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and chondroitin-sulphuric acid are dissolved out with 0'2 to 0"5 per
cent, solution of potash ; the collagen is dissolved out by hot water, being
converted into gelatin in the process; the albuminoid remains undissolved.

(1) The collagen differs from ordinary collagen in only containing
16 '4 per cent, of nitrogen.

(2) The albuminoid, which is found only in late adult life, is a proteid-
like substance of an insoluble natm-e. It contains loosely combined
sulphur. It differs from elastin in its high percentage of sulphur (see p. 73).

(3) Chondromucoid. — This substance has the following percentage
composition :—C, 47-3 ; H, 6-42; N, 12-58; S, 2-42 ; 0,31-28 (Morner).
The sulphiu- is loosely combined. Chondromucoid gives the ordinary
proteid reactions. On decomposition, it yields the usual decomposition
products of proteids, with chondroitin-sulphuric acid in addition; this
latter substance is, on fm'ther decomposition, broken up into sulphiuic
acid and a reducing substance. Schmiedeberg ^ regards chondromucoid
as a union of proteid with chondroitin-sulphinic acid.

(4) Chondroitin-suljjhui'ic acid. — This substance was called chondroitic
acid by Bodeker ^ and Krukenberg ^ (who classed it among his hyahns,

^ Vcrhandl. d. naturh.-med. Ver. zu Heiddhcrcf, Part 5, Bd. i.
• ^ Ztschr. f. 2)hysiol. Chevi., Strassljurg, Bd. xii. S. 396; Skandln. Arch. f. Physiol.,
Leipzig, Bd. i. S. 210.

^ Arch./, exper. Path. n. Pharmakol., Leipzig, 1891, Bd. xxviii. S. 355.
•* Ann. d. Chcm., Leipzig, 1861, Bd. cxvii. S. 111.
^ Ztschr.f. Biol., Miinclien, Bd. xx. S. 307.



NERVOUS TISSUES.



"5



see p. 64). It was first prepared in a pure condition Ijy Mijrner, and
its constitution made out by that observer and by Schniiedeberg. It is
partly found as such in the cartilaginous matrix, but most originates
from the decomposition of chondromucoid.

Morner found that the sulphur in it was all in the form of ethereal
hydrogen sulphate ; hence the name chondroitin-sulphuric acid. It is
almost, but not quite, characteristic of cartilage. Morner ^ separated it
from twenty different varieties of cartilage, from cartilaginous tumours,
and also from the tunica intima of the aorta,^ but from no other tissue
or organ of the body.^

Schniiedeberg ascribes to it the formula CjgHgyNSOj-. On decom-
position, the first products are sulphuric acid, and a nitrogenous sub-
stance chondroitin.

Ci8H,,NS0ij+H,0 = H2S04+Ci,H2,NOj,

(chondroitin -sul- (water) (sulphuric (chondroitin)
phuricacid) acid)

Chondroitin is a gummy material, and a monobasic acid. On
hydration it yields acetic acid, and a new nitrogenous body called
chondrosin.

CisH^^NO^.+SH^O = C^H^O.+Ci^H^iNOji

(chondroitin) (water) (acetic acid) (chondrosin)

Chondrosin is also gummy, and a monobasic acid. It reduces
Fehling's solution even more strongly than dextrose ; it is dextro-
rotatory, and is the reducing substance which so many previous chemists
have obtained in an impure form from cartilage. On further decomposi-
tion it yields glycuronic acid (see p. 5) and glucosamine (see pp. 9 and 75).



Nekvous Tissues.

General composition. — The amount of water varies. It is present
in larger amount in the grey than in the white matter, in early
than in adult life, in the brain than in the spinal cord, in the spinal
cord than in nerves. These facts are illustrated by the followhig
table ^ :—



Portion of Nervous
System.


Percentages op Water.


Grey matter of brain)
White „ i
Spinal cord .
Nerves .


In Fostus
OV.).


Age, 20-.30
(W.).


Age, 70-90
(W.).


(B.)


(P.)


(M.)


(R.)


87-92 1


S3
69


84
72


85

70
73-76
64-72


81)
68/


68
57


86
70



^ ZtscJir. f. loliysiol. Chcm., Strassburg, 1895, Bd. xx. S. 357.

