parts of the mammary gland, where, though 
each constituent of the blood be changed in its 
character to form milk, yet each constituent of 
that secretion retains an obvious type in the 
fluid from which it was eliminated ; thus if we 
compare the blood and this secretion side by 
Blood separating 
into * 
¢ Fibrin 
Clot ? Red particles 
f Albumen 
and Alcoholic extractive, viz. 
lactates 
Serum << Aqueous extractive; albu- 
minate of soda 
Alkaline salts 
Fatty matter 
The similarity of reaction between fibrin, 
albumen, and casein is well known to chemists, 
and the derivation of the latter from the former 
can scarcely admit of a doubt; the only dif- 
ference acknowledged between fibrin and albu- 
men is a physical one, and it would be exceed- 
_ ingly interesting to ascertain whether or not the 
_ easein, which, like fibrin, separates on stand- 
ing, be not physically different from that por- 
tion of casein which remains in the fluid after 
the milk has creamed. The red particles of 
_ the blood certainly bear no analogy whatever to 
_the butyraceous matter of milk in its perfectly 
_ formed state; it is a curious fact, however, 
_ noticed by Sir Astley Cooper, that the colos- 
_ trum at its first appearance is occasionally of a 
ted colour, and the cream which separates is 
of a still deeper red tinge, thus rendering it 
_ probable that the fatty matters are in this in- 
Stance the colouring ingredient. I may men- 
tion, in addition, that it is not easy to obtain 
_hematosine quite free from fatty matter; there 
i ea to exist a natural affinity between 
em. 
The alcoholic extractives of blood and milk 
_ are too obviously analogous to need comment, 
as are the salts and likewise the fatty matters 
_ adherent to the casein of the milk and the 
albumen of the serum. The sugar of milk 
appears in all probability derived from the 
ueous extractive of the blood, an ingredient 
ich has scarcely attracted sufficient attention, 
and the development of whose chemical rela- 
tions may perhaps assist us greatly in obtaining 
a further insight into the true nature of those 
changes which are effected in the minute struc- 
ture of secreting glands. I have thought it 
right to mention the above resemblance, as I 
cannot help thinking that we may be greatly 
assisted hereafter in the physiology of secretion 
by attempting to reduce each proximate ele- 
ment of a secerned fluid to some type in the 
, and afterwards endeavouring artificially 
_ to produce those substances from their ana- 
___ logues in the circulating fluid; as nature may 
perhaps in this way form them for the various 
__-Secretions, by the so-called chemistry of vita- 
lity. Physiologists and chemists are, in the 
present day, too prone to attribute the forma- 
tion of those substances contained in secretions, 
MOLLUSCA. 
363 
side, we remark not only identical physical 
changes to occur, but also the existence of a 
set of proximate elements, which, though proper 
to each, still possess many chemical qualities 
in common ; for instance— 
Milk separating 
into 
Casein : Cain 
Butyraceous matter 
Casein a 
Alcoholic extractive, a i 
lactates and lactic aci : 
Aqueous extractive, with > aa Ik 
sugar of milk eet 
Alkaline salts 
Fatty matter 7 
and which cannot be detected in the blood, 
to changes occurring among the ultimate ele- 
ments of organic matter, changes of a character 
too intimate and obscure ever to be induced 
by those reagents which non-vital chemistry 
applies in effecting transformations. Such a 
conclusion is founded on the assumption that 
organic chemistry is not to make that progress 
which we now see in almost every branch of 
inquiry, a progress marked by the develop- 
ment of new laws which not only become 
valuable as indicators for the ascertainment of 
new facts, but which have a retrospective in- 
fluence on science, rearranging and altering the 
bearings and relations of previously ascertained 
phenomena. 
(G. O. Rees.) 
MOLLUSCA, (Lat. mollis,) Malakia and 
Ostracoderma of Aristotle; Mollusques, Fr. ; 
Weichthiere, Mollusken, Germ.; Mollusks, 
Engl.; invertebrated animals, distinguished by 
Bruguiere from insects by the negative cha- 
racters of the absence of bones, of stigmata, 
and of jointed feet; and first accurately de- 
fined by Cuvier as a ree division of the 
animal kingdom, with the following anatomical 
characters. The Mollusks are animals without 
an articulated skeleton or a vertebral column. 
Their nervous system is not concentrated in a 
spinal marrow, but merely in a certain number 
of medullary masses dispersed in different 
points of the body, the chief of which, termed 
the brain, is situated transversely above the 
cesophagus, and encompasses it with a nervous 
collar. Their organs of motion and of the 
sensations have not the same uniformity as to 
number and position as in the Vertebrata, and 
the irregularity is still more striking in the 
viscera, particularly as respects the position 
of the heart and respiratory organs, and even 
as regards the structure of the latter: for some 
of these respire elastic air, and others water, 
either fresh or of the sea. Their external 
organs, however, and those of locomotion are 
generally arranged symmetrically on the two 
sides of an axis. The circulation of the Mol- 
lusks is always double; that is, their pulmo- 
nary circulation describes a separate and dis- 
tinct circle. The blood of the Mollusks is white 
