434 PHYSIOLOGY CHAP. 



intraocular pressure becomes less than it was at the outset of 

 the experiment. This tends to show that the absorption of the 

 humour is not due to simple filtration. 



The vitreous body consists of a trabecular skeleton of collagenous 

 substance, the meshes of which contain a fluid of approximately 

 the same physical and chemical properties as the aqueous humour ; 

 it also contains a rnucoid. 



It is generally held that the vitreous humour is also a secretion 

 from the ciliary processes. Nothing definite is known as to the 

 circulation and absorption of this fluid; but it is generally 

 assumed that it circulates and is renewed exceedingly slowly. 

 Schwalbe demonstrated that the peri vascular lymph spaces of the 

 branches of the arteria centralis retinae communicate directly 

 with the hyaloid canal. After injecting coloured substances into 

 this canal they reappear in a short time in these lymph spaces 

 (Leber). 



The pressure in the eyeball is, as will readily be understood, 

 one of the most important factors in the correct function of the 

 dioptric systems of the eye. This pressure depends (a) on the 

 tension of the intraocular contents ; (&) on the resistance opposed 

 by the elastic walls of the eyeball. As the latter is constant, or 

 probably varies but little within physiological limits, we must 

 to understand the oscillations of intraocular pressure consider 

 the factors which determine the amount of fluid contained in the 

 eye, and the pressure that prevails in the intraocular blood- 

 vessels. 



The height of intraocular pressure varies under normal 

 conditions in man and animals between 20 and 30 mm. Hg. 



The methods employed in measuring intraocular pressure are of two kinds : 

 manometers (in animals) and tonometers. The former, by means of special 

 cannulae, are brought into direct connection witli the interior of the eye. 

 The latter exert pressure on the outer wall of the eyeball by a small plate, 

 so as to flatten it (A. Tick). By measuring the force necessary to obtain a 

 certain degree of flattening, it is easy to calculate the pressure within the 

 eye. 



According to Bottazzi and Sturchio (1906) the fluids of the eye 

 contribute effectively to the maintenance of the normal intra- 

 ocular pressure, owing to the fact that their molecular concentra- 

 tion is higher than that of the blood. On analogy with the 

 phenomena of turgor in plant cells (Pfeffer) these fluids continu- 

 ously attract water from the outside, that is through the coats 

 of the eye which must accordingly be semipermeable and thus 

 increase in volume. 



As we stated above, the intraocular pressure is also in relation 

 with the pressure in the blood-vessels. In fact, oscillations of 

 intraocular pressure which correspond exactly to those of the 

 blood -pressure have been described. These pulsations are 



