152 



THE CIRCULATION OF THE BLOOD 



oxygen, the nerves cause the muscle fibres to contract. 

 The arterioles as a result become smaller and less blood 

 is allowed to pass. The nerves which control the relaxa- 





Capillaries 



FIG. 92. A, Diagram of circulation from artery through capillaries to vein of 

 frog. The darker corpuscles show change to venous blood. B, Diagram of cir- 

 culation in congestion, showing dilatation of blood-vessels and crowding of 

 corpuscles. (After Warren.) 



tion and contraction are known as the vasomotor nerves. 

 They are a part of that branch of the nervous system 

 which, because it works automatically, is called the 

 sympathetic system. 



EXPERIMENTS AND DEMONSTRATIONS 



Anatomy of the Heart 



Materials: Sheep's heart with pericardium unpunctured ; 

 rabbit, freshly killed if possible ; glass tube *4 in. diameter, 12 

 in. long; scalpels; scissors; forceps. 



1) Demonstrate in rabbit, which has been prepared * by cut- 



1 If the chest is carefully punctured with the points of scissors, 

 previous to the removal of the ribs, the entrance of the air will 

 allow the lungs to collapse, so that they will not be injured by 

 subsequent cutting. 



