CIRCULATION IN THE MESENTERY. 155 



the frog upon it, is then placed on the stage of the 

 microscope (the head of the animal being away from 

 the observer, and the web over the aperture in the 

 diaphragm), and is fixed in this position by the 

 clamps like an ordinary slide. To observe the web 

 a low power is used to see the general features of 

 the circulation, a high power being afterwards em- 

 ployed to observe the parts more in detail. But 

 this should not (unless it is an immersion) be of too 

 short a focal distance, since otherwise the lower 

 glass is apt to become clouded by moisture from the 

 web. It is not advisable to put a piece of covering 

 glass on the latter to prevent the clouding, as the 

 circulation might thereby be interfered with or 

 arrested. 



The web of the frog's foot is the easiest of the 

 vascular membranes to prepare, but has the disad- 

 vantage that, owing to its comparative thickness and 

 its epidermic covering, it is not always easy clearly 

 to make out the intermediate tissue. At the same 

 time, being under almost completely natural condi- 

 tions, the circulation will continue for an indefinite 

 time quite unimpaired. 



Preparation 7. Circulation in the mesen- 

 tery. The mesentery of the frog, and still better 

 of the toad, is admirably adapted by its thinness 

 and perfect transparency, as well as it great vascu- 

 larity, for observations on the bloodvessels and sur- 

 rounding tissues. It is necessary to have a special 

 mesentery-plate for this purpose, which can, how- 

 ever, be readily made from an oblong piece of cork, 

 like that used for the web observation. A round 

 hole about half or three-quarters of an inch in 

 diameter is made at one side with a cork borer, 

 and a small piece of the same material about half 

 an inch thick, and with a segment of a similar circle 

 cut out of its side, is fixed on the first with sealing- 

 wax or small pins (Fig. 26, b). A piece of glass of the 

 same shape as this segment may be fitted into it near 

 the top for the mesentery to rest on. The animal 



