LIVING SUBSTANCE 109 



of the more delicate parts of the organism. A detailed examination 

 of the known reactions which the numerous albuminoid bodies 

 present would lead too far and be superfluous for our purpose.^ It 

 is sufficient here to cite some of the most important members of 

 the group, all of which occur in the solid undissolved state. Such 

 are keratin, which is contained in most horny structures produced 

 by the epidermis-cells of the skin (horns, hoofs, hairs, feathers, and 

 nails) ; elastin, which composes the elastic fibres of the cells of 

 connective tissue and the strong yellow ligamentum nuchce ; collagen, 

 which composes the organic ground-substance of bones and car- 

 tilage, and in boiling passes over by hydrolysis into gelatine ; 

 spongin, the skeletal substance of bath-sponges ; conchiolin, the 

 organic substance of the shells of mussels and snails ; cornein, that 

 of the skeletons of corals ; and many other substances that form 

 skeletons, especially in invertebrates. 



With the albuminoids also is classed a series of highly complex 

 nitrogenous bodies which at least are derivatives of proteids and 

 possess the greatest importance in the life of the organism, 

 especially in digestion. These are the lonformed ferments or 

 enzymes, such as pepsin, produced by the gland-cells of the 

 stomach ; ptyalin, by the cells of the pancreas and the salivary 

 glands ; trypsin, produced likewise by the pancreatic cells ; and 

 many others. The properties of these bodies and their roles in 

 the life of the cell will be considered more fully elsewhere. 



There appear in living substance, as constant accompaniments of 

 proteids, certain decomposition-products of them which can be 

 divided into two groups — the nitrogenous and the non-nitrogenous 

 cleavage-proditcts. The former constitute a series of substances 

 whose chemical constitution is more exactly known. They are 

 the products of retrogressive proteid-metamorphosis. Among 

 them belong especially the substances excreted in considerable 

 quantity by the higher animals in the urine. Among them tcrea, 

 {^13.^,f!i0, holds the first rank ; it is the richest in nitrogen of all 

 the nitrogenous end-products of proteid-decomposition, and its 

 artificial synthesis was accomplished by Wohler in the year 1828. 

 Next to urea, ttric acid, CgH^N^Og, contains the most nitrogen ; 

 next to uric acid come in order hippuric acid, creatin, which 

 originates in the muscles by the decomposition of proteid, and 

 creatinin. Further, the nuclein bases, xanthin, hypoxanthin or 

 sarJcin, adenin and giianin are met with as end-products of the 

 decomposition of nucleins in the living organism. Of these, 

 especially the last in combination with calcium occurs very 

 frequently in the skin-cells of Amphibia and of fishes, in the 

 latter of which its crystals produce the well-known silvery sheen. 

 Finally, there is one more group of nitrogenous bodies, the lecithins, 



' A review of the subject and the bibliography of it may be found in Neu- 

 meister : Lehrhuch der physiologischen Ohemie. 2nd edition, Jena, 1897. 



