128 THE MICROSCOPE. 



" We will now examine these injections. A portion of 

 the intestine of the frog may be removed with scissors, 

 opened, and the mucous surface washed with the aid of 

 the wash-bottle. It may be allowed to soak in glycerine 

 for a short time, and then examined. 



" This portion of the delicate choroidal membrane which 

 has been carefully removed in the same manner shows the 

 vessels perfectly injected, and in this preparation of the 

 ciliary processes you will not fail to observe that all the 

 capillary vessels are fully distended with fluid, although 

 the injection was made so quickly. 



" Of Injecting the Ducts of Glands. The modes of inject- 

 ing which we have just considered, although applicable to 

 the injection of vessels, are not adapted for injecting the 

 ducts and glandular structure of glands ; for as these 

 ducts usually contain a certain quantity of the secretion, 

 and are always lined with epithelium, it follows that when 

 we attempt to force fluid into the duct, the epithelium 

 and secretion must be driven towards the secreting struc- 

 ture of the gland, which is thus effectually plugged up 

 with a colourless material, and there is no possibility of 

 making out the arrangement of the parts. In such a case 

 it is obviously useless to introduce an injecting fluid, for 

 the greatest force which could be employed would be insuf- 

 ficient to drive the contents through the basement mem- 

 brane, and the only possible result of the attempt would 

 be rupture of the thin walls of the secreting structure and 

 extravasation of the contents. As I have before mentioned, 

 partial success has been obtained by employing mercury, 

 but the preparations thus made are not adapted for micro- 

 scopical observation. 



" After death the minute ducts of the liver always contain 

 a little bile. No force which can be employed is sufficient 

 to force this bile through the basement membrane, for it 

 will not permeate it in this direction. When any attempt 

 is made to inject the ducts, the epithelium and mucus, in 

 their interior, and the bile, form an insurmountable barrier 

 to the onward course of the injection. Hence it was ob- 

 viously necessary to remove the bile from the ducts before 

 .one could hope to make a successful injection. It occurred 

 to me, that any accumulation of fluid in the smallest 



