356 



NORMAL HISTOLOGY. 



closely felted. It is, however, not a true membrane, but only a condensa- 

 tion of the vitreous fibres. The vitreous is attached firmly to the retina at 

 the nerve-entrance and at the ora serrata, between these points the hyaloid 

 being indistinct. As the vitreous leaves the retina, the boundary layer be- 

 comes thicker, in some cases to become thin again or absent in the region of 

 the patellar fossa. The adult vitreous ordinarily contains no cells, but some 

 small round ones are occasionally seen near the surface, beneath or on the 

 hyaloid membrane. They are amoeboid, often contain vacuoles and are 

 modified leucocytes. In addition a few branched connective tissue cells may 

 be present, as the remains of the mesodermic elements gaining entrance 

 along with the blood-vessels during fcetal life. The central part of the vit- 

 reous is occupied by a channel, the hyaloid canal, also known as the canal 

 of Stilling or the canal of Cloquet, which is about "I mm. wide and extends 

 from the optic entrance towards the posterior pole of the lens. During fcetal 

 life this canal lodges the arteria hyaloidea, the continuation of the central 

 artery of the retina. 



THE SUSPENSORY APPARATUS OF THE LENS. 



The lens is held in position by a series of delicate bands, which pass 

 from the vicinity of the ora serrata over the ciliary processes to be attached 

 to the periphery of the lens. Near the ora serrata, the hyaloid membrane 



Cornea 



Canal of Schlem 



Sclera 



Ciliary Suspensory 

 processes ligament of lens 



FIG. 408. Meridional section of ciliary region, showing ciliary processes and suspensory ligament of 



lens. X 20. 



splits into two layers; the inner of these adheres to the vitreous as its in- 

 vestment, while the thicker outer layer passes over the ciliary region and 

 bridges over the space between the processes and the lens. The fibres 

 extending between the processes and the lens constitute the suspensory 

 ligament, or Zonule of Zinn, a structure of great importance, not only for 

 the support of the lens, but also in assisting the ciliary muscle in effecting the 

 changes in the curvature of the lens incident to accommodation. The zonule 

 includes a complex of fibres, some of which are firmly attached to the ele-. 

 vations of the ciliary processes, while others pass from the depressions be- 

 tween and along the sides of the processes. They all proceed across the 



