254 PROTOPLASM 



arrangement, we find that the Hst is a long one; it includes 

 starch, gelatin, chitin, rubber, silk, hair, keratin, sinew, muscle, 

 nerve, and brain. It is but a step from these to protoplasm — 

 indeed, muscle, nerve, and brain are protoplasm. 



Not all organic substances that yield spectrograms do so with 

 equal clarity. Herzog finds that, in sharpness, the X ray 

 interference of hair is but fair, sinew better, and silk still better ; 

 muscle and nerve are less clear yet indicate crystalline character. 

 None of the pictures of these substances can be analyzed with 

 the accuracy of those of the best crystals, yet all show fibrous 

 structure with crystalline symmetry, only less perfect, as is to be 

 expected of colloidal matter. 



The case of chitin is particularly interesting. Chitin (a poly- 

 acetyglucosamine of both plant and animal origin) is said to be 

 the most chemically resistant skeletal material known. Its 

 resemblance to cellulose is expressed by the formula 



^^ — 0/ 



where A^ equals OH in cellulose, NHCOCH3 in chitin, and NH2 

 in chitosan. Herzog found that the chitin plate of the crab 

 gives an X ray picture identical with that of sinew in spite of a 

 different chemical constitution. This is true because the 

 chitinous shell in Crustacea is a direct transformation of sinew 

 from which it arose by a slow change. It is completely con- 

 verted chemically but unchanged structurally. Substances 

 so transformed Freundlich calls permutoids. 



X ray studies on nerve have been carried out by G. L. Clark, 

 F. 0. Schmitt, and J. N. Mrgudich, They find that the molecular 

 configuration producing the diagram in nerve is probably due to 

 a system of oriented protein primary valence chains lying 

 parallel to the fiber axis. The equatorial spacing of 17 A. U. 

 corresponds to the direction of the side chains, and the meridional 

 spacing may correspond to the reflection from double amino- 

 acids residues along the fiber axis. The radiation required 

 to produce these patterns has no appreciable effect on the 

 irritability of the nerves. Nerve, then, is essentially a single 

 system of partially oriented primary valence chains probably 

 admixed with unoriented intermicellar protein chains. 



