14 MANUAL OF HISTOLOGY. 



leucin, tyrosin, sarkin, kreatin, kreatinin, glycogen, grape and milk 

 sugar, inosit, and others besides. From a knowledge of these matters, it is 

 not possible at present to arrive at any definite conclusions in regard to the 

 constitution of the protein substances themselves ; we may, however, set 

 it down as being very complex. Owing to their great liability to under- 

 go decomposition, farther, the protein compounds appear to be peculi- 

 arly fitted to act in the economy as ferments, i.e., to effect a metamor- 

 phosis in other substances without at the same time acting through their 

 chemical affinities. We shall refer again to these properties in 12. 



Turning now to the peculiarities of the protein compounds and their 

 formative derivatives, especially important in histogenesis, we must bear 

 the following points in mind : 



1. The fact that the substances in question are not crystallizable, but 

 belong to the colloid group in the ordinary sense, as stated by Graham, 

 This seems to constitute them peculiarly well fitted to assume the specific 

 forms of tissue elements, and to preserve the same. 



2. Their readiness to imbibe water, and to swelL up in the latter 

 into gelatinous masses, seems to render them suited for the formation 

 of the watery, soft, and semi-solid matters of many tissues. Their capa- 

 city for gelatinisation appears greatest in slightly alkaline or acid water ; 

 in solutions of neutral salts less than in pure water. 



3. The remarkable readiness manifested among the protein bodies to 

 change from one modification to another, as well as from the liquid to the 

 gelatinous or coagulated state, and vice versa, renders them capable of 

 becoming deposited from the animal juices in the solid form, or, when 

 previously so laid down, of undergoing re-solution and easy transport to 

 other localities. 



4. While gelatinised protein substances admit of the passage through 

 them of crystallizable matters, they oppose the most determined resist- 

 ance to the diffusion of colloid materials. 



5. Albuminoous principles manifest a readiness to intermix with other 

 bodies, e.g., fats, and phosphate of lime, and to retain these with obstinacy. 

 They may therefore be regarded as the bearers of these substances. 



6. On the other hand, true albuminous substances appear unfavourably 

 constituted to form for any length of time unchanged the elements 

 of composition of a tissue, owing to their great unstableness. They seem 

 to impart to the textures into whose- structure they enter, a certain 

 aptness to undergo physical change, as may be strikingly seen in many 

 instances. This is not the case, however, with many of their derivatives, 

 whose liability to alteration appears to be far more limited as, for 

 instance, keratin, chondrigen, and elastic material. These being pecu- 

 liarly suited for permanent tissues, serve for the formation of indifferent 

 membranes, allowing the transudation of animal fluids, or including the 

 same. 



10. 

 Albumen. 



This most important of all the protein substances in the system, 

 coagulates between 55 and 75 C. from its solutions in the form of flakes, 

 and not spontaneously like fibrin. From very dilute solutions it can 

 only be separated by a much more elevated temperature. 



Like other protein matters it has two forms, a soluble and coagulated. 



