ORGANS OF THE BODY. 623 



may be seen in the eye of any white rabbit), which, seated likewise 

 in the back part of the iris, radiate outwards through the tissue of the 

 latter. 



Not so, however, in man. 



The dilator appears, to be sure, here also to be a continuation of those 

 circular fasciculi of the sphincter. At first, still in the neighbourhood of 

 the latter, we may recognise separate arched interwoven bundles partly in 

 the circular muscle, partly behind it. After passing beyond the limits 

 of the circular muscle, however, these radiating bands unite to form a 

 completely continuous muscular plate, with regular layers of fibres forming 

 the posterior wall of the iris (b). At the ciliary border, at last, a circular 

 layer is formed by the interweaving of thicker and thinner bundles spring- 

 ing from the muscular plate (Iwanoff, Jeropheeff, Merkefy. The muscular 

 substance of the iris, however, is not connected with the ciliary muscle. 



The radiating fibres, then, just mentioned, constitute the dilatator 

 pupillce. 



It is a fact of great interest, that the muscular tissue of the iris, which 

 in man and the mammalia is composed of involuntary fibres, consists in 

 birds and scaly amphibia of transversely striated fibres. 



At its circumference the diaphragm of the eye receives another element 

 of tissue on its anterior surface, in the form of the so-called ligamentum 

 pectinatum iridis (Huek), ( 309). 



The fibres of the latter, originating in a transformation of the membrane 

 of Deseemet, as we have already seen, and covered at first with the normal 

 epithelium of this coat, commence near the border of the cornea as a fine 

 network, which is changed at the edge of the latter into a plexus of 

 stronger bands. These pass free across the peripheral portion of the an- 

 terior chamber, and so reach the anterior surface of the iris, in whose 

 tissue they are lost. The ligamentum pectinatum encloses a circular 

 space crossed, of course, by bands of fibres known as the " canal of 

 Fontana" (fig. 580, at h). 



In regard to the nature of these fibrous masses much difference of 

 opinion still exists. Erom their chemical reactions they would appear in 

 man to be allied to the elastic tissues, without, however, possessing the 

 same power of resistance to reagents, while among the mammalia they 

 manifest rather the constitution of connective -tissue. In birds, again, 

 they give the reactions of elastic tissue. 



In all probability there was originally a cellular network here. 

 On its posterior surface the iris is covered by a laminated coating of 

 pigmentary epithelium cells, and on its anterior aspect by a single layer 

 of colourless polyhedral and roundish elements. These latter are con- 

 tinued in single rows over the bands of the ligamentum pectinatum. 



Reserving the arrangement of the vessels of the uvea for the next 

 section, let us turn now to the nerves of the same. These, the nervi 

 ciiiares, are distributed principally to the iris and ciliary muscle. Their 

 number is from 14 to 18, and they spring for the greater part from the 

 lenticular ganglion. 



After piercing the sclera, these twigs advance through the external 

 layer of the vascular membrane of the eye to the ciliary muscle, giving 

 oil' in this course a few twigs to the choroid itself. The latter form super- 

 ficial and deep plexuses, whose fibres are found to be partly medullated, 

 partly pale, and among them small ganglia are formed by collections of 

 nerve cells (H. Muller, and C. Schweigger, Sdmich). 



