fresh cord is to be hardened in strong spirit, so that 

 extremely thin sections can be made with a very sharp knife ; 

 and such sections, placed on slips of glass, are to be treated with 

 a mixture of one part of acetic acid and three of spirit, which 

 not only makes the fibrous portion more distinct, but also renders 

 the vesicular portion more transparent. If it be desired to 

 preserve such a section, it should be transferred, after maceration 

 for an hour or two in the mixture of acetic acid and spirit, into 

 pure spirit, in which it should be allowed to remain for about 

 the same space of time ; from the spirit it should be transferred 

 to oil of turpentine, which soon expels the spirit, and renders 

 the section perfectly transparent, so that it can be examined with 

 high magnifying powers ; and it may then be mounted in Canada 

 balsam in the usual manner. 



431. Circulation of the Blood. — One of the most interesting 

 spectacles that the microscopist can enjoy, is that which is fur- 

 nished by the circulation 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 ordinaiy structure; and the number of these is very 

 limited. The web of the Frog's foot is perhaps the most suita- 

 ble for ordinary purposes, more especially since this animal is to 

 be easily obtained in almost every locality; and the following is 



' See his Memoir on tliat subject, in " Pliilos. Transact ," 1851. 



