122 THE CHEMISTRY OF THE TISSUES AND ORGANS. 



May's method l of preparing f uscin is to boil retime in alcohol, then in 

 ether, lastly in water. The residue is then subjected to try p tic digestion. 

 Three things remain undigested ; of these iiuclein is got rid of by tritura- 

 tion with alkali; the second, neurokeratin, must be picked out with 

 forceps ; the third is the pigment. 



Fuscin is slowly bleached in the air ; it dissolves by boiling it a long 

 time with concentrated sulphuric acid, or caustic alkalis. 



There is a considerable doubt, as in the case of other melanins, 

 such as those in the skin, whether or not it is derived from 

 haemoglobin. 2 Krlikenberg considers it is more closely related to 

 the lipochromes. It is, however, undoubtedly nitrogenous. It is 

 certainly not a member of the group of pigments occurring in 

 plants named humous substances by* Hoppe-Seyler, 3 since on fusing 

 with alkali it yields no pyrocatechin or protocatechnic acid. 4 The 

 chief interest of fuscin is not, however, chemical but physiological. 

 Such problems as its varying distribution under the influence of light 

 and its relationship to the visual purple of the rods will be treated 

 under " Vision." 



Visual purple or rlwdopsin. AV<i possess very little chemical 

 knowledge of visual purple. Klihne found it to be soluble in certain 

 reagents such as solutions of bile salts, that in the process of bleaching it 

 passes through a yellow stage, that the bleaching occurs at different 

 rates at different temperatures and in different coloured lights, and 

 that spectroscopically it cuts out a very considerable portion of the 

 spectrum. It is destroyed by alcohol, ether, chloroform, ^ and strong 

 alkalis and acids, but not by most oxidising agents. It is perhaps 

 related to the lipochromes. The green, yellow, and red pigments 

 (chromophanes) of the oil droplets in the cones of birds are undoubtedly 

 lipochromes (see p. 20). 



The aqueous humour is lymph. 5 In parts per 1000 it contains: 

 water, 986'87 ; solids, 1313 ; proteids ; 1-22 ; extractives, 4*21 ; inorganic 

 salts, 770 ; sodium chloride, 6'89. 6 It does not clot spontaneously, but 

 does so on addition of serum. The proteids in it are fibrinogen, serum 

 globulin, and serum albumin. 7 Kiilme 8 found a reducing substance 

 among the extractives. This is not sugar. Urea and sarcolactic acid 

 are also present in small quantities. 1 ' 



The vitreous humour. The membranes of the vitreous humour 



worked at especially by v. Jaksch. The following are references to the principal papers 

 on the subject : Miirner, loc. cit., also ibid., Bel. xii. S. 229; Nencki, Arch. f. cs'per. 

 Path. u. PharmaM., Leipzig, Bd. xxiv. S. 17, 27 ; Chem. Centr.-BL, Leipzig, 1888, 

 S. 587; Brandl and Pfeitfer, Ztsclir. f. Biol., Mitnchen, Bd. xxvi. S. 348; v. Jaksch, 

 Ztschr. f. pkysiol. Chem., Strassbnrg. Bd. xiii. ; Abel and Davis, Journ. Exper. Med.. 

 Baltimore, 1896, No. 3, vol. i. ; Schmiedeberg, Arch. f. cxpcr. Path. u. Pharmakol. , 

 Leipzig, 1897, Bd. xxxix. S. 1. 



1 Untersuch. a. d. pliysiol. Inst. d. Univ. Heidelberg, Bd. ii. S. 324. 



2 Delepine has even suggested that, in the case of the skin pigment, haemoglobin is derived 

 from it (Proc. PhysioL Soc., London, Dec. 13, 1890, p. xxvii.). Abel and Davies (loc. cit.} 

 have recently studied the pigment of the negro's skin. The granules contain inorganic 

 matter as well as pigment. The latter contains the merest trace of iron. They conclude 

 that it originates not from haemoglobin, but from the proteids of the tissue juice. 



3 Ztschr. f. physiol. Chem., Strassburg, Bd. xiii. S. 66. 



4 Hirschfeld, ibid., Bd. xiii. S. 407. 



5 Chavvas, Arch. f. d. ges. PhysioL, Bonn, Bil. xvi. S. 143. 



Lohmeyer. See Gorup-Besanez, "Lehrbuch," 4th edition, 1878, S. 401. 



7 Friend and Halliburton, Rep. Brit. Ass. Adi\ Sc., London, 1889, p. 130. 



8 Arch. f. d. ges. PhysioL, Bonn, Bd. xii. S. 200. 



9 Griinhagen, ibid., Bd. xliii. S. 377 ; Pautz, Ztschr. f. Biol., Miinchen, Bd. xxxi. 



