MOLLUSCA. 



363 



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 



vz. 



Fibrin 



{ Red particles 

 '"Albumen 

 Alcoholic extractive, 



lactates 

 Aqueous extractive; albu- 



minate of soda 

 Alkaline salts 

 matter 



Clot 



and 

 Serum 



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 aud albu- 

 men is a physical one, and it would be exceed- 

 ingly interesting to ascertain whether or not the 

 casein, 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 

 red 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 

 haematosine quite free from fatty matter ; there 

 appears to exist a natural affinity between 

 them. 



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 

 aqueous extractive of the blood, an ingredient 

 which 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 

 blood, 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, 



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 



1 r 



Cream 

 and 



Milk or 

 skim milk 



Casein 



Butyraceous matter 



Casein 



Alcoholic extractive, viz. 



lactates and lactic acid 

 Aqueous extractive, with 



sugar of milk 

 Alkaline salts 

 Fatty matter 



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. 0. 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 primary 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 

 oesophagus, 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 



