ENZYMES. 117 



with the white fibers of connective tissue, and yet they contain 

 chemically this new material. The answer to the problem is 

 probably that reticulin is not specially characteristic of reticular 

 fibers, but is present in all white connective-tissue fibers." 



Keratin. This substance is found in all horny tissues, such 

 as hair, nails, and epidermis. It is soluble in water at 150- 

 200 C., and in alkalies, but is unaffected by pepsin or trypsin, 

 and contains a large amount of sulphur. Its percentage-composition 

 in hair is as follows: C, 50.60; H, 6.36; N, 17.14; O, 20.85; 

 S, 5. In the skin the change of the protoplasm of the cells 

 into keratin takes place in two strata which are between the 

 Malpighian layer and the horny layer the stratum lucidum, next 

 to the horny layer, and the stratum granulosum, next to the 

 Malpighian. In the latter the cells contain eleidin, which is 

 regarded as an intermediate stage in the conversion of the pro- 

 toplasm into keratin. 



Neurokeratin. A modified form of keratin, neurokeratin, 

 occurs in the medullary sheath of nerves and in neuroglia. Its 

 percentage composition varies considerably, being in some portions 

 of the nervous system as low as 0.3, and in others as high as 2.9. 



Mucins. Inasmuch as mucin is not a single substance, but 

 consists rather of several varieties, differing in solubility in acid 

 and alkaline solutions, it is more correct to speak in the plural. 

 Mucin is an ingredient of mucus, the product of mucous glands, 

 and it exists also in the ground-substance of connective tissue. 

 Mucins give the characteristic viscidity to the fluids in which 

 they occur ; they are soluble in alkalies, and, when so dissolved, 

 can be precipitated by acetic acid. When they are treated with 

 superheated steam a carbohydrate called animal gum is split off, 

 the formula of which is C 6 H 10 O 5 . When this latter is treated 

 with a dilute mineral acid it is changed into a reducing but not 

 fermentable sugar, whose formula is C 6 H 12 O 6 . It is an interesting 

 fact that from other albuminoids, and also from proteids, carbo- 

 hydrates may be obtained. The percentage-composition of sub- 

 maxillary mucin is: C, 48.84 ; H, 6.80 ; N, 12.32; O, 31.20; 

 S, 0.84. 



Nuclein. This substance has been sufficiently discussed in 

 connection with the nucleoproteids (p. 111). 



ENZYMES. 



There are two varieties of ferments : (1) organized ferments, 

 of which yeast is an example, and (2) unorganized or soluble fer- 

 ments, of which pepsin is an example. It has been proposed to 

 limit the term ferment to the organized class, and to denominate, 

 the changes which its members cause in substances upon which 

 they act as fermentation, and to the soluble or unorganized class 