" Ujjsala Ldkaref. Forh. , Bd. xxix.

^ Oddi [Arch. f. exper. Path. u. Pharmalcol., Leipzig, Bd. xxxiii. ) states he has obtained
it from livers which had undergone amyloid degeneration.

^ In the above table, (W. ) refers to Weisbach (Hofmann's "Lehrbuchd. Zoochenne," Wien
1876, S. 121); (B.) to Bernhart (Gamgee's " PhysioL Chem.," voL i. p. 446); (P.) to
Petrowsky {Arch. f. d. ges. Physiol., Bonn, Bd. vii. S. 367) ; (M.) to Moleschott (GharleS;
"Physiol. Chem.," p. 335); and (R.) to de Regibiis {Jahresh. ii. d. Fortschr. d. Thier-
Chem., Wiesbaden, Bd. xiv. S. 346).



ii6 THE CHEMISTRY OF THE TISSUES AND ORGANS.



Solids. — The solids may l^e divided into the following classes : —

(«) Proteids. — These comprise a very considerable percentage of the
solids, especially in the grey matter (over 50 per cent.).

(&) Neurokeratin and nuclein.

(c) Phosphorised constituents ; especially protagon and lecithin.

{d) Cerebrins. — Nitrogenous substances of unknown constitution.

(e) Cholesterin. — Especially abundant in white matter.

(/) Extractives. — Creatine/ xanthine,^ hypoxanthine,^ inosite,^ lactic
acid,^ leucine,^ uric acid,^ and urea.

{g) Gelatin and Eat. — From the adherent connective tissue.

{h) Inorganic salts. — The total mineral matter varies according to
different writers from Ol to 1 per cent.

Geoghegan ^ gives the following figures in parts per thousand of
brain : —



Total ash


2-9


to 7-1


Chlorine .


0-4 to P2


Potassium


0-6


„ 1-7


PO,


0-9 „ 2-0


Sodium


0-4


„11


CO..


0-2 „ 0-7


Magnesium .


0-0


„ 0-07


so,


0-1 „ 0-2


Calcium


0-005


„ 0-02


re(P0,)2


0-01 „ 009



The grey matter is stated by Schlossberger to be richer in total ash
than the white, but poorer in phosphates ; Petrowsky, on the other hand,
found more phosphates in grey than in white matter.

The following table gives some typical quantitative analyses which
have been made of the proportion in which the principal solids occur in
different nervous structures : —



Portion of Nervous
System.


Proteids.


Lecithin.


Cholesterin
and Fat.


Cerebrins.


Neuro-
keratin.


Other
Organic
Matters.


Salts.


Grey matter of ox
brain ^

White matter of
ox brain ^

Spinal cord ^

Human sciatic
nerve "^


55-37
24-72
23-8
36-S


17-24
9-90


18-68
51-91


0-53
9-55


,


1-45
0-57
1-1


6-71

3-34

. '


75-1


32-57 12-22


11-30


3-07


4-0



The quantitative work I ^ have done on this question may be sum-



1 Miiller, Ann. d. Chem., Leipzig, Bd. ciii. S. 141; Stiideler. Joxmi. f. inalct. Chcm.,
Leipzig, Bd. Ixxii. S. 256.

^ StJideler, Ann. d. Chcm., Leipzig, Bd. cxvi. S. 102; Scherer, ibid., Bd. cvii.
S. 314.

^ Miiller, loe. cit. ; see also Strecker, Ann. d. Chem., Leipzig, Bd. cv. S. 316.

^ ZtHchr. f. liliijsiol. Chem., Strassburg, Bd. i. S. 330.

•'' I'etrowsky, loc. cit.

^ Moles(;hott, loc. cit.

^Josephine Chevalier, Ztschr. f. physiol. Clum., Strassburg, Bd. x. S. 97.

** Halliburton, Journ. Physiol., Cambridge and Loudon, 1893, vol. xv. p. 90,



PROTEIDS OF NERVOUS TISSUES.



117



marisecl in the following table of mean analyses. The organs were from
adult human beings, dogs, cats, and monkeys :—





Water.


Solids.


Percentage
of Proteids
in Solids.


Grey matter of cerebrum .


83-467


16-533


51


White ,, „ . .


69-912


30-088


33


Cerebellum ....


