NERVE 



509 



the urostyle open into the small triangular spaces on either side of the 

 latter. A small transparent lymph-heart will be seen in each space. 



(A) Count the number of their pulsations per minute, and compare 

 their rhythm with that of the blood-heart. Do they correspond? 



(B) Pith the caudal part of the spinal cord and observe that the beat- 

 ings of these lymph-hearts stops, while the blood-heart is undisturbed. 



FIG. 282 



Apparatus as set up by students to compare the beat-rhythms of the blood-heart and of 

 the lymph-hearts in the frog. 



By boring a hole through the frog-board the beat of the blood-heart 

 may be recorded on the drum, the connecting thread passing over the 

 requisite pulleys. With a Morse key arranged to actuate a signal writing 

 just under the pen of the heart-lever, the time of the pulsations of the 

 lymph-hearts may also be recorded. By this simple means the com- 

 parison may be made directly in a graphic way and many altering condi- 

 tions of the lymph-heart rate may be studied. 



VIII. NERVE. 



Very great care must be observed in experiments on nerves that they 

 be uninjured by mechanical or chemical influences. Drying (chemical 

 injury) is especially to be guarded against, and to prevent it a nerve 

 should never be allowed to hang free in the open air. In all experi- 

 ments requiring more than a few minutes the moist-chamber, lined more 

 or less with salined filter-paper, should be used. However inconvenient, 

 these protectors from evaporation are essential in experimental work on 

 isolated nerves. 



