-8 3 



LESSON^ XX. 

 DIGESTIVE TRACT (Continued). 



(a) Longitudinal Section of Hard Tooth. 



By means of a fine saw a longitudinal section is cut from a 

 tooth (incisors or canines best). Grind this on an emery wheel as 

 thin as possible, and then between two hones until it becomes quite 

 transparent. Care must be taken to grind it evenly ; keep the 

 hones well moistened with water. Wash thoroughly, first in water, 

 then in alcohol ; then dry the section between filter paper. Mount 

 in hard balsam. 



Study the preparation first .under low power, observing 

 the shape and size of the pulp cavity, and the relative pro- 

 portion of the dentine, cement, and enamel. The structure 

 of these parts is then to be studied under high power. In 

 the enamel note the enamel prisms; in the dentine the 

 dentinal tubules radiating from the pulp cavity, and in its 

 peripheral portion, the interglobular spaces. The cement 

 shows the structure of bone; very seldom, however, are 

 Haversian canals found. Sketch under low power. 



(b) Cross Section of Decalcified Tooth. 



The canine of a young cat was extracted, the pulp cavity 

 opened from above, and hardened in 50 parts of a saturated aqueous 

 solution of mercuric chloride and 3 parts of formalin, for 2 days. It 

 was then washed in flowing water for 12 hours, and decalcified in 

 20 parts of formic acid, 3 parts of glacial acetic acid, and 30 parts of 

 distilled water. It was again washed in water, dehydrated, 

 embedded in paraffin, and cross-sectioned. The sections were 

 fixed to cover glasses, stained in hrematoxylin and eosin, dehy- 

 drated, and cleared in oil of bergamot and xylol. Mount in balsam. 



Study under high power. The preparation is given to 

 demonstrate the structure of the pulp. Observe the loose 

 connective tissue, consisting of a few fibrils and branched 

 connective tissue cells. A layer of odontoblasts bounds 

 the pulp. These cells have a column-shaped body ; from 



