Mammalia. 157 



6. Just back of the upper angle of the thyroid cartilage find 

 a muscle connected with the base of the epiglottis; pull this 

 muscle to determine what effect its shortening produces on the 

 epiglottis. 



7. Under the thyroid cartilage in front observe a narrow ring 

 of cartilage not much wider than one of the rings of the trachea. 

 Move this up and down to prove that it is distinct from the thy- 

 roid. This is the cricoid cartilage. 



8. Observe the sheet of muscle passing from the cricoid to the 

 thyroid. Again move the cricoid toward and from the thyroid. 

 What does this muscle do ? Cut away this muscle from one side, 

 and see that the cricoid cartilage widens as it passes backward. 

 How are the cricoid and thyroid hinged together ? 



9. Projecting upward and backward from the top of the larynx 

 are two curved yellowish cartilages, the arytenoid cartilages. 

 Move them about to see that they are movable, and that they 

 rest on the upper edge of the back part of the cricoid cartilage. 



10. Move the arytenoid cartilages backward and forward, mean- 

 while watching the inside of the larynx from its lower opening. 

 The projecting ridges, which meet just back of the Adam's apple, 

 are the vocal cords. What effect is produced on the vocal cords 

 by the movements of the arytenoid cartilages? 



11. Observe the connection of the thyroid cartilage with the 

 cricoid by means of a downward projection of the former. Cut 

 away all of this half of the thyroid cartilage. Notice the slender 

 hyoid bone loosely connected with the upper horn of the thyroid. 



12. Examine now the muscles which move the arytenoid 

 cartilages. 



a. On each side of the posterior surface of the cricoid is 

 a muscle passing upward to be attached to the corresponding 

 arytenoid ; this is the posterior crico- arytenoid muscle. Dissect it 

 loose from the cricoid at its origin below. By pulling, determine its 

 action on the arytenoid, and through the arytenoid on the vocal cord. 



b. Arising from the upper edge of the side of the cricoid 

 cartilage, and passing upward and backward to the arytenoid, is 



