PRACTICAL EXERCISES 



LAS 



9. Effect of Muscarine (or Pilocarpine) and Atropine. Paint on the 

 sinus venosus with a small camel's-hair brush a very dilute solution of 

 muscarine (or of pilocarpine). The heart will soon be seen to beat 

 more slowly, and will ultimately stop in diastole. Now apply a dilute 

 solution of sulphate of atropine to the sinus. The heart will again 

 begin to beat. Stimulation of the vagus will now cause no inhibition 

 of the heart, because its endings have been paralyzed by atropine. 

 (Muscarine or pilocarpine has also been applied to the heart, but it 

 could be shown by a separate experiment that atropine by itself has 

 the same effect on the vagus endings p. 164.) 



10. Stannius' Experiment. Pith a frog. Expose the heart in the 

 way described under 2 (p. 191). Ligature the fraenum with a fine silk 

 thread, and use the thread to manipulate the heart. With a curved 

 needle pass a moistened silk thread between the aorta and the superior 

 vena cava, and tie it round the 



junction of the sinus and right 

 auricle (Fig. 86). The auricles 

 and ventricle stop beating as 

 soon as the ligature is tightened. 

 The sinus venosus goes on beat- 

 ing. Now separate the ven- 

 tricle from the rest of the heart 

 by an incision through the 

 auriculo -ventricular groove, or 

 tie a second ligature in the 

 groove. The ventricle begins 

 to beat again, the auricle re- 

 maining quiescent in diastole 

 (p. 165). Occasionally both 

 auricle and ventricle, or only 

 the auricle, may begin to beat. 



11. Stimulation of Cardiac 

 Sympathetic Fibres in the Frog 

 (i) In the v ago- sympathetic 

 after the inhibitory fibres have 

 been cut out by atropine. 

 Arrange everything as in 7 

 (p. 196). Assure yourself, by 

 stimulating the vagus, that it 

 inhibits the heart, and take 

 a tracing during stimulation. 

 Then paint a dilute solution 

 of atropine on the sinus. 



Stimulation of the vagus, which is really the vago-sympathetic (see 

 Fig. 92), will now cause, not inhibition, but augmentation (increase 

 in rate or force, of both), since the endings of the inhibitory fibres have 

 been paralyzed by atropine. The strength of the stimulating current 

 required to bring out a typical augmentor effect is greater than that 

 needed to stimulate the inhibitory fibres. Take a tracing to show 

 augmentation produced by stimulating the nerve. 



(2) By direct stimulation of the cervical sympathetic. Make the same 

 arrangements as in n (i), but, instead of isolating the vagus, dissect 

 out the sympathetic on one side in the manner described in 6 (2) (p. 195)- 

 and do not apply atropine to the heart. Lay the upper (cephalic) end 

 of the sympathetic on very fine and well-insulated electrodes, and 

 stimulate (Fig. 76, p. 165). (To insulate electrodes the points may be 

 covered with melted paraffin. When the paraffin has cooled, a narrow 



Fig. 92. Relation of the Sympathetic to 

 the Vagus in the Frog (after Gaskell). 

 Sym, sympathetic chain ; G, ganglion of 

 the vagus; VS, vago-sympathetic; GP, 

 glosso-pharyngeal nerve; LAS, levator 

 anguli scapulae muscle ; SA, subclavian 

 artery; A, descending aorta; V, vertebral 

 column; OC, occipital part of skull; 1-4, 

 spinal nerves. 



