of the Organs of Vision. 209 



posterior and anterior chambers. The aqueous humour, when 

 lost by an accident or operation, is newly secreted in 24 hours. 

 The next humour behind this is the crystalline humour or lens 

 (l), represented very dark in the diagram, it being more con- 

 sistent than the other two ; but it is, in fact, as its name im- 

 plies, beautifully clear ; it is embedded in the vitreous humour 

 (v), which occupies the larger portion of the ball of the eye. 

 It is as clear as the purest glass (vitrum), more consistent 

 than the aqueous (a), and less so than the crystalline humour 

 (l). It is covered with a fine membranous bag, called the 

 hyaloid membrane (Jiyalos, glass, and eidos, likeness), and it is 

 upon this humour that the retina is expanded. 



Thus much for the chief parts of the eyeball. Of their 

 uses we shall speak hereafter. It impossible, even from this 

 faint sketch, not to see that it is an organ of the most delicate 

 make, and every body has, without doubt, often experienced 

 how exquisitely sensitive it is: the smallest particle of dust, 

 or even a drop of rain suddenly touching the eyeball, causes 

 a sensation of uneasiness almost incredible, had not every 

 individual been in the course of his life an eye-nscitness of 

 the fact, in more senses than one. With a view to its defence, 

 Providence has placed it in a round bony hollow, plentifully 

 lined with a soft cushion of fat, to guard it from violence, and 

 to break the jars which it would otherwise experience from 

 the daily motion of the body, or from accidental falls. The 

 effects of friction are likewise thus obviated, which the con- 

 stant motion of the eyeball would produce. To keep the 

 front or cornea of the eye clear, a constant lotion is provided 

 by the lacrymal gland (l g), situated in a hollow of the orbit 

 or bony case. This fluid is known by the name of the tears, 

 and is constantly, though (under ordinary circumstances) in- 

 sensibly, passing over ; it is carried off, through two small 

 orifices at the inner corner of the eye, into the lacrymal 

 canals {I c), which unite and form the lacrymal sac (I s), and 

 thence into the nostril. Under ordinary circumstances, as it 

 has been already observed, the moisture passes along unper- 

 ceived ; but when the gland is over-excited, either by moral or 

 physical causes, the tears flow top rapidly to be absorbed by 

 the puficta lacrymdlia, and fall in copious drops down the 

 cheek, " aequore fervido " [in a warm stream]. But, besides 

 the emotions of the mind, or any stimulus applied immediately 

 to the eye, other causes produce the same effect, such as snufF 

 taken in at the nose, or when a small sponge " perfusus 

 liquid is urguet odoribus " [imbued with liquid odours excites 

 it]. The cause of this is easily understood : — the nerve 

 which supplies the lacrymal gland is a branch of the fifth 



Vol. IV. — No. 19. p 



