i28 SCIENCE PRIMERS. [§ ix. 



goes away hotter than the arterial which came ; and 

 all the hot blood mingling together and rushing over 

 the whole body keeps the whole body warm. Sweep- 

 ing as it continually does through innutnerable 

 little furnaces, the blood must needs be warm. This 

 is why we are warm. But from some places, as 

 from the skin, the venous blood goes away cooler than 

 the arterial which came, because while journeying 

 through the capillaries of the skin it has given up 

 much of its heat to whatever is touching the skin, 

 and has also lost much heat in turning liquid perspira- 

 tion into vapour. This is why so long as we are 

 in health we never get hotter than a certain degree 

 of temperature, the so-called blood-heat, 98° Fahr., 

 and why we make warm the clothes which we wear 

 and the bed in which we sleep. ^ '' • 



Everywhere oxidation is going on, oxidation either 

 of the blood itstflf or of the structures which it bathes, 

 and whose losses it has to make good. Everywhere 

 change is going on. Little by little, bit by bit, every 

 part .^f the body, here quickly, there slowly, is con- 

 tinually mouldering away and as continually being 

 made anew by the blood. Made anew according 

 to itF ovTi nature. Though it is the same blood 

 which is rushing through all the capillaries, it makes 

 different things in different parts. In the muscle 

 it makes muscle ; in the nerve, nerve ; in the bone, 

 bone ; in the glands, juice. Though it is the same 

 blood, it gives different qualities to different parts: 

 out of it one gland makes saliva, another gastr.'^ 

 juice : out of it the bone gets strength, the brain 

 power to feel, the muscle power to contract. 



When the biceps muscle contracts and raises the 



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