CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 787 



in some way or other, we should have an image of the nutrition of 

 such a tissue as muscle, in which the proteid constituent is in 

 excess of the others. 



$ 518. That both the secretion and ejection of milk are under 

 the control of the nervous system is shewn by common experience, 

 but the exact nervous mechanism has not yet been fully worked 

 out. While erection of the nipple ceases when the spinal nerves 

 which supply the breast are divided, the secretion continues, and 

 is not unvoted even when the sympathetic as well as the spinal 

 nerves are cut. 



502 



