CILIARY NERVES. 417 



These veins as well as the long ciliary, discharge finally at the 

 back part of the eye into the ophthalmic veins or sinuses.* 

 The arteries that run to the ball are very flexuous, and 

 anastomosing, so that the impetus of the heart may be over- 

 come, and the retina undisturbed by the pulsation of any of 

 the vessels. The great vascularity of the choroid appears to 

 be for the purpose of secreting the pigment in abundance, so 

 as to absorb the rays of light that enter from the pupil, and cut 

 off those that penetrate on the side of the semi-opaque sclerotic 

 coat. 



The nerves which appear on the choroides, come from the 

 ophthalmic ganglion. They pass very obliquely through the 

 sclerotica, and run forwards, having a very flat appearance in 

 proportion to their size : five or six of them are of this descrip- 

 tion ; some others are very small. There are some filaments 

 which come from the nasal nerve, without passing through the 

 ganglion. 



Fig. 190.f The internal surface of this coat is 



covered with a black paste, denomi- 

 nated Pigmentum Nigrum. This 

 appears to be spread most thickly on 

 the anterior part of it, and thinner be- 

 hind, near the optic nerve ; it is said 

 to be thinner and less dark in old per- 

 sons. It seems to resemble the matter 

 of the rete mucosum of negroes, when 

 that is softened by putrefaction. It is 

 asserted that the color of this pigment has never been changed by 

 the ordinary chemical agents, or by moderate heat. When it is 

 washed away, the internal surface of the choroides appears to 

 be villous. 



There is also a portion of this substance on the external sur- 

 face of the choroides. It is said that in very recent subjects, 

 this matter appears to be inherent in the structure of the inter- 



* Zinn, De vasis Subtilior Oculi, 1790. 



f A representation of the nerves of the choroid coat and the iris. 



