THE SKIN. 69 



What is meant by milk-curdling ferments? 



Certain ferments will act upon milk to cause its separation into 

 curd and whey. These are called milk- curdling ferments. They 

 are found especially in the secretions of the stomach, pancreas, and 

 intestine. 



How is a milk-curdling ferment utilized in the arts? 



In cheese manufacture the fourth stomach of the calf (rennet) 

 is used to curdle milk. 



What causes a scum to form upon boiled milk ? 

 The serum-albumin is solidified by heat. 



What changes occur in milk at different periods of lactation ? 



The milk of mothers prior to and just after parturition is known 

 as colostrum, and is deficient in casein and contains an excess of 

 serum-albumin, as well as colostrum-corpuscles. Toward the end 

 of lactation there is also said to be the same excess of serum-albu- 

 min and lack of casein. 



How are the fat-globules of milk secreted ? 



It seems probable that two methods are possible : (1) The epithe- 

 lial cells of the gland seem capable of causing a metabolism of pro- 

 teids to form fat ; (2) the cells themselves may undergo a fatty 

 metamorphosis. 



What is the value of milk as a food ? 



As the food of all young animals it has to be considered of the 

 greatest importance ; besides this, as a direct food, both in the 

 natural state and in the derived forms of butter and cheese, it is 

 one of the most used articles of adult diet. The causes which lead 

 to its choice in the dietary are easily seen : it contains all the ele- 

 ments necessary to sustain life water, proteids, carbohydrates, and 

 salts. As an exclusive diet it will probably sustain life better than 

 any other substance. 



THE SKIN. 

 Describe, roughly, the structure of the skin. 



There are two layers of tissue which form the entire thickness 

 of the skin. The superficial epithelial layer is known as the epider- 

 mis or cuticle; the deeper stratum, in which lie the active functional 

 elements, is called the cntis vera, coriunij or derma. 



