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APPENDIX. 
CHAPTER XIII. 
The beat of the heart is very conveniently studied in the 
frog. Put a live frog into a glass bowl with a piece of cotton 
batting or of cloth saturated with chloroform and cover the 
bowl. In a few minutes the animal will have become motion¬ 
less and insensible. Remove it from the bowl; with a sharp 
knife divide the skin and cartilage at the base of the skull, thus 
making an opening into the brain cavity; into the latter thrust 
a wire, and by twisting it about destroy the brain. The frog 
will probably struggle, but its motions are reflex, and it has no 
consciousness of pain. The heart may now be exposed by 
making an incision through the skin and muscles of the upper 
part of the abdomen and removing the cartilaginous part of the 
breastbone. The heart will be seen beating inside the pericar¬ 
dium. The latter may be removed and the heart freely exposed. 
After studying the movements of the organ it may be removed 
from the body by cutting the blood-vessels close to their junc¬ 
tion with the heart, and placed on a plate of glass or in a watch 
glass containing .75 per cent, salt solution. Its movements will 
continue a long time after its removal from the body. The 
heart may afterward be opened and the relation of its ventricle, 
auricles, and the connecting veins and arteries studied (Fig. 76). 
The heart of the pig, sheep, or calf may be used to show the 
structure of the mammalian heart. It is best to procure at the 
meat-shop several “ plucks ”— i. e ., heart, lungs, and trachea all 
attached together. Instructions should be given the butcher 
that the parts are to be left intact, otherwise they will be found 
to be punctured with knife cuts. Dissect out the blood-vessels 
for some little distance from the heart in order to get their re¬ 
lations. Open some of the hearts lengthwise, others crosswise, 
to show the internal structure (Fig. 74). Pour water into the 
cavities to show the action of the valves. The flow of blood 
through the heart may be illustrated by connecting the aorta 
with the venae cavae by means of rubber or glass tubing to rep¬ 
resent the systemic circulation, and the pulmonary artery with 
the pulmonary veins to represent the pulmonary circulation, 
then filling the heart with water or a colored fluid and compress¬ 
ing the organ with the hand (Fig. 76). 
The circulation may be studied in the web of the frog’s hind- 
