THE PHYSIOLOGY OF EXERCISE. 323 



already encamped, in the question how inflammation and vascular 

 paralysis are distinguished. Our case is also distinguished by the fact 

 that the skin, protected by callus like the practiced muscles, now af- 

 fords better service under similar circumstances. The callus, m par- 

 ticular cases, represents an improvement in the grasping organ. For- 

 mative stimulation also occurs in the muscles ; the contents of the 

 primitive bundle are moved to nucleation through local stimulation, 

 yet . the advantageous stimulation by exercise seems to be almost en- 

 tirely, or chiefly, of a nutritive sort. 



In like manner as the skin fortifies itself against the repeated touch 

 of hot bodies by means of local calluses, it adapts itself to the heat of 

 the sun through erythema and a change consequent upon it which is 

 accompanied with the development of pigment, although pigment 

 favors the absorption of the sunbeams. The fact is, perhaps, con- 

 nected with this, that it is advantageous to animals to have the side 

 that is turned toward the light of a dark color. Hence, as Moseley 

 observed on the Challenger, JEcheneis rernora has the belly dark, the 

 back light. Heat from artificial sources of a relatively lower temper- 

 ature, which is deficient in refrangible rays, has a remarkably differ- 

 ent effect from sunlight. Workers by the fire are pale. It is still to 

 be seen whether the electric light will take the place of the sunlight 

 in its effect on the skin as it does in the case of plants. 



Horny structure becomes unfit for its purpose with insufficient 

 use. A remarkable example of this is the cessation of the growth of 

 the hoofs of horses and cattle on the soft turfs of the Falkland Islands, 

 mentioned by Darwin. On the other hand, the hoofs of horses harden 

 on dry, stony soils, as Xenophon teaches in his school for horsemen ; 

 and colts brought up on such soils need no protection. 



The so-called rider's bones, the exercise-bones, which have not be- 

 come rarer since the introduction of the new armor and the modified 

 drill, but have moved from the left to the right, may be considered 

 as a kind of inner callus, the development of which affords a new ex- 

 emplification of the Osteo-blasten theory. These bones hardly bring 

 any advantage to their possessor, and can not be included among the 

 instances of self-improvement through exercise. It would be too far 

 fetched and groping in a too dark quarter for me to do more than 

 mention here that Ludwig Fick believes that the well-adapted form of 

 the joints may have been derived from exercises during the fetal pe- 

 riod and the earliest days of life. Is it not possible that the splendid 

 formation of the spongy bone-substance in the epiphyses, which was 

 discovered by Hermann Meyer, and further investigated by Julius 

 Wolff, depends on nutritive and formative stimulus in the direction of 

 the greatest pressure and strain ? The injurious effect of insufficient 

 use is shown in this region by the non-growth of the teeth of rodents 

 when they are fed on too soft food, or after the trigeminus has been 

 cut. 



