742 



INTRAOCULAR PRESSURE. 



one wall to the other are similarly covered. According to Axel Key and Retzius, these lymph- 

 spaces communicate anteriorly with the perichoroidal space. 



The aqueous humour closely resembles the cerebro-spinal fluid, and contains albumin and 

 sugar ; the former is increased, and the latter disappears after death. The same occurs in the 

 vitreous. The albumin increases when the difference between the blood-pressure and the intra- 

 ocular pressure rises. Such variations of pressure, and also intense stimuli applied to the 

 ere, cause the production of fibrin in the anterior chamber (Jesner and Griinhagcn). 

 ' Intraocular Pressure. The cavity of the bulb is practically filled with watery fluids ;> which, 

 daring life, are constantly subjected to a certain pressure, the " intraocular pressure." Ulti- 

 mately, this depends upon the blood- pressure within the arteries of the retina and uvea, and 

 must "rise and fall with it. The pressure is determined by pressing upon the eyeball, and 

 ascertaining whether it is tense, or soft and compressible. Just as in the case of the arterial 

 pressure, the intraocular pressure is influenced by many circumstances ; it is increased at every 

 pulse-beat and at every expiration, while it is decreased during inspiration. The elastic tension 

 of the sclerotic and cornea regulates the increase of the arterial pressure by acting like the air- 

 chamber in a fire-engine ; thus, when more arterial blood is pumped into the eyeball, more 

 venous blood is also expelled. The constancy of the intraocular pressure is also influenced by 

 the fact that, just as the aqueous humour is removed, it is secreted, or rather formed, as rapidly 

 as it is absorbed ( 392). [Fick has invented an instrument for the direct measurement of the 

 intraocular pressure, a small plate of known size is pressed against the eyeball, and the pressure 

 exerted is registered by means of a spring and index.] 



The secretion of the aqueous humour occurs pretty rapidly, as may be surmised from the 

 fact, that haemoglobin is found in the aqueous humour half an hour after dissolved blood (lamb's) 

 is injected into the blood-vessels of a dog. It is rapidly reformed, after evacuation, through a 

 wound in the cornea. According to Knies, the watery fluid within the eyeball is secreted, 

 especially from thechorio-capillaris, and reaches the suprachoroidal space, in the lymph-sheaths 

 of the optic nerve, and partly through the network of the sclerotic. It saturates the retina, 

 vitreous, lens, and for the most part passes through the zonula ciliaris into the posterior 

 chamber, and through the pupil into the anterior chamber. The movements of the fluid with- 

 in the eyeball have been recently studied by Ehrlich, who used fluorescin, an indifferent sub- 

 stance, which, on being introduced into the body, passes into the fluids of the eyeball, and in a 

 very dilute solution may be recognised by its green fluorescence in reflected light. From obser- 

 vations on the entrance of this substance into the eye, Scholer and Uhthoff regard the posterior 

 surface of the iris and the ciliary body as the secretory organs for the aqueous humour. It 

 passes through the pupil into the anterior chamber^; some passes into the lens, and along the 

 canal of Petit into the vitreous humour (Pfliiger). Section of the cervical sympathetic, and still 

 more of the trigeminus, accelerates the secretion of the aqueous, but its amount is diminished. 

 If the substance is dropped into the conjunctival sac, it percolates towards the centre of the 

 cornea, and through the latter into the anterior chamber {Pfliiger). 



A current passes forwards from the vitreous humour around the lens, and there is an outflow 

 along the central artery of the retina backwards through the optic nerve to the cavity of the 

 skull {Gifford). The current in the spaces between the sheaths flows from the brain to the eye 

 {Quincke). 



The outflow of the aqueous humour, according to Leber and Heisrath, takes place chiefly 

 between the meshes of the ligameutum pectinatum iridis (fig. 514, m, m), and the canal of 

 Schlemni (*, k), into the anterior circular veins (p. 737). A small part of the aqueous humour 

 diffuses into the posterior layers of the cornea, to nourish it {Leber). None of the water is 

 conducted from the eyeball by any special efferent lymphatics {Leber). Under normal circum- 

 stances, the pressure is nearly the same in the vitreous and aqueous chambers, but atropin 

 seems to diminish the pressure in the former and to increase it in the latter, whilst Calabar 

 bean has an opposite action {Ad. Weber). Arrest of the outflow of the venous blood often 

 increases the pressure in the vitreous, and diminishes that in the aqueous chamber. Compres- 

 sion of the bulb from without causes more fluid to pass out of the eye temporarily than enters 

 it. The diminution of the intraocular pressure is well-marked after section of the trigeminus, 

 while it rises when this nerve is stimulated. The statements of observers regarding the effect 

 of the sympathetic nerve upon the pressure vary. Interruption to the venous outflow increases 

 the pressure, while an imperfect supply of blood, the outflow being normal, diminishes the pres- 

 sure. The innervation of the blood-vessels of the eye is referred to at 347. 



385. DIOPTRIC OBSERVATIONS. The eye as an optical instrument is comparable to a 

 camera obscura ; in both, an inverted diminished image of the objects of the external world 

 is formed upon a background, the field of projection. Instead of the single lens of the camera, 

 however, the eye has several refractive media placed behind each other cornea, aqueous 

 humour, lens (whose individual parts capsule, cortical layers, and nucleus, all possess different 

 refractive indices), and vitreous humour. Every two of these adjacent media are bounded by 

 a refractive surface," which may be regarded as spherical. The field of projection of the eye 

 is the retina, which is coloured with the visual purple {Boll, Kuhnc). As this substance is 



