SENSITIVE SYSTEM. 387 



everywhere a black surface ; internally, only the anterior parts 

 are black : the posterior concave part, about one half of the 

 entire internal superficies, being of a brilliant variegated green 

 tint. The black part is called the nig/Km pigmeittum : the 

 green, the tapetiim luctdum. The basis of the structure of the 

 choroides consists in vascularity : arteries and veins in count- 

 less numbers enter into its composition, dispersed through an 

 intertexture of fine cellular web. Though this fact has been 

 ascertained, however, the arrangement of the vessels is a question 

 that does not seem to this day to have been satisfactorily de- 

 termined. It appears certain that there are two strata or sets of 

 vessels ; but, some say an external one of veins, which they call 

 the vena vorticosa, and an internal one of arteries, to which has 

 been given the name of tunica Rui/schiana ; while others place 

 the veins upon the inside. — " Les arteres," says Girard, " dis- 

 tribuees a la surface externe forment une couche premiere, unie 

 a une lame interieure, dans laquelle se rendent les veines." It 

 is, perhaps, sufficient for us to know, that this tunic is (with least 

 difficulty in the situation of the tapetum) separable into two 

 layers ; that these layers consist almost entirely of bloodvessels, 

 wliich cross and intercross one another so as to form a vascular 

 network very difficult if not impossible to unravel ; and that its 

 arteries come from the ciliary, and its veins pour their blood 

 into the ophtliahnic vein. In order to examine the structure of 

 the choroides it should be freed from the colouring matter ad- 

 hering to it ; and this cannot be done without suftering it to 

 remain some days in water, which is very apt to impair its 

 texture. 



Pigment. — This adventitious substance is spread over both of 

 the choroid surfaces ,• but the inner layer is thicker and more 

 consistent and determinable in its limits and disposition than the 

 outer. The outer, the entirely black one— the veritable nigrum 

 pigmentum — begins just behind the ciliary circle, and appears to 

 be uniformly continued over the whole of the posterior hemi- 

 sphere ; the inner one, partly black and partly green, lines the 

 dark chamber, fills up the interstices between the ciliary pro- 

 cesses, and coats the posterior surface of the iris. The coloured 

 part, called the tapetum or carpet, or from its brightness the 

 tapetum lucidum, is not found in the eyes either of men, birds, 

 or fishes, and in quadrupeds it is seen of different colours : in 

 tlie horse, it is pea-green ; in the cat, it is yellow ; in the stag, 

 blue. In all, however, its relative situation is the same : it makes 

 a circular sweep to a certain extent around the entrance of the 

 optic nerve ; in the horse, it spreads wider superiorly than in- 

 feriorly, occupying the posterior half of the dark chamber — that 



