.1 WOULD OF BILLWyS 



13 



moved. The sudden redness of a young 

 girl's "^'blushing"' cheeks is caused in the 

 same way. 



Again, when we have eaten a meal, 

 the stomach needs a very large and 

 sudden increase in its supply of blood 

 to elaborate the large quantity of gas- 

 tric juice required for digestion. In- 

 stead of a rather dirty looking brownish 

 yellow appearance, as when it is empty, 

 the stomach (like the eye) at once be- 

 comes of a bright red color from the 

 dilated blood vessels. As soon as the 

 meal has been digested, the nerves 

 stimulate the circular muscles of the 

 arteries. By their contraction these 

 immediately lessen the caliber of the 

 blood vessels, and less and less blood 

 reaches the stomach until at length the 

 supply is only enough for the ordinary 

 demands of the stomach in its period 

 of inactivit}^, and it has reverted to its 

 original brownish yellow color. 



These sympathetic nerves are dis- 

 tributed not only to the muscular coat 

 of the blood vessels but to many other 

 muscular fibers which are involuntary, 

 that is, not controlled by the will. We 

 have an excellent illustration of this in 

 the iris, the circular colored disk in the 

 eye with a hole in the center which we 

 call the pupil. This is constructed like 

 a wheel with a circular hub and radi- 

 ating spokes. The "hub" consists of 

 circular muscles which, by contracting, 

 narrow the pupil to a pin point. The 

 spokes are radiating muscular fibers 

 which pull the pupil wide open. We go 

 out into blinding blazing sunlight. For 

 a moment we wink and blink, or have 

 to shade the eyes by the hand or by 

 closing the eyelids. But in a few sec- 

 onds the muscular fibers of the hub 

 contract strongly, the pupil narrows 

 down to a pin point, and the eye is 

 automatically protected from the in- 

 jury which it would otherwise suffer. 

 On the contrary, we go into a darkened 

 room. At first we may stumble over 

 the furniture which we cannot see on 

 account of the small amount of li^jit 



oiiloring the eye through the minute 

 pui)il. In a few minutes the pupil 

 has been dilated by the "spokes," or 

 radiating fibers, to a maximum, and 

 we can see everything with perfect clear- 

 ness. 



All these changes, be it observed, are 

 perfectly involuntary. They are ac- 

 complished for us, and in fact, in spite 

 of ourselves. We do not order the 

 l)lood vessels to dilate when a meal is 

 oaten, or order them to contract when 

 it is digested. We do not order the 

 pupil to contract or dilate according to 

 the brightness or the dimness of the 

 light. In fact we cannot by any possi- 

 bility control the caliber of the blood 

 vessels of the stomach, or the size of the 

 pupil. It is all done for us whether we 

 wish it or not. And it is most fortu- 

 nate that this is so. Suppose it had 

 to depend on our conscious volition. 

 Suppose, tired out and hungry, we ate 

 a hearty meal and fell asleep ! The 

 stomach would get no increase of blood, 

 there would be little or no gastric juice 

 secreted, no digestion would take place. 

 What endless confusion there would be ! 

 Before long we should surely perish! 

 In the case of the pupil, the bright 

 light might easily seriously damage the 

 eye, or we could never see in a dimly 

 lighted room. 



In this rather long discussion we 

 have almost forgotten that we were con- 

 sidering the differentiation of the rap- 

 idly multiplying cells into bone cells, 

 muscle cells, and cells forming the 

 l)rain, spinal cord, and nerves to con- 

 trol the voluntary muscles, and the 

 sympathetic nerves to control the in- 

 voluntary muscles. 



If we examine a chicken embryo, we 

 shall find even as early as the third day 

 the well-recognized and differentiated 

 parts of the organs of sight, of hearing, 

 and of smell. When we remember the 

 ultimate excessive complexity of these 

 organs, especially of the eye and the ear 

 in num, this faculty of differentiation 

 causes our wonder and astonishment to 



