Duration of Yield. 63 



lives shortened by this conflict* of opposite demands, is 

 equally clear and certain. 



Naturally, cows must dry up their yield to enable them 

 to breed. In breeding, they originate milk yield, and milk 

 yield is necessary to complete the vital breeding process, 

 while drying is necessary to initiate the preliminary breed- 

 ing process ; so that, though disregarded, breeding pow- 

 er seems too important to be entirely ignored even in dairy 

 cows. 



During embryo breeding the cow feels the hunger of 

 the embryo through their united nerves, and supplies the 

 demand which hunger indicates as long as the hunger con- 

 tinues, if possible. To do this she must control the flow 

 of her breeding blood, through her nerve action, causing 

 its conveyance to the embryo before calving, and to the 

 udder after that event. This control by nerve influence, is 

 exerted by means of contractile power in the udder sup- 

 ply arteries, which enables her to close these arteries du- 

 ring the embryo breeding process, thereby diverting the 

 breeding blood to the udder after the calf is born. The 

 evidences of this power are three : natural contractility in 

 the artery walls, the fact of cows drying themselves by 

 the use of this power in closing their udder supply arteries; 

 and the_^other fact, that, without this necessary power, the 

 supply of blood to the embryo, through the placental cir- 

 culation, could not be increased, as it necessarily must be, 

 according to increasing embryo size, and its correspond- 

 ing demand for nutrition. 



If this power of self-drying remained unimpaired, cows 

 would naturally dry according to reduction of blood sup- 

 ply to the udder, to thereby supply the constantly increas- 

 ing embryo demand, and the duration of yield would cor- 

 respond with the period of drying. 



*See chapter on Alternate Breeding and Milking. 



