392 SENSITIVE SYSTEM. 



lens of a bullock's eye muscular fibres with intersecting tendons, 

 to which he ascribed the power of augmenting its sphericity : 

 indeed, Mr. Hunter had a notion that it possessed a power of 

 varying its figure, and had made some allusions to a fibrous 

 structure. These suppositions, however, have arisen from exa- 

 minations of the lens in an opaque and altered condition : so 

 long as it remains pellucid, though its substance does come away 

 in spherical laminae, there appear no grounds for ascribing a 

 fibrous composition to it. Notwithstanding the physiological 

 ingenuity, therefore, of these hypotheses, all that we are absolutely 

 warranted in offering upon the subject of its composition is rather 

 the result of chemical inquiry than any anatomical knowledge 

 we possess of its organization. In the living animal, it is as 

 transparent as the clearest crystal : after death it gradually 

 loses its pellucidity, and more rapidly when immersed in water, 

 by which it becomes converted into an opaque pulpy mass. 

 Even if it is simply squeezed between the fingers, it turns 

 opaque : a change rationally explained by saying, that we have 

 destroyed its organization and extravasated its fluids. Acids and 

 alcohol take the same effect that boiling water does upon it : they 

 convert it into an opaque, white, and more solidified body, such 

 as we see it in the head of a boiled fish. It is found to be chemi- 

 cally composed of albumen, gelatine, and water ; and this seems 

 to be the sum and substance of our present knowledge of its 

 conformation. Neither bloodvessels nor nerves have ever been 

 traced into it ; nor are we certain that it is organized at all, 

 unless we receive as proofs the ordinarily assigned tests of organ- 

 ization — the phenomena of growth and morbid changes. 



Vitreous Humour. 



The vitreous humour fills the posteiior concavity of the 

 globe — the dark chamber — occupyingnearly four-fifths of its whole 

 interior. It is of the consistence of thin jelly, and from its pellu- 

 cidity and glassy appearance has got its name. It is moulded to 

 the form of the cavity containing it, and lies in contact every- 

 where with the retina, though there is no connexion whatever 

 between them. Beyond the boundary line of the retina it is 

 covered by the corpus ciliare ; and in front is shaped into the 

 form of a hollow bed for the reception of the lens. Let this 

 tremulous mass be pricked or otherwise wounded, and a limpid 

 fluid drops from it, very like water, leaving behind a thick and 

 gelatinous part which at length becomes resolved into a mem- 

 branous substance : this membranous residue is said to be dis- 

 posed in small cells, varying in figure and size, and is called the 

 tunica vitrea vol hj/aluidea. Oj^posite to the corpus ciliare, this 



