PHYSIOLOGIC APPARATUS 



771 



receives a glass shade, which covers the whole. The air of the chamber is 

 kept moist by placing in it pieces of blotting-paper saturated with water. 



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 attachment 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 



FIG. 347. MOIST CHAMBER. 



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 relaxation, 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 elas- 

 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 



