DISSECTION OF A RAT. 03 



and, beginning at the angles of the mouth, carefully cut 

 backwards through the cheeks, so as to allow the lower jaw 

 to be bent back. In the mouth-cavity study the teeth. 

 In front are the incisors, and further back the molars. 

 How many of each in each jaw. "With a knife test the 

 hardness of the front and back surfaces of the incisors. 

 Which is the harder ? Why are these teeth always sharp ? 

 Is there any such arrangement in the molars ? 



Between the molars is the hard palate, its surface with 

 transverse folds. Farther back is the soft palate, bounded 

 behind by the place (internal narial opening) where the 

 nostrils communicate with the back part of the mouth- 

 cavity. How many of these openings do you find ? Slit 

 soft palate with the scissors and see how this arrangement 

 is brought about. 



Opposite the internal narial opening (i.e., on the floor 

 of the pharyngeal region) is an opening the glottis, sur- 

 rounded by a raised rim, which is enlarged in front into 

 a soft epiglottis. Inside of the glottis may be seen two 

 folds (vocal chords), which narrow the opening. Insert a 

 probe into the glottis. Where does it appear ? 



Split the skin down the back, and remove it from the 

 body, and then with the bone forceps break through the 

 cranial walls at the back of the head,* taking pains not to 

 injure the underlying structures, When the opening is 

 made enlarge it by removing the skull bit by bit with a 

 strong knife from the dorsal surface and right side. Then 

 continue the process back in the neck region as far as the 

 shoulders. 



* The points relating to the brain can be made out more easily on 

 the cat or sheep, but with a little pains the directions here given can 

 be followed on the rat. 



