130 



ELEMENTARY PHYSIOLOGY 



7. Where is the tricuspid -valve ? What is its use ? How would you illus- 



trate its action ? 



8. Describe the structure of the mitral valve. 



9. Where are the semilunar valves ? Describe their structure and their 



mode of action. 



10. W 7 here and what are the coronary arteries and the coronary veins' 1 . 



11. If you were shown a sheep's heart, how could you tell which was the 



right side and which was the left ? 



12. How do the semilunar valves differ from the other valves on the same side 



of the heart ? 



LESSON XXIII. 



THE BLOOD-VESSELS AND CIRCULATION. 



THE system of blood-vessels through which the blood circulates 

 consists of arteries, capillaries, and veins. 



The arteries, as we have al- 

 ready learnt, have strong, tough, 

 and elastic walls, and convey the 

 blood stream from the heart. 



These vessels have three dis- 

 tinct coats : 



1. The outer strong and tough 

 layer of areolar tissice, containing 

 elastic fibres. 



2. The middle muscular and 

 elastic coat. This layer is so thick 

 in the large arteries that it forms 

 the greater part of the wall. The 

 muscular fibres encircle the vessels. 



3. The internal coat formed of 

 layers of elastic tissue, lined with 

 a delicate layer of epithelium. 



The large arteries divide into 

 branches as they proceed from the 

 heart, and these branches again 

 divide and subdivide into smaller 

 and smaller arteries, till at last they 

 give rise to the smallest vessels 

 the capillaries. 



The capillaries (Lat. capillus, 

 a hair) are very minute vessels, 

 having an average diameter of 

 about g^V^ of an inch. They are 

 generally arranged like the meshes of a network, though the form 



Fig. 121. Transverse Section 

 through a small Artery and 

 Vein. 



A, artery ; v, vein ; e, epithelial lining ; 

 m, middle muscular and elastic coat, 

 thick in the artery, much thinner in the 

 vein ; a, outer coat of areolar tissue 

 (magnified 350 diameters). 



