122 PHYSIOLOGY. 



SENSE OF VISION. 



By the sense of sight, we become acquainted with the existence of 

 light; and by the medium of that agent, we take cognizance of the 

 form, size, colour, position, &c. of bodies that transmit or reflect it. 

 A knowledge of the laws of light and optics, is essential to an un- 

 derstanding of the functions of the different parts of the organ of 

 vision ; for these the student is referred to the works on natural 

 philosophy. 



In the globe of the eye we recognise, as the most essential parts, 

 the expansion of the optic nerve, called the retina; and in front of 

 this the transparent refracting media, which transmit the light so 

 as to bring it to a focus upon the nervous expansion. The optic 

 nerve, at its entrance into the eye, splits up into numerous fibrils, 

 which spread themselves out, and inosculate freely with each other, 

 so as to form a net-like plexus. This plexus comes into relation 

 with numerous vessels and a layer of ganglionic cells, like those in 

 the cortical part of the brain. This layer of cells constitutes the 

 internal layer of the true retina. 



In order to protect the retina, the slightest change in whose form 

 would be attended with injury to its function, the whole is enveloped 

 in a dense tissue called the sclerotic, which is opaque, with the ex- 

 ception of its front, where it becomes modified to allow the light to 

 enter, and is called the cornea. Between the sclerotica and the re- 

 tina is a layer of dark pigment contained in a delicate membrane 

 called the choroid, the use of which is to absorb the rays of light 

 after they have made their impression upon the retina. In the albino 

 it is entirely wanting, and in others it becomes gradually lighter in 

 colour as they advance in life, so as to increase the stimulus applied 

 to the retina, by reflecting the rays again from one part of its sur- 

 face to another. In front of the retina are the transparent media. 

 These are the vitreous humour, the crystalline le^is, and the aqueous 

 humour. The vitreous humour lies immediately within the cup formed 

 by the retina, and seems destined to give it the necessary support 

 inside, which the sclerotic gives outside. In the anterior part of the 

 vitreous humour is imbedded the crystalline lens, which comes 

 nearly up to the cornea in front, leaving a small cavity, however, 

 which contains the aqueous humour. Across this cavity, and divid- 

 ing it into an anterior and posterior chamber, hangs a vertical curtain- 

 like process of the choroid, called the iris. This is perforated in its 

 centre by a circular aperture called the pupil, which is capable of 

 being enlarged or diminished ; a condition permitted by the fluidity 

 of the aqueous humour.* The contraction of the pupil under the 

 stimulus of light, seems to be effected by a sphincter muscle sur- 

 rounding the orifice of the pupil, and put in action by the third pair 



* Todd and Bowman. 