79-809


20-191


42


Spinal cord as a whole


71-641


28-359


31


Cervical cord ....


72-529


27-471


31


Dorsal cord ....


69-755


30-245


28


Lumbar cord ....


72-639


27-631


33


Sciatic nerves ....


61-316


38-684


29



This table illustrates the fact that the amount of grey matter, of
water, and the percentage of proteid in the solids, vary directly the one
with the other. This is very well seen in the different regions of the
spinal cord. The percentage of proteid in the white matter of the
brain is a little higher than in the spinal cord; this exception is
perhaps to be explained by the high percentage of neurokeratin ^ in
white matter, which, according to the methods used, would be included
with the proteids.

Reaction of nervous tissues. — Heidenhain- and Gscheidlen^ state that
the normal reaction of the axis cylinder is alkaline ; on death or on
long-continued activity the reaction becomes acid. They further state
that the grey matter is acid even during life. O..Langendor£f* found
the reaction of the central nervous system alkaline during life ; the
alkalinity rapidly diminishes after death, or on stoppage of the circula-
tion. S. Moleschott and Battistini ^ found both central and peripheral
portions of the nervous system acid, especially the grey matter ; this was
increased by activity.

In my own work I found in animals that the fresh tissues were
invariably alkaline, but they became rapidly acid, especially the grey
matter. In the human brains I received from the post-mortem room the
reaction of the grey matter was always, of the white matter often, acid.
This I put down to changes after death, for at least twenty-four hours
had always elapsed since death.

The acidity is due to lactic acid ; but, according to Miiller and
Gscheidlen, it is not sarcolactic acid but the fermentation lactic acid
(optically inactive ethylidene-lactic acid). Miiller also obtained traces
of formic acid.

Proteids of nervous tissues. — The large quantity of these, especi-

^ The percentage of neurokeratin is in grey matter, "3 ; in white matter, 2 "2 to 2 -9 ; and
in nerve, 0-3 to 0-6 (Klihne and Chittenden, Ztschr. f. Biol., Mlinchen, Bd. xxvi. S. 291).

^ Ccntralhl.f. d. med. JFissensch. , Berlin, 1868, S. 833.

■* Arch. f. d. ges. Physiol.. Bonn, Bd. viii. S. 171.

■* Neu7'ol. Centralbl., Leipzig, 1885, No. 14 ; Ccntralhl. f. d. mcd. Wissensch., Berlin,
1886, No. 25.

^ Arch. ital. de hiol., Turin, vol. viii. p. 90 ; Chem. Centr.-BL, Leipzig, 1887, S. 1224.



ii8 THE CHEMISTRY OF THE TISSUES AND ORGANS.

ally in the grey matter, has l^een already alluded to. Petrowsky, in
the investigation just mentioned, describes a globulin somewhat
resembling myosin, and an albumin especially abundant in grey matter
which is coagulated at a temperature of 75° C. Baumstark,^ in a more
recent research, speaks of the chief proteid matter in nervous tissue as
resembhng casein ; this is so, for it is a nucleo-proteid. My own con-
clusions ^ on the subject are as follows : —

The proteids present are three in number. The first is a globulin,
coagulated by heat at 47" C, and analogous to the cell globulin deriv-
able from nearly all cellular tissues. The second and most abundant is
nucleo-proteid. In a saline extract of nervous tissues it is mixed with
the other proteids; attempts to prepare it by the sodium-chloride
method failed. It may, however, be prepared in large quantities by
precipitating an aqueous extract of brain by weak acetic acid (Wool-
dridge's method). The supply obtainable from white matter is small. It
is coagulated at 56°-60° C. ; it contains 0'5 per cent, of phosphorus, and
gives the general reactions of nucleo-proteids, production of intravascular
coagulation included. The third proteid is a globulin, coagulated by
heat at 70°— 75° C, and analogous to a similar globuhn separable from
liver cells (see p. 86). Peptone, proteose, myosin, and all^umin are not
obtainable.

Protagon. — In the year 1865, Liebreich^ separated from the brain
a material he called protagon ; he further found that, when decomposed
by baryta water, it yielded two acids — stearic acid and giycero-phosphoric
acid — and a base choline.

