180 OF SIGHT. 



M. Majendie has found the matter of the tarsal or Meibomiau 

 glands to be not sebaceous but albuminous, and soluble in the 

 tears : hence we discover why, during sleep, it accumulates on 

 the tarsi ; because its solvent, tlie tears, are not sufficiently 

 abundant to remove it. 



(C) In Albino animals, whether the rabbit, pigeon, or mouse, 

 the sclerotic and chorioid are nearly transparent, the latter losing 

 its blood after death, and the image formed upon the retina may 

 be readily seen without removing a portion of the sclerotic. From 

 observations of this kind M. Majendie has declared that whether 

 the eye be presented to a neighbouring or to a distant object, the 

 image upon the retina is equally distinct, and therefore that all 

 the explanations of this circumstance which have been hitherto 

 given, founded on changes which can occur only during life, fall 

 to the ground, whether founded on pressure of the ball by the 

 recti muscles, motion of the crystalline, contraction of the crys- 

 talline or ciliary processes, &c. The iris, however, dilates when 

 we look at a distant, and contracts when at a near, object ; as 

 may be distinctly observed by holding up a finger and desiring a 

 person to look alternately at it and at a distant object which 

 stands in the same line. We are conscious of this adjustment 

 of the iris : w r e move the muscle (if it may be so called) volun- 

 tarily, and the act is painful if quickly repeated. 



M. Majendie also discovered that the escape of a little of the 

 aqueous or vitreous humour, or the total removal of the former 

 or of the cornea, impaired the distinctness of the image ; the 

 total removal of the aqueous humour or of the crystalline also 

 increased the size of the image j the removal of the humours 

 prevented the formation of any image ; an increase produced in 

 the pupil by a circular incision of the iris produced an increase of 

 the image.* 



(D) Mariotte's experiment was to make two spots upon a wall, 

 to fix the right eye upon the left spot, the other being closed, 



- T * ^ ' - . . . , 



*' JW:w E'emcntaire de Physiologic.. T. i. p. 61 sc^. 



