90 PHYSIOLOGICAL, CHEMISTRY. 



and presents for the most part similar chemical reactions. It differs 

 from gelatine in being precipitated from its watery solution by both 

 acetic acid and lead subacetate. It rotates the plane of polarization to 

 the left 213.5. 



4, Elastine. 



The fibres of all the yellow elastic tissues, as in the middle coat of 

 the larger arteries, the elastic ligaments of the spinal column, and the 

 ligamentum nucha3, consist mainly of a homogeneous substance dis- 

 tinguished by its refractory nature toward chemical reagents. It is 

 obtained by boiling the elastic tissues successively with alcohol, ether, 

 water, acetic acid, dilute soda solution, and dilute hydrochloric acid. 

 The elastine, thus purified from other ingredients, is not itself soluble 

 in either of the above liquids. It is not converted into gelatine even 

 by long boiling ; and it is dissolved, but at the same time decomposed, 

 only by the concentrated acids and alkalies. The slender elastic fibres 

 mingled with connective tissue, and the sarcolemma of the striped 

 muscular fibres, are probably composed of the same substance. Elas- 

 tine contains no sulphur. 



5. Keratine. 



This is the exceedingly resisting and indestructible substance of the 

 hair, nails, epidermis, feathers, and all horny tissues. It is unaffected 

 by boiling with alcohol, ether, water, or the dilute acids. By continu- 

 ous boiling in a Papin's digester at 150 C. it is liquefied and partly 

 decomposed. It is distinguished from the preceding substance by con- 

 taining sulphur as an ingredient ; and when decomposed by boiling 

 under pressure or with concentrated alkalies, it gives rise to hydrogen 

 sulphide vapors. 



Source, Changes, and Destination of the Albumenoid Substances. 

 The source of albumenoid substances in the animal body is in the 

 food. Herbivorous animals take them ready formed in the juices and 

 parenchyma of plants, and the carnivora are supplied with them in 

 still greater quantity in the animal tissues. Man obtains them from 

 "both sources, and all nutritious articles of food contain them in greater 

 or less abundance. According to the estimates of Payen, which cor- 

 respond very closely with our own observations, an adult man requires 

 a daily supply of about 130 grammes of albumenoid matter to provide 

 for the wants of the system ; and this quantity is actually contained in 

 the food consumed. 



But although albumenoid matter is thus abundantly supplied to the 

 system from without, yet the particular substances characteristic of the 

 various tissues and fluids are formed within the body, by transformation 

 of those introduced with the food. None of the albumenoids contained 

 in the food of an herbivorous animal are precisely identical with those 

 of his own body. All the tissues and juices of the embryo chick are 

 formed from the albumen of the egg ; and the nourishment of all the 

 organs of the infant is provided at the expense of caseine, the albumen- 



