74 ANATOMY. 



artificially obtained from blood. The so-called Ticematln is a deep red, 

 almost black, substance, insoluble in water, alcohol, and ether, unless 

 these be acidulated ; it is readily soluble in alkalies. It contains a 

 very large quantity of iron, nearly 7 per cent., but its color does not 

 depend on the presence of this mineral. Haematin is now supposed to 

 be a product of decomposition of the true coloring substance of the 

 blood, which is named cruorin, and has a peculiar action on the solar 

 spectrum. The green and yellow coloring matters of the bile, biliver- 

 din and bilifulvin, seem to be somewhat allied to haematin. 



Black or brown pigment, as found in the eye and in the negro's 

 skin, is also peculiar in its composition. Both it and the haematin of 

 the blood, have their color discharged or bleached by chlorine. 



Casein exists in solution in fresh milk, and forms the substance of 

 the curd of milk, and therefore of cheese (caseum), in which, however, 

 it is mixed with more or less fatty matter or butter. Casein is very 

 like albumen in its general properties, but, when pui*e and by itself, it 

 will not coagulate by boiling. Its solubility appears to depend greatly 

 on the presence of some alkaline or earthy s*alt having an excess of 

 base. The pellicle which forms on the top of boiling milk is, in some 

 way, owing to the action of the atmospheric oxygen. Acids, even 

 when much diluted, readily coagulate it, hence the curdling of sour 

 milk, in which lactic acid is developed ; but its characteristic reaction 

 is rapidly to coagulate by the addition of rennet, or the prepared mu- 

 cous membrane of the fourth stomach of the calf: this is what takes 

 place in the manufacture of cheese. 



Pepsin is a very remarkable and potent albuminous substance, 

 which exists in the gastric juice or secretion from the mucous mem- 

 brane of the stomach of man and arrimals, and also in that membrane 

 itself. From this it may be extracted by cold water, in which it is 

 sparingly soluble. When slightly acidulated, especially by dilute 

 hydrochloric acid, a solution of pepsin in water, at a low heat, rapidly 

 brings about the solution of coagulated albumen, blood, and muscle- 

 fibrin, meat, fish, cheese, and many other aliments. It does not dis- 

 solve the epidermoid or horny tissues, or the yellow elastic tissue, or 

 pure fat. Its marvellous solvent property, or property of causing 

 albuminous substances to dissolve in weak acids is, as we shall here- 

 after see, the source of the digestive power of the gastric juice. The 

 peculiar properties of pepsin are destroyed by a boiling temperature, 

 and by alkalies. Pepsin is precipitated in whitish flocculi, and ren- 

 dered inert by alcohol: but the precipitate regains its solvent power 

 when washed or again soaked in large quantities of water. 



Salivin, or ptyalin, is a peculiar albuminoid substance found in the 

 saliva. It has the very remarkable property (not altogether peculiar 

 to it, however), of almost instantaneously converting starch into a kind 

 of gum called dextrin, the dextrin into a form of sugar, and this sugar 

 very soon into lactic acid. Pancreatin is the active principle of the 

 secretion of the pancreas, which seems to possess the power of emulsi- 

 fying fat. 



The extractive matters mentioned above, in the list of azotized sub- 

 stances of the body, are yet but imperfectly understood. Amongst 