Hoppe-Seyler, and Diaconow * working under Hoppe-Seyler's direc-
tion, denied the existence of this substance, and considered that it was a
mere mechanical mixture of lecithin with a nitrogenous non-phosphor-
ised substance called cerebrin. Diaconow's analyses were, however, far
from convincing.

The subject was taken up in this country by Gamgee and
Blankenhorn,^ who showed that protagon is a perfectly definite
crystalline substance of constant elementary composition. They also
showed that even prolonged treatment with alcohol and ether will not
extract lecithin from protagon, as alleged by Diaconow. When protagon
is digested with alkalis it yields cerebrin or cerebrins, and the decom-
position products of lecithin. This work has been confirmed by
Baumstark,^ Euppel,'^ and Kossel and Freytag.^

Protagon is prepared as follows : — The brain is digested with alcohol
at 45° C. ; the extract is filtered warm, and cooled to 0° C. It then
deposits a white precipitate of protagon mixed with cholesterin, which
is dissolved out by means of ether. The protagon is dried, redissolved
in warm alcohol, and crystallises out on cooling. The empirical formula,
calculated from their analytical results, is given as Ci6oIl3(,8N5PO.j5 l)y
Gamgee and Blankenhorn.

^ Ztschr. f. physiol. Chem., Strasslnirg, Brl. ix. S. 145.
'^ Jfmrn. Physiol., Cambridf^e and London, 1893, vol. xv. p. 100.
" Ann. d. Clicm., Leipzig, Ijd. cxxxiv. S. 29.
■* Centralhl. f. d. mcd. TVissensch., Berlin, 1868, S. 97.

^ Journ. Physiol., Cambridge and London, vol. ii. p. 113; Gamgec's "Physiol. C'lietn,,"
vol. i. p. 427.

'^ ZL^chr. f. physiol. Chem., Strassburg, Bd. ix. S. 329.

■^ Ztschr.f. Biol., Miinciien, Bd. xxxi. S. 8G.

^ Ztschr. f. physiol. Chem,, Strassburg, Bd. xvii. .S. \'-\\.



THE CEREBRINS OR CEREBROSIDES.

The percentage composition is seen in the following table : —



119



Elements.


LlEBRElCII.


Gamgee and
Blankeniiorn.


Bau.mstark.


KOSSEL.


RUITEL.


Calculated

FROM

Formula.


Ox.


Human.


C
• H

N
P
S



66-74

11-74

2-80

1-23


66-39
10-69
2-39
1-068

19-462


66-48

11-12

2-35

1-02

.18-701


66-25

1113

3-25

0-97
0-51


66-29
10-75

2-32

1-13

0-090


66-51

10-88

2-55

1-138


66-45

10-66

2-42

1-07

19-40



An elaborate research by Thudichuni ^ led him to the conclusion that
there are three groups of phosphorised substances in the brain, which he
termed kephalines (very soluble in ether), myelines (less soluble in
ether), and lecithins (characterised by their extreme instability). In
each of these ill-defined groups several members with their empirical
formulcC are described. Thudichum's work has been so far confirmed
by that of Kossel, in that he has shown that protagon is not a single
substance, but that there is more than one protagon. They yield either
one or two or perhaps three derivatives (cerebrosides), called cere-
brin, kerasin or homocerebrin, and encephalin ; and, further, probably
several lecithins are obtainable from the different protagons. The
constitution of lecithin is discussed on p. 22, and there it will be seen
that the existence of several lecithins {i.e. containing different fatty acid
radicles) is mooted. The protagons, according to Kossel, resemble each
other in the following points : —

1. They contain carbon, hydrogen, nitrogen, oxygen, and phosphorus.
Elementary analysis gives practically the same results as those obtained
by other ojjservers. But the existence of sulphur in some varieties of
protagon is a new point.

2. By oxidation with nitric acid they yield higher fatty acids (palmitic
and stearic).

3. By the action of boiling sulphuric or hydrochloric acid a reducing
carbohydrate is formed.

4. By the action of alkalis they yield cerebrosides (formerly called
cerebrins).

5. The cerebrosides are the source of the reducing carbohydrate
mentioned above.

6. The carbohydrate formed is galactose.

7. Other decomposition products of the cerebrosides are ammonia, and
a complex material which on fusion with potash yields higher fatty acids.

