AND PHYSIOLOGY. 



417 



the eye be used in too strong light, weakness or partial pa- 

 ralysis of the optic nerve may be produced ; and if the ear 

 receive too violent a sound, deafness may follow, and so with 

 the other senses. Therefore care should be taken in the use 

 of the senses, for when the function of one of the nerves 

 of special sensation is gone, it is very rarely if ever restored. 



775. — 2. Sudden Extremes of Sensations to be Avoided. 

 — With some of the senses, and especially that of vision, great 

 care must be taken to avoid, if possible, sudden transitions. 

 Next to very excessive use and mechanical violence, no greater 

 trial can be given to the optic nerve than a sudden contact 

 with powerful light. To a student this inference is of great 

 value, and especially to those who would rise early and study 

 by candle-light, before daybreak. This practice is often a 

 source of confirmed weak eyes. One reason is, that during 

 sleep the pupil is dilated, and when the eye is opened does 

 not readily contract. This statement, however, is not an ar- 

 gument against early rising, which for other reasons has been 

 shown to be healthy to the student. A good rule for students 

 in sound health during the long nights of the year is to rise 

 before daybreak, and after washing the eyes in pure cold water 

 to take a good share of the exercise needed for the day. 

 Then, after an early breakfast, when food and exercise are 

 both obtained, he maybe ready for a day's work of hard study 

 by the congenial light of the sun. If, however, circumstances 

 are such that we must study in the morning by lamp-light, 

 we should commence and work moderately until the eye is 

 accustomed to the light. 



775. a. — 3. Cleanliness of the Organs of Sensation Nec- 

 essary, — Perfection of sensation requires that all the organs 

 should be kept clean. The eye needs occasional washing, not 



774. Why do the senses all require moderation in their use? What if a nerve of special 

 sense is destroyed ? 775. What is said of sudden extremes of sensation? Why should 

 students especially avoid all extremes of light and darkness as far as possible ? How does 

 early rising affect the eyes ? 775. a. How does cleanliness affect all the organs of the 

 senses t 



