LABORATORY WORK 



187 



Root Nodules. A fully-grown lupin plant is dug up and the 

 roots washed in water. Numbers of tubercles of various sizes 

 will be seen. 



Salts 



The importance of calcium for physiological processes may be 

 shown with the frog's heart. The canula of Symes is the most 

 convenient (Fig. 14). 



The apex of the ventricle (see anatomy of the heart, p. 188 below) 

 has a tiny bent pin passed through it ; a light clip is better. A 



FiG.14. 



A, frog heart tied on the end of the glass canula B, which has a side tube connected by 

 rubber tubing to a siphon C, which dips into the solution D in a Mariotte's bottle. 

 E, a light straw lever pivoted at F. 

 G, enlarged view of the end of the canula. 



thread is attached to this clip, and to a straw lever, which magnifies 

 the beats. A tracing may be obtained by making a paper point, 

 fixed on the end of the lever, to write on a glazed paper gummed 

 around a cylinder and then smoked. The cylinder is slowly 

 rotated by clock-work or electric motor. Such apparatus is 

 supplied by makers of physiological apparatus. The paper is 

 smoked by a gas flame, fed with coal gas which has passed over 

 cotton-wool on which benzene has been dropped. During the 

 smoking the drum is rotated quickly by hand. To fix the tracing, 

 it is removed from the drum by a vertical cut in an appropriate 

 place and passed through a dilute spirit varnish, or a solution of 

 paraffin wax in petrol. It is then hung up to dry. 



