508 GLUCOSIDES, AND ORGANIC ACIDS. 



(3.) Glucosides containing Nitrogen. 



In addition to chondrin, the following glucosides containing nitrogen, when sub- 

 jected to hydrolytic processes, may combine with water, and form sugar and other 

 substances: 



Cerebrin (see Nervous Syslem) = C 57 'H.i- lo N 2 25 (Geoghegan). 



Protagon occurs in nerves, and contains phosphorus. 



CMtin, 2(C 1 5H 26 N 2 Oio), is a glucoside containing nitrogen, and occurs in the 

 cutaneous coverings of arthropoda, and also in their intestine and trachea?; it is 

 soluble in concentrated acids, e.g. , hydrochloric or nitric acid, but insoluble in 

 other reagents. According to Sandwick, chitin is an amm-derivative of a carbo- 

 hydrate with the general formula n(Ci2H2oOio). The hyalin of worms is closely 

 related to chitin. (Solanin, amygdalin, p. 49, and salicin, &c., are glucosides of 

 the vegetable kingdom.) 



(4.) Colouring Matters containing Nitrogen. 



Their constitution is unknown, and they occur only in animals. They are in 

 all probability derivatives of hemoglobin. They are (1) hcematin (p. 33) and 

 hcematoidin (p. 33). (2) Bile-pigments (p. 357). (3) Urine-pigments (except 

 ludican). (4) Melanin, C. 44'2, H3, N. 9'9, 0. 42'6, or the black pigment, which 

 occurs partly in epithelium (choroid, retina, iris, and in the deep layers of 

 epidermis in coloured races) and partly in connective-tissue corpuscles (Lamina 

 fusca of the choroid). 



II. Organic Acids free from Nitrogen. 



(1.) The fatty acids with the formula C n H 2 n-iO(OH) occur in the body 

 partly free and partly in combination. Free volatile fatty acids occur in decom- 

 posing cutaneous secretions (sweat). In combination, acetic acid and caproic acid 

 occur as amido-compounds in glycin ( =: amido-acetic acid), and leucin ( = amido- 

 caproic acid). More especially do they occur united with glycerine to form neutral 

 fats, from which the fatty acid is again set free by pancreatic digestion (p. 343). 



(2.) The acids of the acrylic acid series, with the formula C n H 2 n-30(HO), are 

 represented in the body by one acid, oleic acid, which in combination with 

 glycerine yields the neutral fat olein. 



251. Fats. 



Fats occur very abundantly in animals, but they also occur in all plants; in the 

 latter more especially in the seeds (nuts, almonds, cocoa nut, poppy), more rarely 

 in the pericarp (olive) or in the root. They are obtained by pressure, melting, or 

 by extracting them with ether or boiling alcohol. They contain much less O 

 than the carbohydrates, such as sugar and starch ; they give a greasy spot on 

 paper, and when shaken with colloid substances, such as albumin, they yield an 

 emulsion. When treated with superheated steam, or with certain ferments 

 (p. 507, c), they take up water and yield glycerine and fatty acids, and if the latter 

 be volatile they have a rancid odour. Treated with caustic alkalies they also 

 take up water, and are decomposed into glycerine and fatty acids; the fatty acid 

 unites with the alkali and forms a soap, while glycerine is set free. The soap- 

 solution dissolves fats. 



