80 On Diapedesis. 



used; but the study of the capillary circulation of mammalia is 

 attended with such great difficulty that they are seldom employed. 



We will dismiss the fish and mammalia in a few words. An 

 oblong box of gutta-percha, with glass top and bottom, is generally 

 employed for studying the circulation in fish. A constant supply of 

 fresh water is conveyed to, and the deoxygenated water from, 

 the box, by means of two pipes. As regards mammaha, there is 

 but one external part — viz. the wing of the bat (used by Paget) — 

 sufficiently transparent for observation; hence it is necessary to 

 employ some internal structure. The mesenteries of small rodents 

 have been used by Strieker ; but these are not to be compared with 

 the omentum, and particularly with that of the guinea-pig. It 

 differs from that of man in consisting of only two layers of peri- 

 toneum, in being much more delicate in structure, and containing 

 very little fat. The observations on these serous membranes have 

 been productive of no good results, for the injurious effects of 

 exposure are much greater than those which occur in Batrachians. 



Therefore, to overcome these difficulties, it is necessary to have 

 recourse to complicated apphances and expensive apparatus, and 

 even then so vulnerable a tissue as that of the peritoneum cannot 

 be exposed even for a few minutes without injury, so that, although 

 the greatest care is taken in demonstration, only a momentary 

 glimpse can be obtained. 



Batrachians and their larvae. Four parts of these animals may 

 be used, as follows: — (1) The web of the frog's foot. (2) The 

 mesentery of the frog or toad — preferably of the toad, whose mesen- 

 tery is longer. (3) The tongue of the toad or frog. (4) Tadpole's 

 tail. These animals may or may not be curarized. If curara be 

 employed, one drop of one-sixth per cent, solution is injected under 

 the skin of the back with an ordinary hypodermic syringe. If not 

 curarized, the brass frog-plate sold by opticians, or a large flat piece 

 of cork with a hole at one end, must be employed; but these 

 arrangements are only fitted for examining the web. 



1. The web of the foot must be extended over the glass in the 

 brass plate, or the hole in the cork, by means of thread tied to the 

 toes and fastened to the plate, or by means of pins to the cork ; but 

 the web is not well fitted for observing diapedesis. The epithelium 

 soon becomes dulled by the action of reagents, and often the 

 extension of the web impedes or entirely prevents the circulation. 



2. The mesentery. Although the preparation of the mesentery 

 is not so simple as one might anticipate, yet it is well suited for our 

 purpose. The animal must be under the infiuence of curara, and 

 the use of a stage frog-plate is necessitated, the construction of 

 which is as follows : — A piece of cork, having a circular hole in the 

 middle, is fastened to one end of a plate of glass or cork with a 

 corresponding hole ; a circular piece of thin glass is fixed to the 



