d 



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 "^jMMff SffilWIIW-fi 

 of the two forms of blood (j©TOlm M/JMIkm-'C 

 vessels is the elastic and 

 muscular coat thickest? 

 Why should you expect 

 this condition from the 



method of flow of blood ^^°- 34.— cross section of an Artery: a, 



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



m eacn .' W nat is tne nectlve tissue; d, small artery to nourish 



special advantage of the i^^g««^«- 



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? 



XL VII. — 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 the heart pulses. 



