776 TEXT-BOOK OF PHYSIOLOGY 



electro-negative auricles and electro-positive ventricles gives an upward loop 

 to the tracing, one centimeter of height being made to correspond to an elec- 

 tro-motive force of one millivolt on suitable adjustment of the quartz filament. 

 Various forms of string galvanometers, based on the principle stated in 

 a previous paragraph, have been devised. The apparatus shown in Fig. 351 

 is known as the Cambridge Electro-cardiograph. 



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 gastrocnemius, 

 the sartorius, the semimembranosus, the gracilis, and the hyoglossus. 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 purpose the gas- 

 trocnemius muscle and sciatic nerve are employed, constituting the so-called 

 "nerve-muscle preparation." 



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

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

 of the posterior extremities. 



Preparation of the Frog. Destroy the frog by plunging a pin 

 through the skin and soft tissues covering the space between the occipital 

 bone and the first vertebra until the point is stopped by the vertebra. Turn 

 the pin toward the head and push it into the brain cavity; move it from side 

 to side and destroy the brain. Pass the pin into the spinal canal and destroy 

 the spinal cord. With a stout pair of scissors cut off the body behind the 

 fore limbs. Remove the viscera and the abdominal walls. Draw the 

 hind-legs out of the skin. Place the legs on a glass plate, back uppermost, 

 and moisten them freely with normal saline solution. 



Observe on the outer side of the dorsal surface of the thigh the follow- 

 ing muscles (Fig. 355, 356). The triceps femoris (tr), made up of the 

 rectus anticus (ra), the vastus externus (ve), and the vastus internus (vi), 

 not seen from behind; on the inner side, the semi-membranosus (sm) and 

 the rectus internus minor or gracilis (ri"). Between these two groups, 

 note the biceps femoris (b). Above the thigh observe the gluteus (gl), the 

 ileo-coccygeus (ci), and the pyriformis (p). 



In the leg observe the gastrocnemius (g) with its tendon (the tendo- 

 Achillis), the tibialis anticus (ta), and the peroneus (pe). 



Turn the frog on its back and note the muscles on the ventral surface 

 of the thigh, the rectus internus ma jor (ri'), and minor (ri"), the adductor 

 magnus (ad"), the sartorius (s), the adductor longus (ad'), and the vastus 

 internus (vi). In the leg, in addition to those already seen from behind, 

 note the tibialis posticus (tp) and the extensor cruris (ec). 



Note in the abdominal cavity the three large spinal nerves, the seventh, 

 eighth, and ninth. 



Dissection of the Sciatic Nerve. The sciatic nerve is composed 

 of the seventh, eighth, and ninth spinal nerves. After its emergence from 



