T4O ELEMENTARY PHYSIOLOGY 



7. Describe the condition of the heart and its valves (i) just before the con- 



traction of the ventricles, and (2) just after this contraction. 



8. What is the use of the chorda tendinece ? 



9. Describe fully the action of the semilunar valves. When are they shut, 



and when open ? What shuts them ? 



10. What do you mean by the beating of the heart ? At what rate does the 



heart beat ? 



11. Describe the sounds of the heart. How are they produced ? 



12. What is the pulse? What causes the pulsation of blood in the arteries? 



Why is there no pulsation in the capillaries and the veins ? 



13. How does the blood flow from (i) a cut artery, and (2) a cut vein ? 



14. Describe the course of the blood as it passes from the right to the left side 



of the heart ; and also as it flows from the left to the right side. 



1 5. What is the shortest course the blood can take ? 



1 6. Give proofs that the blood circulates. 



LESSON XXIV. 



THE TRACHEA AND LUNGS. 



THE trachea consists of a series of cartilaginous hoops, shaped 

 like the letter C, and arranged so as to be complete before and 

 deficient behind. These rings are surrounded externally by 

 fibrous connective tissue, which also joins their extremities, thus 

 completing the tube behind ; it also fills in the spaces between the 

 rings. It will thus be seen that the trachea is a resistant and 

 elastic tube, which, though always open for the passage of air, will 

 yield with the bending of the neck, and will also allow of the 

 distension of the oesophagus, which lies against its posterior portion 

 where the cartilage is absent. 



The two upper cartilaginous rings, which are much larger than 

 the others, form the framework of the larynx. The first, called 

 the thyroid cartilage (Gr. thureos, a shield ; and eidos, form), is very 

 wide in front, forming the prominence known as Adam's apple, 

 but is open behind, and is connected by means of ligaments and 

 muscles with a small U-shaped bone, the hyoid bone (Gr. letter -u 

 and eidos, form), situated just above it. The second cartilage is 

 the only complete ring in the windpipe. It is called the cricoid 

 cartilage (Gr. krikos, ring ; and eidos, form). Its broadest portion 

 is behind, while in front it is so narrow that a space is left between 

 it and the first, this space being occupied by the fibrous tissue. 

 Of course, this second ring will not so freely admit of the disten- 

 sion of the oesophagus behind it, and this will account for the 

 sharp pain we experience in the upper part of the throat when we 

 swallow a large and hard substance, such as a solid piece of food 

 which has not been properly masticated. 



