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on its side, and a vertical incision is made from the axilla 

 downwards for about an inch. The knife must be used care- 

 fully as the lung is approached ; when the last layer is punc- 

 tured, a small hernial protrusion is seen ; the opening is now 

 enlarged by a probe-pointed knife, and the whole lung springs 

 out through the thoracic wall. With a low power, the cir- 

 culation is seen very prettily by transmitted light. 



2. In the Toad the tongue and mesentery may be exposed 

 in the same manner as in the frog. 



3. In the Mouse and other small Mammals, the circulation 

 may be studied in the mesentery with advantage. The white 

 mouse is preferable to the common one, being less liable to 

 die of fright, and more easily handled. From six to ten 

 minims of the chloral solution should be injected. Operation 

 performed as in the frog. 



II. — Circulation in Animals breathing by Gills. 



1. Fishes. — The caudal fin in certain small species shows 

 the circulation with great beauty. The difficulty of keeping 

 the fish alive on the stage of the microscope for more than 

 a few seconds, and its disposition to contract the fin when 

 out of water, have hitherto prevented the general study of 

 the fish's circulation. Being convinced of its value as a 

 field for observation and experiment, I attempted the con- 

 struction of an apparatus for supplying the fish with a 

 stream of water while on the stage, and for keeping its tail 

 fin still and stretched ; after many unsuccessful efforts, the 

 fish-trough shown in the sketch was produced, and has 

 been found to answer perfectly. 



A A is a rectangular piece of common brown gutta-percha, 

 3 in. by 2^, and ^ in. in thickness, having a circular aperture 

 about 1 in. in diameter, in its centre ; c is a trough of the 

 same material, l^in. in length, 1 in. in breadth, and ^ in 

 depth, placed on the plate, sloping diagonally from the 

 central opening in the plate to one corner ; it is partly covered 

 in, as shown in the drawing ; D D is a little square glass 

 stage, raised ^ in. from the plate over the opening, and in a 

 line with the trough ; e is an inclined plane, connecting the 

 surface of the glass stage with the floor of the trough ; r is 

 a small india-rubber tube, conveying water to the trough 

 from a tap ; g is a second tube, conveying the water away, 

 and placed at such a level that the trough is always nearly 

 full, but cannot overflow : h h' is a loop of silk cord, which 

 passes through two little holes, K K, in the plate. 



The fish is placed in the trough (as shown in dotted out- 

 line), with its tail on the glass stage. The upper part of the 



