FORMATION OF THE TISSUES CORNEA AND CRYSTALLINE. 47!) 



commence, vessels are prolonged into it ; and a distinct line of demarkation is 

 seen betwixt the vascular portion, which is to be converted into Bone, and 

 the non-vascular part, which is to remain as Cartilage. At this period, the 

 Articular Cartilage is nourished by a plexus of vessels spread over its free 

 surface, beneath its synovial membrane, as well as by the vessels with which 

 it comes into Contact at its attached extremity. Towards the period of birth, 

 however, the sub-synovial vessels gradually recede from the surface of the 

 articular cartilage ; and at adult age they have entirely left it, though they 

 still form a band which surrounds its margin. At the same time, the line of 

 demarkation between its attached surface and the subjacent bone becomes more 

 distinct, by the formation of a thin lamella at the surface of the latter; which 

 covers the subjacent cancelli, and is of extremely solid texture, not containing 

 any perceptible foramina through which vessels could pass. The vessels of 

 the cancelli, however, are very large ; and have the same dilated or varicose 

 character with those which, at an earlier period, cover the surface of the car- 

 tilage. It appears that the Articular Cartilage is gradually becoming ossified 

 through the whole of life ; in old age it is sometimes almost completely con- 

 verted into Bone. From Mr. Toynbee's researches it further appears, that 

 the Fibrous cartilages are somewhat vascular ; but that the vessels do not ex- 

 tend to the cellular portions, where such exist. No vessels can be traced 

 (according to Mr. T.) into the substance of the true Cornea ; which, contrary 

 to the statement of Muller, is a cellular rather than a fibrous cartilage. The 

 cells are not so numerous as are those of the articular cartilages ; and they are 

 surrounded by a plexus of bright fibres, laxly connected together so as to re- 

 semble areolar tissue. Two sets of vessels, a superficial and a deep-seated, 

 surround the margin of the cornea. The arteries of the former are prolonged 

 for a short distance upon the Conjunctival membrane, which forms the outer 

 lamina of the cornea ; but they terminate in veins at from 5 to a line from 

 its margin. The deep-seated vessels belong to the Cornea proper ; but they 

 do not enter it, the arteries terminating in veins just where the tissue of the 

 Sclerotic becomes continuous with that of the Cornea. In diseased conditions 

 of the Cornea (as of the articular cartilages), both sets of vessels extend them- 

 selves through it ; the superficial not unfrequently form a dark band of con- 

 siderable breadth round its margin ; whilst the deep-seated are prolonged into 

 its entire substance. Notwithstanding the absence of vessels in the healthy 

 condition of this structure, incised wounds commonly heal very readily, as is 

 well seen after the operation of extraction of Cataract ; but the foregoing de- 

 tails make evident the importance of not carrying the incision further round 

 than is necessary; since the corneal tissue should not be cut off from the sup- 

 ply of nourishment afforded by the vessels in its immediate proximity. 



627. In connection with the cornea, it is natural to allude to the Crystalline 

 lens and Vitreous humour, which have a structure essentially the same. The 

 structure of the crystalline lens has long been known to be fibrous ; and Sir 

 D. Brewster has shown, by the aid of polarized light, the very beautiful 

 manner in which the fibres are arranged.* They are united into laminae, by 

 means of numerous teeth or sinuosities at their edges, which lock into one 

 another. That these fibres originate in cells, has been clearly ascertained ; but 

 the nature of the metamorphosis has been differently stated by two eminent 

 observers, Schwann and Barry. By the former, the. fibres are considered to be 

 prolonged cells : whilst the latter regards them as rather formed upon the plan 

 of the tubes of muscular fibre ( 642), several cells coalescing into one ; in this 

 he is supported by Mr. Toynbee, who states that he has frequently seen the 

 fibres, towards the margin of the lens, made up of such cells. After it n fully 



* Philosophical Transactions, 1833. 



