THE CIRCULATORY SYSTEM 173 



the boys under their care are addicted to smoking, simply 

 by the comparative inferiority of their appearance, and 

 by their intellectual and bodily indolence and feebleness. 

 After full maturity is attained the evil effects of com- 

 mencing the use of tobacco are less apparent ; but com- 

 petent physicians assert that it cannot be safely used by 

 those under the age of forty. 



DEMONSTRATIONS AND EXPERIMENTS 



75, Dissection of the Heart. Order of a butcher the heart of a calf, 

 sheep, or pig. Explicit directions should be given that the entire 

 "pluck" be saved, i.e. heart, lungs, and larger blood vessels intact; 

 otherwise mutilated specimens will be received. By inflating the 

 lungs through the trachea their structure and general relationship 

 to the heart can be shown. Observe that the heart lies in a sac, the 

 pericardium. Cut open the latter and notice the pericardial fluid. 

 Before proceeding to cut open the heart identify as many as possible 

 of its parts and connected blood vessels. Observe the blood vessels 

 connecting the heart and the lungs, and distinguish between the 

 pulmonary artery and the pulmonary veins. The aorta and the two 

 venae cavae can be distinguished with very little trouble. After sever- 

 ing the vessels connecting the heart with the lungs, the course of the 

 blood through the heart can be shown, and a difference between 

 the arteries and the veins can be demonstrated by forcing water into 

 the vessels. Water injected through the venae cavse into the auricle 

 emerges from the pulmonary artery. Injected in the opposite direc- 

 tion the flow is retarded by the auriculo-ventricular valve. In this 

 way one can identify vessels of which he is in doubt, and demonstrate 

 the functions of the valves. Demonstrate the internal structure of 

 the Heart'by making incisions in the walls of the auricles and ventri- 

 cles, and identifying the parts as described in the text. 



76. Demonstration of the Organs of Circulation. The principal ves- 

 sels of the circulatory system can be readily dissected out in the body 

 of a cat, dog, or rabbit. If the teacher is familiar with methods of 

 injecting the circulatory system with some colored substance, the 

 vessels can be more easily traced out, and also preparations can be 



MACY'S PHVS. 11 



