336 SOLID PARTS OF ANIMALS. 



Externally it is covered by cellular tissue and fatty matter, but 

 its inner side is smooth and shining, with a pearly or almost sil- 

 very lustre. When boiled in water it is converted into gelatin, 

 and is therefore similar in its nature to the skin. If we cut it 

 into small pieces, and digest it in water, the liquid assumes a yel- 

 low colour, and holds in solution an extractive matter similar 

 to that obtained from muscle. When the sclerotic coat thus treat- 

 ed is boiled in water, the jelly obtained is colourless, but contains 

 mixed with it numerous fragments of blood-vessels. Muriatic 

 acid causes the sclerotic coat to contract, and dissolves it rapidly 

 when raised to a boiling temperature. No gas escapes during 

 the solution. Acetic acid also causes it to contract, deepens the co- 

 lour, and when boiled on it renders it semitransparent, though 

 it does not dissolve it ; but if we add water, and boil it, a solu- 

 tion takes place which gelatinizes on cooling. Potash and prus- 

 siate of potash do not precipitate this solution. Hence it folio w s 

 that the sclerotic coat contains no fibrin.* The tendons of the 

 muscles of the eye being spread upon the sclerotic coat must in 

 some measure modify its chemical properties. 



3. The cornea is a transparent membrane, which occupies the 

 fore-part of the eye, and is inserted into the sclerotic somewhat as 

 a watch-glass into a watch. It adheres firmly to the sclerotic, 

 so that long maceration is necessary to separate them. It is 

 composed of thin lamellae, and in the living eye is quite transpa- 

 rent ; but after death it acquires a grey colour and a semitrans- 

 parency, and when plunged into water it becomes opaque and 

 white like coagulated albumen. When boiled in water it swells 

 very much, then softens, and is gradually dissolved. The solu- 

 tion on cooling coagulates into a jelly. It is soluble in muriatic 

 acid. In acetic acid it swells without becoming transparent. 

 When we digest it in acetic acid the liquid acquires the property 

 of being precipitated by prussiate of potash. This shows that 

 the cornea besides gelatin contains also albumen. 



4. The choroid coat lies immediately within the sclerotic, to 

 which it is attached by cellular tissue. It is soft and dark-colour- 

 ed, loose in its texture, and consists of two lamellae, which are 

 separable behind, though connected before. It is essentially vas- 

 cular in its structure, being composed of minute arteries and 

 veins united by cellular tissue. The veins, for the most part, 



* Berzelius, Traite de Chimie, vii. 449. 



