CONCLUSIONS AS TO NATURE OF FUNCTIONAL ACTIVITY. 293 



tion, by afferent impulses to the motor centers: a feature 

 which the rapid formation of milk after nursing and the in- 

 fluence of emotions upon its flow suggest. In framing our 

 summary of the functional mechanism of the mammary gland, 

 therefore, it is not the difference between active and passive 

 activity that we have in mind, but the manner in which fluct- 

 uations of activity are brought about. The actual process 

 involved, even here, however, is primarily a mechanical one, 

 since filtration of the blood-plasma into the secreting elements 

 represents no mean expense of force. As blood-pressure to- 

 tally independent of the normal systemic pressure must 

 account for this, we are relegated to the vessel-walls and their 

 nerve-supply as the intermediaries through which this is car- 

 ried out. 



Fluctuations in the functional activity of the mammary 

 gland during lactation are brought about, if our views are well 

 founded, in the following manner: 



Each of the motor nerves distributed to the mammary glands 

 divides into two main branches. One branch, the "extrinsic vaso- 

 constrictor" (ex-sympathetic), is distributed to the arteries outside 

 the secreting structures. The other branch, the excito-regulator, 

 subdivides into two branches; one of these, the "excitor," supplies 

 fibers to the acini; the other, the "intrinsic constrictor" sends 

 fibers to the glandular arterioles, but not to those which supply 

 capillaries to the secreting elements. 



During exacerbations of activity the branches distributed to 

 the arterioles, the "intrinsic constrictors," constrict the latter, thus 

 forcing the bulk of the blood through the free arterioles and thence 

 into the capillaries of the secreting acini, while the "excitor" branch 

 excites and governs the metabolism of the acini. 



The rapidity of the blood-flow through the intrinsic blood- 

 supply arterioles, capillaries and through the secretory elements, 

 is concurrently augmented through increased contraction of the 

 extralobular mammary arteries by the "extrinsic vasoconstrictor" 

 (sympathetic) branches distributed to them. 



GENERAL CONCLUSIONS AS TO THE MECHANISM OF FUNC- 

 TIONAL ACTIVITY. A number of general questions have been 

 referred to in this chapter the discussion of which has to be 



