132 NUTRITION OF FARM ANIMALS 



passing through* the heart is increased and a more abundant 

 supply of it reaches the active tissues. Second, there may 

 be a partial shunting of the blood supply from one region 

 of the body to another as one set of organs or another calls 

 for a larger amount. This is accomplished through the agency 

 of the middle or muscular coat of the arterioles, controlled by 

 the so-called vaso-motor nerves. When a larger supply of blood 

 is called for in the muscles, for example, these fibers relax and 

 allow the arterioles to enlarge, thus reducing the resistance 

 offered to the blood flow and allowing the arterial pressure to 

 force blood into the capillaries more rapidly. To compensate 

 for this there is a contraction of the arterioles of the internal 

 organs, especially of the abdominal organs, resulting in a di- 

 minished blood supply. The effect of the performance of work 

 upon digestion, discussed in Chapter XVI (721), is possibly 

 connected with this effect upon the blood flow. On the other 

 hand, after a hearty meal the arterioles of the digestive tract 

 relax, while the superficial blood vessels tend to contract and 

 the blood supply is partially diverted from the surface tissues 

 to the internal organs. This power of the body to regulate 

 the supply of blood to different regions is of special importance, 

 as will appear later (321), in connection with the regulation of 

 the body temperature. 



2. RESPIRATION 



188. The oxygen supply. By means of the processes 

 described in the preceding section the nutritive materials de- 

 rived from the feed and taken up by the intestinal capillaries 

 and lacteals are distributed to the various tissues and cells. 

 Equally necessary with a supply of feed materials to the living 

 cells, however, is a supply of oxygen, and this another set of or- 

 gans, those of respiration, are engaged in furnishing to the blood 

 through another set of capillaries for transmission to the cells. 



189. The lungs. The transfer of oxygen from the air to the 

 blood is effected in the lungs, which, with the heart and large 

 blood vessels, fill the cavity of the thorax, or chest. This cav- 

 ity is enclosed on the sides by the ribs and their connections, 

 forming the chest walls, and is separated from the abdominal 

 cavity, containing the digestive organs, by a strong, arched, mus- 



