1166 THE CILIARY MUSCLE. [BOOK m. 



channel, has direct connections with neighbouring veins, and under 

 certain circumstances becomes filled with blood. 



718. The Ciliary Muscle. This occupies the space behind the 

 ligamentum pectinatum between the sclerotic on the outside, and 

 the ciliary processes and root or attachment of the iris on the 

 inside. Just outside the ligamentum pectinatum the circular, 

 equatorial bundles of the sclerotic are well developed, and just to 

 the inside of the canal of Schlemm lies a small mass of denser 

 tissue (Fig. 140 #); these structures serve as a point of attachment 

 and sort of tendon for a number of small bundles of plain muscular 

 fibres which run thence in a radiate meridional direction back- 

 wards. These interlace with other similar bundles having a 

 similar direction and thus constitute all round the ring of the 

 ciliary region a flat radiate meridional muscle, the longitudinal 

 or meridional ciliary muscle (Fig. 140 l.c.m.), ending eventually in 

 the front part of the choroid. 



Between this and the ciliary processes and iris, is seen, at least 

 in some eyes, a number of bundles of plain muscular fibres arranged 

 circularly, equatorially, both ends of each bundle being attached to 

 the loose connective-tissue forming the outer part of the ciliary 

 processes and iris. These bundles constitute the circular ciliary 

 muscle, or muscle of Miiller (Fig. 140, c.c.m.). This is as a rule 

 less conspicuous than the longitudinal muscle and in some animals 

 is absent. Moreover the two muscles, longitudinal and circular, 

 are not sharply defined from each other, many of the inner bundles 

 of the longitudinal muscle curving round so as to take an 

 equatorial direction (Fig. 140, y) ; and it is perhaps best to 

 speak of the whole mass as forming one muscle, "the ciliary 

 muscle ". 



719. The Lens. This, as we have said, is of epithelial origin, 

 and at an early stage presents itself as a sac with a wall consisting 

 of a single layer of epithelium, supported by a scanty amount of 

 mesoblast. The cavity, often from the first potential rather than 

 actual, is soon obliterated by the elongation and growth of the cells 

 of the hind half, and the sac is thus transformed into a solid more 

 or less ellipsoidal mass in which one can distinguish an anterior 

 part formed of short cubical and a posterior part formed of 

 elongated epithelial cells; in a section the short anterior cells 

 may at the edge of the mass be traced gradually lengthening 

 into the long posterior cells. 



The anterior cells remain throughout life as a single layer of 

 cubical cells, often spoken of as the anterior epithelium of the lens. 

 The whole of the lens except this thin layer is developed out of the 

 posterior cells. These grow into long transparent flattened prismatic 

 bands or fibres with a hexagonal transverse section, and with serrated 

 edges by which each fibre locks into its neighbours. In the course 

 of development the fibres assume a special disposition, being ar- 

 ranged in concentric lamellae like the coats of an onion, the 'fibres' 