The cerebrins or cerebrosides. — These substances, the glucoside
constitution of which has just been alluded to, form a group of ill-
defined, nitrogenous substances, existing especially in the white sub-
stance of nervous tissue, and also in the yolk of egg, pus corpuscles,
and spleen cells.^

^ Rep. Med. Off. Privy Council, London, 1874, p. 113 ct seq.
- Hoppe-Seyler, "Phj'siol. Chem.," S. 720, 788.



I20 THE CHEMISTRY OF THE TISSUES AND ORGANS.

Miiller ^ obtained cerebrin by rubbing brain up with baryta water,
so as to form a milky fluid ; this is boiled, and the resulting coagulum
extracted with boihng alcohol ; on cooling, the alcoholic solution deposits
cereljrin and cholesterin. The latter is removed by ether, and the former
is purified by repeated crystallisation from iDoiling alcoliol. According to
Miiller, its formula is CiyHggNOg; according to Parens,^ C8oHi6(,K20i5.
Parens also obtained two other similar substances (homocerebrin and
encephalin) with different formulge. Adopting a slightly different modus
oijerandi, Geoghegan' obtained a substance with the formula Cg-H^ioNgOag.
Thudichum* separated three cerebrins, which he named cerebrin
(C34H6o]Sr208), phrenosine^ (Cg^Hg-ISrOg), and kerasine (C46H9iN09).'
Gamgee ^ found that, while protagon cannot be separated by the simple
action of solvents into lecithin and cerebrm, yet such non-phosphorised
substances do exist by its side in the brain, and one which he called
pseudo-cerebrin (C44H92NOg) can be obtained from protagon by the
action of caustic baryta.

The fact that the cerebrins are glucosides was known to Liebreich,"
Diaconow, Otto,^ Geoghegan,^ and Tliudichum,!*' but it was only within
quite recent years that the sugar was identified as galactose, almost
simultaneously in this country and in Germany.^^

The most recent work on the subject is that by Kossel and Freytag,^^
who adopt the very a^^propriate name of cerebrosides for these bodies.
They find that these substances are constituents of the medullary
sheaths rather than of the axis cylinders. They have especially worked
at two, which they obtained by the decomposition of protagon crystals,
namely, cerebrin and kerasm. The analyses of these agree very well
with those previously published by Thudichiun, Parens, and others.
Their molecular weight was investigated by Beckmann's boiling method,
and by the examination of theu' barium and bromine compounds. By
treatment with nitric acid they yield not only galactose but also a
fatty acid recognised as neurostearic acid by Thudichum, and correctly
analysed but not identified by Miiller. It is, in fact, stearic acid, three
molecules of which are formed from cerebrin for every two atoms of
nitrogen. From all these considerations, the formula given to cere-
brm is C^oHi^olSTgOig, and to kerasin (the homocerel)rin of Parens),

C7oHl38N20l2-

Similar substances occur in other parts of the body ; thus two
separated from pus are named 2yyosin, Cj-HiioNgOu, or CggHi^o^aOis,
and 'pyofjcnin, CgsHiosNgOig. These bodies and sunilar ones separated
from testicular cells are components of the cell protoplasm, not of the
nucleus (Kossel and Freytag).

^ Ann. d. C'hem., Leipzig, Bd. ciii. S. 131 ; ev. S. 361.

" Journ. f. prakt. C'hem., Leipzig. Bd. exxxii. S. 310.

'■' Ztsclir. f. 'physiol. Chew,., Strassburg, Bd. iii. S. 'PjZ'I.

* Loc. cit.

'■> For recent papers on phrenosine, see Tlnidiclmm, Jov,rn. f. iirald. Clicm., Leipzig,
Bd. liii. S. 49 ; Kossel, iUd., 1896, Bd. liv. S. 215.

" Loc. cit.

' Virchov/s Archiv, Bd. xxxix. S. 183.

^Ibid., Bd. xli. S. 272.

" Geoghegan stated that tlie rcdnciiig substance liad the formula CooHj^O^ ; he termed
it cetylid ; cetylid was no doubt a mixture of galactose and fatty acids.
^" Journ. /. 2>rakf. Chem., Leipzig, Bd. xxv. S. 23.

