312 TUNICA VASCULOSA, BY PKOF. A. IWANOFF. 



a thickness of 0'25 of a millimeter. It is situated towards the 

 posterior surface of the iris, and is separated from the uvea 

 only by a thin layer of connective tissue, and some extremely 

 delicate muscular fasciculi belonging to the dilatator. 



The dilatator pupillse (fig. 365, 6) is developed from the fasci- 

 culi of the sphincter, of which it constitutes the unbroken 

 continuation. It commences in a series of arcuate interwoven 

 fasciculi that are partly situated in the interior of the sphincter, 

 and partly lie on its posterior surface, between it and the pigment 

 layer. These several fasciculi, after they have passed beyond the 

 boundary of the sphincter, unite to form a continuous muscular 

 lamina, extending over the whole posterior surface of the iris 

 (fig. 362, /), all the fibres of which lie parallel to one another, 

 and are all directed radially from the pupillary to the ciliary 

 border. 



Fig. 365. 



Fig. 365. Segment of the iris, seen from the surface, a, Sphincter ; 

 6, dilatator. 



At a distance of half a millimeter from its insertion, the 

 muscle breaks up into separate fasciculi, which are arranged in 

 two layers, one lying upon the other (fig. 366, a a). The fibres 

 of these fasciculi having arrived at the ciliary border, imme- 

 diately change their direction, curve round (6), and form by 

 their interlacement a thin muscular plexus (c), which runs cir- 

 cularly round the ciliary border of the iris (fig. 362, g). 



The literary history of the dilatator pupillse leads ns irresistibly to 

 the conclusion that, up to the time of Henle, the existence of this muscle 

 in Man was admitted rather in obedience to urgent physiological re- 

 quirements than from its having been actually demonstrated. That it 



