508 APPLIED BIOLOGY 



(D) Carbon Dioxide in Expired Air. Connect a bellows, bicycle- 

 pump, or atomizer bulb to a glass tube, and blow air into some lime- 

 water (or barium-water). Note the effect. If much cloudiness 

 (precipitate) appears, compare with air pumped out-of-doors directly 

 into the lime-water. 



Now, exhale air from the lungs through a glass tube into lime- 

 water. Compare with the lime-water mixed with fresh air in above 

 experiment. 



434. Excretion of Nitrogen and Water. This work of 

 the kidneys is carried on by the tubules which are numerous 

 in these organs. A section of a kidney prepared for micro- 

 scopic study shows tubules cut transversely, longitudinally, 

 and obliquely; and surrounding the tubules are abundant 

 blood-capillaries. The cells composing each tubule extract 

 nitrogenous excretions and water from the blood, and then 

 eliminate them into the cavity or lumen of the tubule. The 

 water washes the excretion out of the tubule into the ureter or 

 kidney-duct, which in all mammals extends from each kidney 

 to the bladder. This is a reservoir for the temporary storage 

 of the excretions (urine) of the kidneys, and in all mammals 

 is connected with the exterior by a duct known as the urethra. 



435. Skin in Excretion. Under conditions of high tem- 

 perature the human skin-glands eliminate water (" sweat " 

 or " perspiration ") in which is dissolved salt and small 

 quantities of other substances commonly excreted by the 

 kidneys. However, all the water and other substances 

 could be excreted by the kidneys, and so the skin is not a 

 necessary organ of excretion. It will be explained in 447 

 that the skin eliminates water in order to get rid of excess 

 heat, and hence excretion of water is not the primary work 

 of the skin-glands. 



SUMMARY 



436. Functions Serving the Cells. (1) Digestion and 

 absorption of foods are functions for getting dissolved foods 

 into the blood, which directly or through the lymph distributes 