^^ Thierfelder, Ztschr. f. 2^Jfysiol. Chem., Strassbui'g, Bd. xiv. S. 209 ; Brown and
Morris, Proc. Chem. Soc. London, 1889, p. 167.

^" Zl^chr. f. 'phijdol. Chem., Strassbin'g, Bd. xvii. .S. 431.



THE EYE.



The Eye.



The cornea. — A thousand parts of corneal tissue contam 242 of
sohds, of which 204 consist of collagen, 28 of other organic matters, and
10 of ash.^

The erroneous idea that the cornea, like cartilage, contains a specific
substance called chondrin (Miiller), was first combated by Morochowetz,^
who showed that chondrin here as elsewhere is a mixture of gelatin and
a mucinoid material. This latter substance is named by C. T. Morner;'^
cornea-mucoid ; its percentage composition is C, 50-16 ; H, 6"97 ; N,
12-79 ; S, 2-07 ; 0, 28-01. It resembles other mucoids very closely in
its properties (see p. 63). The gelatin obtained from the collagen
resembles that found elsewhere. The same mucoid and collagen are
present in the sclerotic.

Descemet's membrane is resistant to reagents. Morner terms its
chief constituent rmiiibranin. It belongs to the mucoid group. The lens
capsule has a similar chemical structure.

The choroid and iris are principally of chemical interest from con-
taining the black pigment which is identical with or nearly related to
that in the pigment layer of the retina.

The retina. — Calm '^ gives the following table of the quantitative
composition of the retina? of geese : —

per cent.



A¥ater .....




86 to 89


SoHds .....




14 „ 11


Proteids (globulin coagulating at


50°


C,


albumin and mucin C?) ) .




4„6


Gelatin . . . . .




13„17


Cliolesterin .....
Lecithin . . . . .




0-3 „ 0-8
1-0 „ 2-9


Fat'




0-05 „ 0-5


Salts




0-7 „ 1-2



The pifpnents of the retina. — The black pigment of the retinal
epithelium is called /z^scm. In some animals the epithelium is free from
pigment in part ; this constitutes the tapetum lucidum. In some fish this
contains crystals of guanine ; in the ox and sheep it does not.^

Fusein is one of the group of black pigments, termed melanins. It
was investigated by Berzelius, and by Heintz, who found it contained a
small quantity of iron, by Scherer, who found no iron, and also by Eosow^
and Sieber. The percentage composition obtained by the various
observers shows great discrepancies, and this, taken into account wdth their
methods of preparing the pigment, renders it probable they were dealing
with impure substances. The failure to find iron was due to the ' fact
that hydrochloric acid was employed at one stage of the operations,
and this dissolves out nine-tenths of the iron.^

^ His, quoted by Gamgee, "Physiological Cliemistiy, " vol. i. p. 451.

^ VerJiandl. d. naturli.-ined. Veo\ zu Heidelberg, pt. 5, Bd. i.

^ Ztselir. f. iphysiol. Clicm., Strassbnrg, Bd. xviii. S. 213.

■* Hoppe-Seyler, "Physiol. Chem.," S. 699.

^ Klihne and Sewall, Verhandl. d. naturli.-med. Ver. zu Heidelherg , N. F., Bd. ii.
Heft 5.

'^ K. A. H. Morner, Ztsclir. f. 2)hy.nol. Chem., Strassb\;rg, Bd. xi. S. 66. The pigment
in the skin of negroes, and in melanotic sarcomata, is closely allied to fusein. It appears
to contain iron. In melanotic sarcomata, Berdez and Nencki named the pigment phyma-
torusin ; in those of horses, hippom^lanin. The subject of melanin in the urine has been



122 THE CHEMISTRY OF THE TISSUES AND ORGANS.

May's method ^ of preparing f nscin is to boil retinte 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.

i\iscin is slowly bleached in the aii' ; it dissolves by boiling it a long
time with concentrated sulphuric acid, or caustic alkahs.

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
hEemoglobin.^ 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,^ since on fusing
with alkali it yields no pyrocatechin or protocatechnic acid.* 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 iniTiple or rliodoi^sin. — 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 l^leaching 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
{cliTomo'plianes) of the oil droplets in the cones of birds are undoubtedly



Online LibraryE. A. (Edward Albert) Sharpey-SchäferText-book of physiology; (Volume v.1) → online text (page 17 of 147)