40 COMPARATIVE PHYSIOLOGY. 



Finally, and of special interest to the medical student, are 

 the laws of rhythm in disease. Certain fevers have their regu- 

 lar periods of attack, as intermittent fever ; while all diseases 

 have their periods of exacerbation, however invariable the 

 symptoms may seem to be to the ordinary observer or even to 

 the patient himself. 



Doubtless the fact that certain hereditary diseases do not 

 appear in the offspring at once, but only at the age at which 

 they were manifested in the parents, is owing to the same 

 cause. 



Let us now examine more thoroughly into the real nature of 

 this rhjrthm which prevades the entire universe. 



If a bow be drawn across a viohn-string on which some small 

 pieces of paper have been placed, these will be seen to fly off ; 

 and if the largest string be experimented upon, it can be ob- 

 served to be in rapid to-and-fro motion, known as vibration, 

 which motion is perfectly regular, a definite number of move- 

 ments occurring within a measured period of time ; in other 

 words the motion is rhythmical. In strings of the finest size 

 the motion is not visible, but we judge of its existence because 

 of the result, which is in each instance a sound. Sound is to us, 

 however, an affection of the nerve of hearing and the brain, 

 owing to the vibrations of the ear caused by similar vibrations 

 of the violin-strings. The movements of the nerves and nerve- 

 cells are invisible and molecular, and we seem to be justified in 

 regarding molecular movements as constant and associated 

 with all the properties of matter whether living or dead. 



We see, then, that all things living and lifeless are in con- 

 stant motion, visible or invisible ; there is no such thing in the 

 universe as stable equilibrium. Change, ceaseless change, is 

 written on all things ; and, so far as we can judge, these 

 changes, on the whole, tend to higher development. Neither 

 rhythm, however, nor anything else, is perfect. Even the mo- 

 tions of planets are subject to perturbations or irregularities 

 in their periodicity. This subject is plainly boundless in its 

 scope. We have introduced it at this stage to prepare for its 

 study in detail in dealing with each function of the animal 

 body. If we are correct as to the universality of the law of 

 rhythm, its importance in biology deserves fuller recognition 

 than it has yet received in works on physiology ; it will, ac- 

 cordingly, be frequently referred to in the future chapters of 

 this book. 



