CIRCULATION. 149 



the bicuspid valve will be 1 seen to be lifted up, and to prevent its 

 escape ; or, what is better, the air may be made to take the place 

 of the water, by drawing in the breath and blowing forcibly, in 

 quick succession, through the aorta. The action of the valve during 

 life may thus be shown with tolerable accuracy. 



When this has been examined the heart may be cut through 

 transversely, two or three inches above the apex, to show the 

 greater thickness of the left than the right ventricle ; the left 

 ventricle and aorta may then be cut up to show their internal 

 surface the fleshy columns and tendinous cords, which assist the 

 heart in contracting, the appearance of the bicuspid and semilunar 

 valves, the grooves leading to the branches from the aorta, &c. 



The action of the semilunar valve may now be shown by cau- 

 tiously cutting away the right ventricle, till the valves at the mouth 

 cf the pulmonary artery are exposed. Take a pig's bladder, and 

 cut about two inches off each extremity. Sew the narrower end 

 round the inner surface of the pulmonary artery ; pour a jugful of 

 water quickly into the bladder, and the action of the valves, in 

 preventing its return, will be seen. 



By mixing Paris plaster (which may be got from any plasterer) 

 bulk for bulk with the water, casts of the pulmonary valves, of the 

 left ventricle, &c., may be made, and cut out when dry. To take 

 a cast of the right ventricle, the pulmonary valves must be broken 

 down, as above. These make very instructive preparations, when 

 the valves, &c., are distinguished by being colored. 



Attention should also be directed to the great difference in mus- 

 cularity between the auricles and ventricles, and to the sounds of 

 the heart as they can be heard by applying the ear to the left side 

 of the chest of a ttiin person. The first dull sound is supposed to 

 be produced principally by the contraction of the ventricles, the 

 succeeding sharp sound by the falling back of the blood on the 

 semilunar valves. In disease, these sounds become louder and 

 much altered in some cases resembling the blowing of bellows, in 

 others the rasping of a file, &c. The contraction of the heart is 

 called its systole, the time it rests its diastole. 



To show the fibrin of the blood, get some from the butcher (who 

 extracts it by turning his fingers in the blood while coagulating), 

 and wash it till it is pure white. The coagulability of the serum 

 should be shown by heating it in a Florence flask. 



As mentioned in the text, the microscope shows the circulation 

 in a frog's foot. 



