14 PRACTICAL PHYSIOLOGY. [ll. 



(I.) Add tannic acid = copious white precipitate, insoluble in 

 excess. 



(m.) Add picric acid (saturated solution) = yellowish-white pre- 

 cipitate, which disappears on heating and reappears on cooling. 



(n.) It is precipitated by alcohol, and also by platinic chloride. 



2. II. Chondrin is obtained by the prolonged boiling of cartilage, 

 which largely consists of the substance " Chondrigen." 



Preparation. Costal cartilages freed of their perichondriurn and cut into 

 small pieces are boiled for several hours in water, when an opalescent fluid, 

 which gelatinises on cooling, is formed. 



(a.) Add acetic acid = a white precipitate, soluble in great excess. 



(>.) Dilute mineral acids = white precipitate, readily soluble in excess. 



(c.) It is not precipitated by acetic acid and potassic ferrocyanide. 



3. III. Mucin, see "Saliva." It is also found in the ground 

 substance of connective tissue and tendon. There are probably 

 several mucins. On heating with dilute H 2 S0 4 they yield a reduc- 

 ing sugar, and they are regarded as glucosides, compounds of a 

 proteid (globulin 1) with animal gum. 



(a.) .They make fluids viscid and slimy. 



(6.) Cut a tendon into pieces and place it for 3 days in lime-water. The 

 lime-water dissolves the mucin. Add acetic acid = precipitate of mucin. 



4. IV. Elastin occurs in elastic tissue, ligamentum nuchae, and 

 ligamenta subflava, <fcc. 



PitEPAitATiON*. Boil the fresh ligamentum nuchse of an ox successively in 

 alcohol, ether (to remove the fats), water (to remove the gelatin), and finally 

 in acids and alkalies. This substance must be previously prepared so that 

 the student can test its reactions. 



(a.) It is insoluble in water, but is soluble in strong caustic soda. 



(Is. ) It gives the xanthoproteic tests. 



(c.) It is precipitated from a solution by tannic acid. 



5. V. Keratin occurs in epithelial structures, e.g., surface layers 

 of the epidermis, hairs, horn, hoof, and nails. It is characterised 

 by the large percentage of sulphur it contains ; part of the latter 

 is loosely combined. It is very insoluble and resists putrefaction 

 for a long time. A closely-allied body, Neuro-Keratin, is found in 

 nerve fibres and the central nervous system. 



(a.) Burn a paring of horn, and note the characteristic smell. 

 (b.) Heat a paring of nail or horn with strong caustic soda and 

 lead acetate = black or brown colouration, due to lead sulphide. 

 (c.) Test for the presence of sulphur. (Lesson I. 2.) 



