WITH THE MICROSCOPE. 8/ 



ON MAKING INJECTIONS. 



The arrangement of the minute vessels or capillaries distributed 

 to the various textures of man and animals is not to be demonstrated 

 in all instances by the usual methods of investigation, in consequence 

 of the transparency of the walls of the tube. Indeed, in an ordinary 

 examination of a tissue in the microscope, one often fails to detect 

 the least trace of any structure, although it may be almost entirely 

 composed of distinct tubes and vessels. Some even yet maintain 

 the opinion, that the capillaries are to be looked upon in the light 

 of mere channels in the interstices of the tissues, rather than as tubes, 

 with their own proper walls. If, however, this view were correct 

 we should not meet with the perfectly circular outline which the 

 section of an injected capillary vessel frequently presents ; nor should 

 we be able to obtain capillaries completely isolated from other 

 tissues. 



164. Of Natural anil Artificial Injections. Sometimes the capillary 

 vessels remain turgid with blood after the death of the animal, and a 

 natural injection results. Natural injections, however, are accidental 

 and cannot be obtained in the case of every texture. In order, 

 therefore, to investigate the arrangement of the vessels, it is neces- 

 sary to resort to the process of artificial injection, by which a certain 

 quantity of coloured material is forced into a vessel of convenient size. 

 After passing along a large arterial trunk, the injection penetrates into 

 the smallest vessels and sometimes even returns by the veins. The 

 colouring matter employed may be opaque or transparent. In the 

 first case the injected preparation can only be examined by reflected 

 light as an opaque object, p. 18, while transparent injections may be 

 subjected to examination by transmitted light, p. 18, as well as by 

 reflected light. Examples of opaque and transparent injections in 

 which different substances have been employed as colouring matters, 

 can be purchased at all the microscope makers. See list at the end 

 of the volume. Every student is, however, strongly recommended to 

 learn to make injected preparations for himself. 



165. Instruments required for making Injections. The dif- 

 ferent instruments required for making artificial injections are the 

 following : 



An injecting syringe, of about the capacity of one ounce or even 

 half an ounce, pi. XXIII, fig. 151. The piston of the injecting 

 syringe should be covered with two pieces of leather, which may 

 be very easily removed and replaced, fig. 148. The first piece, a, is 

 applied and screwed down with a brass button, b. The piston is 



