CIRCULATION AND THE BLOOD SYSTEM 75 



middle layer consisting of a mixture of muscle and elastic 

 fibers, and the outside layer or coat of connective tissue 

 bundles. Make careful 

 drawings of the two prep- 

 arations, showing the lo- 

 cation and form of these 

 layers, and label the above- 

 mentioned parts. In which 

 of the two forms of blood 

 vessels is the elastic and 

 muscular coat thickest? 

 Why should you expect 

 this condition from the 



method Of flow Of blood FlG - 34. Cross Section of an Artery: a, 

 . endothelium; 6, muscular layer; c, con- 



in each? What IS the nective tissue; d, small artery to nourish 



special advantage of the lar eone - 



elastic fibers in the artery? In what way do they aid to 

 keep the capillaries filled at the end of an artery pulsa- 

 tion? Is the pressure greatest in arteries or in veins? 



XLVII. STRUCTURE OF THE HEART. 



Apparatus. Sheep's heart from the butcher with pericardium 

 attached, bristle seekers, dissecting instruments. 



Directions. Locate the parts named below, and make 

 drawings to show their position. 



A. Note that the heart moves easily inside a loose sac. 

 Cut this pericardium open and observe its slippery inner 

 coat. Note a similar coat on the outside of the heart. 

 What lies between these two coats? This liquid and the 

 slippery coats prevent friction when tne heart pulses. 



