PHYSIOLOGIC APPARATUS. 687 



From the under surface of the platform there descends a rod, which, 

 by means of a double binding screw, supports a horizontal rod, modified 

 at one end to carry the delicate axis of a light stiff recording lever. The 

 end of this lever is pointed, to enable it to write on a smoked glass or paper. 

 Beneath the axis is a strip of brass, carrying a screw, which gives support 

 to the lever until the instant the contraction of the muscle begins. This 

 screw, the after-loading screw, also enables the lever to be placed in a 

 horizontal position. The portion of the lever near the axis is provided 

 with a double hook, the lower portion of which serves for the attach- 

 ment of the weight by which the muscle is counterpoised. 



In some experiments, as in the registration of a muscle contraction 

 under varying conditions, it is necessary to give the lever mass by attaching 

 weights directly beneath the muscle. This, however, introduces certain 

 errors in the movements of the lever, which somewhat deform what would 

 otherwise be the normal curve. If the weight be attached, not opposite 

 to the muscle attachment, but close to the axis of the lever, the undesirable 

 acceleration of the lever movement, during both contraction and relaxa- 

 tion, is largely prevented. The lever may be a straw, a strip of celluloid 

 or aluminium. It should be as light as possible. The writing point may 

 be made of stiff paper, a piece of tinsel, glass or aluminium. It should 

 have sufficient elasticity 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 irrita- 

 bility 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 elec- 

 trometer. 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 respond quickly to the influence 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 influ- 

 ence of the earth's magnetism is eliminated, and the multiplication of 

 the number of turns of the wire in the coils which surround the needle. 



The tangent galvanometer, or boussole, as constructed by Wiedemann, is 

 the form most frequently employed in physiologic investigations (Fig. 350). 

 It consists primarily of a 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 



