PHYSIOLOGIC APPARATUS. 719 



ticity to keep it in contact with the cylinder during the excursion of the 

 lever. The writing point should be placed as nearly parallel as possible 

 to the surface of the cylinder. 



Normal Saline Solution. To prevent drying and a loss of irritability 

 the tissue under investigation should be kept moist with the normal saline 

 solution (NaCl, 0.6 per cent.). This solution very largely prevents either 

 absorption or extraction of water from the tissues and thus retards chemic 

 changes in their composition. 



Ringer's solution, largely used for the same purpose, is made by 

 saturating 0.65 per cent. NaCl solution with calcium phosphate and then 

 adding 2 c.c. of a i per cent, solution of potassium chlorid to each 100 c.c. 



The Galvanometer and Capillary Electrometer. In the detection 

 and investigation of the electric currents of muscles, nerves, and other 

 tissues, the physiologist is limited to the galvanometer and capillary electro- 

 meter. The principle of the galvanometer is based on the fact that an 

 electric current flowing through a wire parallel in direction with a magnetic 

 needle will tend to set the needle at right angles to the direction of the 

 current. The essential requisite of any galvanometer used for physiologic 

 purposes is that it will re- 

 spond quickly to the influ- 

 ence of extremely weak 

 currents. This is realized 

 by the use of small light 

 needles, the adoption of 

 the astatic system, or some 

 similar device by which 

 the directive influence of 

 the earth's magnetism is 

 eliminated, and the multi- 

 plication of the number of 

 turns of the wire in the coils 

 which surround the needle. 



The tangent galvano- 

 meter, or boussole, as con- 

 structed by Wiedemann, 

 is the form most frequently Fia 37o.-Wi E D E MANN's BOUSSOLE. 



employed in physiologic investigations (Fig. 370). It consists primarily 

 of ^ thick copper cylinder, through which a tunnel has been bored. 

 Within this tunnel is suspended a magnetized ring, just large enough 

 to swing clear of the sides of the chamber. The object of making 

 the magnet ring-shaped is to increase its strength in proportion to its size, 

 and to get rid of the central inactive part. Connected with and passing 

 upward from the magnetized ring through the copper cylinder is an alumin- 

 ium rod, surmounted by a circular plane mirror. Above the mirror rises a 

 glass tube,^ which carries on top, on an ebonite support, a little windlass, cap- 

 able of being centered by three small screws. On the windlass is wound a 

 single filament of silk, which passes down the tube and is attached to the 

 mirror. The magnet can, by this contrivance, be raised or lowered and 

 centered in the copper chamber. Deflections of the mirror from currents 



