122 THE CHEMISTRY OF THE TISSUES AND ORGANS. 
May's method x of preparing fuscin is to boil retime in alcohol, then in 
ether, lastly in water. The residue is then subjected to tryptic digestion. 
Three things remain undigested ; of these nuclein 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 Krukenberg considers it is more closely related tit 
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 rhodopsin. — We possess very little chemical 
knowledge of visual purple. Kiihne 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 
(chromopha,nes) 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, 13 -13 ; proteids L'22 ; extractives, 4-21 : inorganic 
salts, 7"70 ; 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 Kiihne s found a reducing substance 
among the extractives. This is not sugar. Urea and sarcolactic acid 
are also present in small quantities. 9 
The vitreous humour. — The membranes of the vitreous humour 
worked at especially by v. Jaksch. The following are references 1" the principal papers 
on the subject: — Mbrner, loc. cit., also ibid., Bd. xii. S. 229 : Nencki, Arch. f. <</<■/■. 
Path. a. Pharmakol., Leipzig, Bd. xxiv. S. 17, 27 ; Chem. Centr.-BL, Leipzig, 1888, 
S. 587; Brandl and Pfeiffer, Ztschr. f. Biol., Mtinchen, Bd. xxvi. S. 34S : v. Jaksch, 
Ztschr. f. physiol. Chem., Strassburg, Bd. xiii. ; Abel and Davis, Journ. Bacper. Med.. 
Baltimore, 1896, No. 3, vol. i. ; Sclnniedeberg, Arch. f. exper. Path. a. Pharmakol., 
Leipzig, 1S97, Bd. xxxix. S. 1. 
1 Untersuch. <>. d. physiol. Just. J. Univ. Heidelberg, Bd. ii. S. 324. 
2 Delepine lias even suggested that, in the case of the skin pigment, hamioglobin isderived 
from it (Froc. Physiol. Soc., London. Dee. 13, 1890, p. xxvii.). Abel and Davies (lor. cit.) 
have recently studied the pigment of the negro's skin. The grannies 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. Bd. xvi. 8. 143. 
,; Lobmeyer. See Gorup-Besanez, "Lehrbuch," 4th edition, 1S7S, S. 401. 
7 Friend and Halliburton, Rep. Brit. As*. Adv. Sis., London, 1SS9, p. 130. 
6 Arch. f. d. ges. Physiol., Bonn, Bd. xii. S. 200. 
9 Grlinhagen, ibid., lid. xliii. S. :'>77 : Pautz, Ztschr./. Biol.. Mtinchen, Bd. xxxi. 
