240 THE MICROSCOPE. 



supersedes the necessity of any other kind of force foi 

 driving it into the vessels. The mercurial injecting 

 apparatus consists of a glass tube, about half an inch in 

 diameter and twelve inches in length, to one end of which 

 has been fitted a steel screw to which a steel injecting pipe 

 may be attached. The pipes and stopcocks must be made 

 of steel, for otherwise they would be destroyed by the 

 action of the mercury. 



" Injecting the Lower Animals. — The vessels of fishes are 

 exceedingly tender, and require great caution in filling 

 them. It is often difficult or quite impossible to tie the 

 pipe in the vessel of a fish, and it will generally be found 

 a much easier process to cut off the tail of the fish, and 

 put the pipe into the divided vessel which lies imme- 

 diately beneath the spinal column. In this simple manner 

 beautiful injections of fish may be made. 



"Mollusca. — (Slug, snail, oyster, &c.) The tenuity of the 

 vessels of the mollusca often renders it impossible to tie 

 the pipe in the usual manner. The capillaries are, how- 

 ever, usually very large, so that the injection runs very 

 readily. In different parts of the bodies of these animals 

 are numerous lacunae or spaces, which communicate 

 directly with the vessels. Now, if an opening be made 

 through the integument of the muscular foot of the 

 animal, a pipe may be inserted, and thus the vessels may 

 be injected from these lacunse with comparative facility. 



"Insects. — Injections of insects may be made by forcing 

 the injection into the general abdominal cavity, when it 

 passes into the dorsal vessel and is afterwards distributed 

 to the system. The superfluous injection is then washed 

 away, and such parts of the body as may be required, 

 removed for examination. 



" Of the Practical Operation of Injecting. — I propose now 

 to inject a frog and the eye of an ox, in order that you 

 may see the several steps of the process. We must bear 

 in mind that a successful injection cannot be made until 

 the muscular rigidity which comes on shortly after death, 

 and which affects the muscular fibres of the arteries as 

 well as those of the muscles themselves, has passed off. 

 In some few instances in which the fluid does not neces- 

 sarily pass through arterial trunks before it reaches the 



