242 THE AMEEICAN MONTHLY [Novemi)er, 



1. That the cells are evenly distributed through the film. 



2. That no air-bubbles or foreign matter is enclosed in the film. 



If these conditions are fulfilled, the counting is to be made with a 

 power of two hundred. The counting is to be made in a systematic 

 manner. 



The surface of one of the squares equals 1-400 of a square millimetre ; 

 the thickness of the film of blood is i-io of a millimetre ; therefore, the 

 cubic contents of one of the squares is 1-400 X i-io = 1-4000 of a cubic 

 millimetre. If the dilution of the blood be represented by the propor- 

 tion I : a, and if in n squares there be found z cells, then, as the cubic 

 contents of a square equals ^'^r)u ^^ '^ cubic millimetre, the cubic con- 

 tents of ?i squares equals — of a cubic millimetre; one cubic milli- 



4000 



metre of diluted blood will contain "^ — cells, and a cubic millimetre 



n 



of undiluted blood "^ cells. 



n 



Having found in a dilution of blood of i to 200, 1215 cells in 150 



, 4000 X 200 X121C ^n 1 11- u- 



squares we have ^- ^ = d.4No,ooo red cells m a cubic 



150 



millimetre of undiluted blood. 



For counting the white cells a dilution of the blood in the proportion 

 of 1:10 is used, and a J"o solution of hydric acetate employed for dilut- 

 ing in place of the 3/0 solution of sodium chloride. This solution de- 

 stroys the red cells, leaving the white cells unaltered. The method of 

 counting is the same as for the red cells. 



Circulation of the Blood. — For studying the circulation of the 

 blood the frog is the animal most commonly used. In order that the 

 animal shall be perfectly quiet during the observation it is paralyzed 

 with curara. A slight nick is made in the skin over the posterior poi"- 

 tion of the head with a pair of scissors, and two or three drops of an 

 aqueous solution of curara, i to 1200, are then injected into the dorsal 

 lymph sac by means of a slender glass pipette introduced through the 

 above nick. The exact amount of the solution to be used depends upon 

 the quality of the curara and the size of the frog, and can onlv be de- 

 termined by experiment. As a general rule, it is better to administer 

 small doses at intervals of an hour until the animal becomes paralvzed. 

 The action of the curara is to suspend all voluntarv motion, while the 

 vegetative functions continue, the necessary amount of oxvgen being 

 supplied by the cutaneous respiration. 



On account of the thickness of the web of the frog's foot and of the 

 presence of numerous pigmented connective tissue-cells, this portion of 

 the animal has been abandoned for viewing the circulation, and in its 

 place we make use of the thin mesentery, bladder, or lung. 



An improvement has also been made on the old-fashioned frog plates. 

 The most convenient forms now in use are those devised by Prof. Thoma 

 of Dorpat, which are shown in Fig. 7. The centre plate is for the tongue ; 

 that on the left for the mesentery ; that on the right for the bladder 

 and lung. These plates consist of a bed-plate, a, of sheet brass, covered 

 with a thin sheet of hard rubber. At B is an opening, which in the 

 tongue-plate is rectangular, in the others, circular. These openings 

 are covered with pieces of thick wdiite glass, on which the organ to be 



