712 



THE AMERICAN MUSEUM JOURNAL 



to multiply itself five hundred times in 

 every twenty-four hours. Yet all goes 

 on in an orderly manner, according to 

 the law of its being, but with incredible 

 swiftness, day after day. 



In the development of a human or an 

 animal body the increase is far more, 

 however, than a mere numerical in- 

 crease, although that in itself is a won- 

 derful phenomenon. Among these mul- 

 titudinous cells a wholly new factor 

 soon comes into view. They do not all 

 continue merely as multiplied dupli- 

 cates of their predecessors but they be- 

 gin to differentiate among themselves. 

 Some will become first cartilage, that 

 is, gristle cells, and later, when the 

 blood carries to these cartilage cells 

 lime and phosphorus and other mineral 

 ingredients, these cells bathed in the 

 liquid portion of the blood will pick out 

 each its own required mineral com- 

 pounds and will turn from soft, flexible 

 cartilage to stifi', yet elastic bones — the 

 skeleton of the animal. 



Other cells will remain soft but will 

 select those ingredients which are nec- 

 essary to form the muscles — that is, the 

 red flesh of animals or the white and 

 dark meat of birds. These muscles 

 acting on the bones as levers will move 

 them so that we can lift the ribs and 

 by the diaphragm depress the floor of 

 the chest and thus aid further in ex- 

 panding the lungs for inspiration ; then 

 we reverse the action, depress the ribs, 

 compress the abdominal viscera, and so 

 push up the diaphragm, thus compress- 

 ing the lungs for expiration. Thus the 

 alternate processes of breathing — a 

 continuous function essential to life — 

 are carried on. We are also enabled to 

 regulate our breathing and our vocal 

 cords in such a manner that we can 

 speak or laugh, swallow or sing. 



Still other muscles attached to our 

 jaws permit us to eat, others to our legs 

 allow us to move about, others to our 

 arms and fingers fit them for labor or 

 other useful functions. 



But these muscles do not move of 



themselves. They require a stimulus 

 connected with the center for our men- 

 tal organization. Thus then the need 

 for a nervous system — in fact, as I 

 shall show, for two interdependent and 

 correlated nervous systems. The first 

 of these two systems is composed of the 

 brain, the spinal cord, the nerves 

 emerging from the latter, and the spe- 

 cial end-organs in which the nerves 

 terminate. From the skin by means 

 of these special organs at the ends of 

 the nerve fibers, impulses start and are 

 carried to the brain. In the brain they 

 are appreciated, for instance that the 

 finger is in contact with a source of too 

 great heat. The brain quickly decides 

 that the finger must be withdrawn, and 

 sends down to the proper muscles the 

 order to retract at once. In such a 

 case, long before the whole of this brief 

 description has been read, the muscles 

 have executed the order. This is but 

 one example of a voluntary movement 

 resulting from a sensation. 



The second nervous system consists 

 of the sympathetic system. At certain 

 important points collections of nerve 

 cells are gathered into small bodies 

 called ganglia. These ganglia are con- 

 nected with one another by a large 

 number of nerves. The familiar ^'solar 

 plexus" just in front of the spine at 

 the level of the "pit of the stomach" is 

 the largest and most notable collection 

 of such ganglia. 



The nerves of the sympathetic sys- 

 tem are distributed to the muscular 

 cells in the walls of every artery in the 

 body. This muscular coat consists 

 chiefiy of circular fibers, which, by their 

 contraction and relaxation change the 

 caliber of the arteries and thus regu- 

 late the supply of blood to all parts of 

 the body according to its needs. 



For example, the eye is a very fa- 

 miliar instance. When a cinder gets 

 into the eye, in a short time the white 

 of the eye becomes very red or 'l^lood- 

 shot," but returns to its normal white- 

 ness as soon as the foreisrn bodv is re- 



