APPENDIX 493 







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. 271). 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 

 represent the systemic circulation, and the pulmonary artery 

 with the pulmonary veins to represent the pulmonary circula- 

 tioivthen rilling the heart with water or a colored fluid and 

 compressing the organ with the hand (Fig. 273). 



The circulation may be studied in the web of the frog's hind 

 foot. Procure a thin board large enough to lay the frog upon ; 

 in one end make a hole about a half-inch in diameter, over 

 which the web may be stretched; anaesthetize the frog with 

 ether or chloroform ; as soon as the animal becomes insensible 

 lay it on the board, with its body covered with a moist cloth ; 

 over the larger toes of the foot to be examined slip nooses of 

 thread, and fasten these in slits around the edge of the board 

 in such positions as to spread the web between two of the toes 

 over the hole in the board. Put a drop of water on the web, lay 

 on the cover glass, place the board on the microscope, and ex- 

 amine with a one fifth or a one sixth objective. The anaesthetic 

 must be renewed from time to time, otherwise the struggles of 

 the animal will interfere with observation (Fig. 263). 



CHAPTER XVII 



The gross structure of the frog's lung may be studied in 

 specimens which have been removed from the body, inflated 

 with air blown through a small glass tube inserted through the 

 glottis, and placed in alcohol a few hours to harden. When 



