690 



TEXT-BOOK OF PHYSIOLOGY. 



devised by M. v. Frey, is shown in Fig. 351. This consists of a glass 

 tube, A, forty millimeters in length, three millimeters in diameter, the lower 

 end of which is drawn out to a fine capillary point. The tube is filled with 

 mercury and its capillary point immersed in a 10 per cent, solution of 

 sulphuric acid. The vessel containing the acid 

 is filled to the extent of several millimeters with 

 mercury also. The mercury in the tube is put 

 in connection with a platinum wire (a), and the 

 acid in the vessel with' a second wire (b). When 

 a constant current passes into the apparatus in 

 the direction from b to a the mercury is pushed 

 up the tube, and, upon the breaking of the cur- 

 rent, it may or may not return to the zero-point. 

 For the purpose of measuring in millimeters of 

 mercury the pressure necessary to compensate this 

 change in the capillary constant produced by the 

 electro-motive force of polarization, the apparatus 

 is provided with a pressure-vessel, H, and a 

 manometer, B. This electrometer can be applied 

 to any microscope having a reversible stage. The 

 oscillations of the mercury can then be observed 

 with the microscope provided with an ocular 

 micrometer (Fig. 352). The special advantage of 

 the electrometer is, that it will respond instantly to 

 any variation in the electro-motive force, and indi- 

 cate a difference of potential, according to Lipp- 

 mann's observation, as slight as the roTgr f a 

 Daniell. These rapid oscillations can be recorded 

 by photographic methods. 



In using either the galvanometer or the elec- 

 trometer for detecting the existence of electric. 



currents or differences of potential in living tissues, it is absolutely 

 essential that non-polarizable electrodes be employed in connection with it 



FIG. 352. CAPILLARY 

 ELECTROMETER. R. 

 Mercury in tube; 

 capillary tube. s. 

 Sulphuric acid. q. 

 Hg. B. Observer. 

 M. Microscope 



DISSECTION OF THE HIND-LEG OF THE FROG. 



Much of our knowledge of the physiologic properties of muscles and 

 nerves has been derived from the study of the muscles and nerves of the 

 cold-blooded animals, especially of the frog, for the reason that in these 

 animals the tissues retain their vitality under appropriate conditions for a 

 considerable period of time after death or removal from the body. The 

 muscles generally employed for experimental purposes are the gastroc- 

 nemius, the sartorius, the semi-membranosus, the gracilis, and the hyo- 

 glossus. The nerve generally employed is the sciatic. Both muscle and 

 nerve may be studied independently of each other, or they may be studied 

 together, as when in their usual physiologic relation. For this latter pur- 

 pose the gastrocnemius muscle and sciatic nerve are employed, constituting 

 the so-called "nerve-muscle preparation." 



For these, and many other reasons, the student should familiarize 

 himself with the general anatomy of the frog, and especially with the 

 anatomy of the posterior extremities. 



