144 LECTURE VII. 



bers of this group, at least to the extent of recognizing the nature of the 

 individual decomposition products. 



Hammarsten * has isolated a peculiar substance from the albuminous 

 gland of the Roman snail (Helix pomatia), which is possibly a protein, 

 difficult to classify, or, more probably, a proteid. It contains a Isevo- 

 rotary carbohydrate, which yields a dextro-rotary substance on boiling 

 with acids. This material also contains phosphorus. It does not belong 

 to the nucleo-proteids, because it possesses no xanthin bases. To this 

 group, called " phospho-glucoproteids," is assigned the substance ichtulin, 

 which has been obtained from fish eggs. 



The proteins and proteids already mentioned do not, by any means, 

 exhaust the list. We have mentioned only those which have been fairly 

 well characterized, and which are starting points for further investiga- 

 tion. The albuminous substances, and especially some of the albumin- 

 oids, isolated from plant seeds and the animal fluids, are undoubtedly best 

 known. We frankly admit that our knowledge is very vague concern- 

 ing the albumins which participate in the construction of the various 

 organs. It is very probable that the number of proteins and proteids 

 occurring in the tissues is extremely large as is indicated by the descrip- 

 tion of the various tissue-albumins. On the other hand, we must remem- 

 ber that when proteins are attacked but slightly their properties are 

 changed completely so that they may be mistaken for new kinds of pro- 

 tein. It is also probable that the tissues, and especially the cell proteins, 

 are continually undergoing changes. It is still undecided whether the 

 albuminous constituents of the tissues are to be considered as stable sub- 

 stances, to a certain degree, or as undergoing continual decomposition 

 and reconstruction. From this question arises the extremely important 

 problem of the whole subject of protein metabolism, about which we are 

 still in the dark. 



We have, so far, neglected to mention a group of nitrogenous com- 

 pounds, which are closely related to the proteins. These are the 

 melanins, which are very widely distributed in the animal kingdom. 

 They are found in the very large number of pigments occurring in the 

 hair, feathers, choroidea of the eyes, skin, etc. Their presence in tumors 

 is very interesting; and their unusually large occurrence in the melano- 

 sarkoma of horses especially of white horses has drawn much atten- 

 tion to them. In the muscles of these animals (for instance, the glutaei), 

 there are often embedded very large tumors, which appear, according, to 

 age, as very black, firm masses, or like a cyst, containing an inky, finely 

 granulated fluid. It is certainly significant that these masses of pigment 

 are obtained in animals whose hair has no pigments. The pure white 



1 O. Hammarsten: loc. cit. Pfliiger's Ann. 36, 373 (1883). 



