42 DIGESTION. 



What are the requirements for good ventilation? 



It is generally accepted as a fact that about 1000 cubic feet of 

 air-space per head must be allowed in sleeping quarters, and suffi- 

 cient facilities for exchange of air to allow complete change in each 

 hour. This ventilation must be accomplished without exposure to 

 draughts. 



What effect has respiration upon the circulation ? 



Each inspiration causes a partial vacuum in the thoracic cavity, 

 and hence it, so to speak, sucks the blood to the heart from the 

 great veins. Each expiration does not, however, check the flow 

 of blood, for the expiration does not increase the air-pressure, but 

 simply returns to the normal. The inspiration, then, increases 

 arterial tension by facilitating the venous return. 



DIGESTION. 

 What is digestion? 



By this term we indicate the process by which food is introduced 

 to the body and prepared in such way that it becomes suitable for 

 absorption and tissue-nutrition. The process may be divided logi- 

 cally and conveniently into mastication, insalivation, deglutition, 

 stomach digestion, intestinal digestion, and defecation. 



Describe mastication. 



When a mass of food enters the mouth it is caught by the tongue 

 and moved to a position such that it may be crushed and ground 

 between the upper and lower teeth. This process is favored by 

 the action of the tongue and of the cheeks, which not only crush 

 the softer food-masses, but bring the less tractable portions re- 

 peatedly under the action of the teeth. 



Describe the teeth. 



There are during life two sets of teeth, temporary and permanent '. 

 In the first set are 20 teeth, and in the second 32. The perma- 

 nent set are arranged as follows : 



Molar. Bicuspid. Canine. Incisor. Canine. Bicuspid. Molar. 



Upper. 3^ 2 1 4 1 2 



Lower. if 2 I 4 I ~~2 3 



The tooth-structure resembles bone in anatomical as well as chemi- 

 cal view. The central portion, or cement, is true bone, and about 

 it is a somewhat harder layer called dentine. Outside of all is a 

 very dense layer of enamel. 



