120 Mr, Paget, [April 8, 



passes through changes which are as exactly regulated in time as they 

 are in visible characters. The changes in this vesicle are, moreover, 

 indicative of a coincident succession of events in, or produced by, the 

 virus inserted, which, in the blood of the vaccinated person, ^increases, 

 and, incorporating itself in the vesicle, reaches its highest develop- 

 ment and greatest inoculating power on the eighth day, and then 

 degenerates. 



The vaccine might, in most essential points, be regarded as a type 

 of morbid poisons, i.e., of such as are the products of disease. Whether 

 inserted in the blood by inoculation, or bred therein, they commonly 

 occupy definite periods of time in their development, and increase, and 

 decline ; as with a life which is chronometric in all its phases and in its 

 total length. 



The instances of morbid poisons would supply examples of organic 

 processes timed to various numbers of days ; and many that are com- 

 pleted in a day, or in given portions of a day, are traceable in the 

 events of sleep and waking in animals (and, perhaps, also in plants), 

 in the daily variations of the pulse, and of breathing, the returns of 

 hunger and thirst, the regulated times of the digestive functions, &c. 

 In man, indeed, consciousness and will are so concerned in some of 

 these functions, that they may seem to lack that regularity which 

 belongs to merely organic processes ; but, if studied generally, and in 

 other species as well as man, they all tell of such processes accomplished 

 with regular measurement of time, and not determined by the external 

 events or conditions of the day or night. Thus, for sleep and wak- 

 ing, and the times of hunger and thirst, man's independence in regard 

 to day and night, or light and darkness, and the habits of different 

 species whose times of activity are, severally, in the early or later day, 

 in twilight or at night, may prove that the earth's diurnal changes are 

 not the causes of these diurnal peculiarities of animal life. The very 

 cause of sleep, and of that which is yet more mysterious, waking, may 

 be unknown ; but they are evidently connected and correlated with 

 those alternating conditions of the structures, of which men, and pro- 

 bably all animals that sleep and wake, are conscious in the sensations 

 of fatigue and of refreshment. The ordinary activities of one portion 

 of the twenty-four hours, the activities, especially, of the muscles and 

 nervous centres and the senses, produce an amount of structural, or 

 chemical, change which is exactly repaired in rest during sleep. In 

 other words, the organic processes for the repair of structures changed 

 (as all structures are) by exercise, are adjusted to such a rate, that, in 

 general, and on an average, in the time of sleep, they may completely 

 restore the parts that are impaired in the activity of waking time. 

 And so, of that replacement of substances in the several structures and 

 in the blood, which is the purpose of feeding ; the processes of diges- 

 tion and of the several stages of assimilation are so timed, as to accord 

 exactly with the times of daily taking food. 



The most minute observances of time in organic processes might 

 be noted in organs that have rhythmic motions, as in hearts and breath- 

 ing muscles, cilise, the vacuoles of certain zoospores, as A^olvox and 



