624 MANUAL OF HISTOLOGY. 



But the nervous supply of the ciliary muscle is much more abundant. 

 Before their entrance into the latter the ciliary nerves have undergone 

 repeated subdivision. "Within the muscle they then form a ganglionic 

 annular plexus, the orbiculus gangliosus, in which, according to Krause 

 and //. Mutter's observations, ganglia are to be found. 



This plexus supplies twigs to the tensor choroidece, besides those 

 branches to the cornea already mentioned, p. 619, the iris, likewise, is 

 innervated by it. 



In the latter structure, twigs made up of medium-sized and fine dark- 

 edged fibres may be observed to course inwards from the whole periphery, 

 dividing as they go dichotomously. Converging while still in the peri- 

 pheral portion of the iris, they commence the formation of an extremely 

 complex nervous plexus, presenting rows of transversely anastomosing 

 twigs. From this, then, in the first place, recurrent branches are given 

 off towards the ciliary border, and then others directed more inwards. 

 These, again, form an irregular network of nerve fibres, about 0'0045- 

 0*0023 mm. in diameter, at first medullated, but later on losing their 

 medulla, which at many of the nodal points presents triangular expan- 

 sions. This plexus is then prolonged farther into an interlacement of the 

 finest species of filaments, only -0020-0 "00 18 mm. in diameter. It is 

 still an unsettled point whether this is a terminal network or not. 



While this nervous interlacement just- described belongs more parti- 

 cularly to the posterior wall of the iris, another is spread over its anterior 

 surface. Its broader elements are probably sensory. 



Finally, a plexus is found traversing the substance of the sphincter, 

 whose fibres are at first still medullated, but later on pale in appearance. 



(L) A very peculiar appearance is frequently to be found in the eyes of 

 some mammals. This proceeds from the tapctum lucidum, a colourless glittering spot 

 situated behind the choriocapillaris, between the most internal lamella of the choroid 

 containing its capillaries and the middle coat, in which the larger vessels are con- 

 tained. Among the ruminants, in the horse and elephant, &c., it consists of a deli- 

 cate and regularly undulating arrangement of connective-tissue bundles, which, from 

 the uniformity of their disposal in wavy lines, gives rise to a play of prismatic colours. 

 In the carnivora, and in seals also, it consists, on the contrary, of smooth, blunt- 

 angled nucleated cells. The cell-substance of these presents, according to Schultze, 

 a very remarkable structure. It consists, namely, of extremely fine-pointed double- 

 refracting crystals, arranged in groups within the cell. Each of these groups reflects 

 the light from a particular angle of incidence into another, producing colour. Over 

 the tapetum the epithelial cells are usually devoid of pigment molecules. 



311. 



The vascular system of the uvea (fig. 582) has been the subject of much 

 study both in earlier and later times. It has been lately investigated in 

 the most thorough manner by Leber. 



Owing to its extreme complication, it will require a very minutely 

 detailed description, during which, however, we shall have an opportunity 

 of referring again to the blood-vessels of the sclerotic and cornea already 

 noticed cursorily ( 309). 



The choroid, corpus ciliare, and iris, all receive their supply from the 

 ciliary arteries as they are called. Of the latter, those springing directly 

 from the ophthalmica are known as the posterior, those given off from the 

 arteries supplying the recti muscles as the anterior. 



The first break up again into the short and long posterior ciliary 

 vessels. 



1. The short posterior ciliary arteries (a, b), three or four in number, 



