210 



fine the majority of the branches in a lax manner to its vicinity. A true 

 idea of this system may be gained by conceiving that the veins and 

 arteries here existing in the foetal eye have in after life been deve- 

 loped according to the growth of the body, but also metamorphosed 

 into these light, peculiar, globule-holding, transparent vessels, and 

 deprived of all foreign support except at their roots and a part of 

 their capillary loops. The intricate ramifications of these vessels 

 have the mechanical effect of in a great degree restraining the rela- 

 tive motion of the humour which fills the hyaloid capsule, and com- 

 pelling it to concur in the various movements of the eye-ball, so as 

 to obviate the risk of concussion from eddies of the fluid in rapid 

 movements of the eye, and consequent disturbing effects on the lens, 

 the retina and its vessels. 



The paper goes on to take this subject up in detail ; supplies the 

 dynamical laws which must be kept in view in the application of 

 previously obtained optical methods to the required examination ; 

 shows that it is the system of ramifications described which has 

 given rise to the peculiar appearances simulating concentric lamellse 

 in the vitreous humour previously subjected to chromic acid, so dif- 

 ferently interpreted by microscopists. Here too the hitherto vagrant 

 muscce volitantes are, for the first time, invested with form, disposi- 

 tion and office. They are now shown to be the essential element in 

 the structure of the vitreous body ; and certain radical misconcep- 

 tions, as to the nature of these appearances and the constitution of 

 the vitreous body, are pointed out. 



The Optical Anatomy of the Retina. 



The existence of the vasa centralia retinas, in the substance of the 

 retina, and the movements of the blood therein, occasion diversified 

 phenomena. We may examine these vessels in our own eyes, in 

 their minutest distributions, by means of a pin-hole, lens, &c., in 

 movement across the eye's axis, in virtue of a physiological law here- 

 after determined. Currents of blood in these vessels, by pressure 

 upon the nervous matter at their sides, produce remarkable pheno- 

 mena, differing for the superficial and deep vessels (that is, according 

 to the place of the vessel in the five layers of the retina lately disco- 

 vered by microscopists). These phenomena may all be distinguished 

 from one another, and assigned with precision each to its cause. 



