! .\.\.\II.\ATIO.\ HI' Till' LYEHM.L 



the ciliary muscle. ( >n this surface numerous white Derve-COrda are visible running forwards. 

 I ih cry.' thai 1 1 ic- iris, I ! .-diary pi I tC., and the chonoid urc all di linen I part - n| I he 



game oeular tunic mere local modi ! it. Similarly the sclerotic and con n to 



blend together to form one outer rout. 



An eyeball slioulil now lie placed lor half all hour in a free/.inj* iniMuri' of crushed i< . 



salt. It will thus IM-COI pule liaril, and should at onee ! divided into two pail.- liy cullint; it 



antero-poateriorly through the cent re of I iie i OJ aea and the optic nerve. We thus gain another 



view of the relations of parts, the position of the lens between the aqueoiu and vitreoUB chanil 



etc. < In removing the len -., vitreous, and retina, and lirushini; oil its pijmenl. the liylit mark- 

 ing com ['"inline; In the ehorioidal veins (venje vorticosas) should lie noted, and their d 

 hution studied. I -ually lour vorticc., or fountain-like markings are found in the whole ehorioid. 

 their ]>oint.s of junction situated ai approximately equal ihsiaiices from one another at aliout the 

 line where the po-terior and middle thinls of the s;lolie meet. These sections .should lie kept lor 

 reference while following the further description of the ocular tunics. 



1. The outer, fibrous coat of the eye is formed by the selenitic and comca. 

 which pass into one another at the sdcra! suleus. It consists throughout mainly 

 of fine connective-tissue fibres, arranged in interlacing bundles, with small lymph- 

 xpaces at intervals bet ween them. The naked-eye appearance of the two divisions 

 of this fibrous coat is, however, quite different, the cornea being transparent, while 

 the sclerotic is white and opaque. 



The sclerotic encloses the posterior five-sixths or so of the eyeball, but there is 

 a hole in it at the entrance of the optic nerve (foramen sclerae), only partially 

 bridged across by fibres from its inner layers forming the lamina cribrosa. The 

 fibre-bundles composing the sclerotic are arranged more irregularly than in the 

 cornea, and run mainly in two directions, viz., from before backwards, and circularly; 

 the circular fibres are particularly well developed just behind the suleus. It is 

 thickest (about 1 mm.) posteriorly, where it is strengthened chiefly by the outer 



Fio. 726. EQUATORIAL SECTION OP EYEBALL: ANTERIOR SEGMENT VIEWED FROM 



BEHIND. (After Mcrkel.) 



PlIC/C CILIARE3 



PUPIL 

 CILIARY PROCESSES 



sheath of the optic nerve, and partly also by the tissue surrounding the ciliary 

 vessels and nerves. It becomes gradually thinner as it passes forwards, up to 

 the line of insertion of the recti muscles, in front of which line it is again reinforced 

 by their tendinous fibres becoming incorporated with it. In children the sclerotic 

 is often so thin as to allow the underlying ehorioidal pigment to show through, 

 appearing then of a bluish white. In the aged, again, it is sometimes yellowish. 

 It always contains a few pigment cells, but these are in the deep layer (lamina 

 fusca) . and only become visible externally where the sclerotic is pierced by vessels and 

 nerves going to the ehorioid. It is itself almost non-vascular, but quite at its ante- 

 rior end a large venous sinus (canal of Schlemm) runs in its deeper layers circu- 

 larly around the cornea. Just. in front of this sinus, at the corneal limbus, the 

 sclerotic merges into the cornea, its deep layers changing first, and finally the SUJXT- 

 licial ones. 



The cornea is, thickest at its periphery, and becomes gradually thinner towards 

 its centre; the curvature of its posterior is consequently greater than that of its 

 anterior surface, but even the latter is more curved than the surface of the sclerotic. 

 In the cornea proper, fibre-bundles are arranged so as to form a series of super- 

 posed lamella?, each of which is connected here and there to the adjacent ones by 

 fibres passing from one to the other, so that they can only be torn apart with diffi- 

 culty. The corneal lymph-spaces communicate with one another by very fine 

 canals, and thus not only is a thorough lymph-circulation provided for, but the 

 protoplasm with which these spaces are partially occupied may be also regarded 

 as continuous throughout. It contains no blood-vessi Is. with the exception of 

 a rich plexus at its extreme' periphery, on which its nutrition is ultimately dependent. 

 65 



