402 LECTURE XVII. 



in the way of the general conception which prevails concerning its cause. 

 According to the theory, however, that it is not the function of synthesizing 

 the material which is disturbed, but rather the function of decomposing 

 the material containing the iron, it is easy to understand the favorable 

 action, of iron salts, for in them the organism receives material which for 

 some reason it cannot itself obtain. At all events, in cases of iron thera- 

 peutics it should be borne in mind that hemoglobin cannot be formed from 

 iron alone, so that care must be taken to supply the remaining material 

 necessary, in the form of meat, eggs, and green vegetables. 



The importance of iron for the tissues has been in the past almost 

 forgotten in the discussion of the relations of iron to the formation of 

 blood. Here again iron plays an important part, but unfortunately we 

 now know but little in regard to the way it is combined in the tissues and 

 cells. It evidently occurs in different forms. Thus, iron compounds 

 have been prepared from the liver, spleen, and muscles of man by P. 

 Marfori l and O. Schmiedeberg. 2 According to these investigators, the 

 iron is present in much the same state of combination as in hematin. The 

 substances thus prepared were apparently absorbed by dogs after they had 

 been starved, or fed upon a diet free from iron. The exact nature of 

 these iron compounds has by no means been fully explained. 



Copper plays, in the case of invertebrates, a similar part to that 

 taken by iron in the vertebrates. 3 It is found combined with albumin. 

 The compound which corresponds to hemaglobin is called hemocyanin. 

 It is found especially in many mollusks and crustaceans. The blood of 

 the cephalopoda has been examined chiefly in this connection. The 

 arterial blood of these animals is blue, the venous blood colorless. In 

 certain other mollusks (Pinna squamosa, Doris, Patella, Chiton) manga- 

 nese 4 replaces the copper. 



We have, up to this point, failed to mention certain inorganic salts 

 which occur in milk, and unquestionably are indispensable foods. Thus, 

 milk always contains some magnesium. This element forms an integral 

 part of plant and animal cells and also of the animal fluids, blood and 

 lymph. The amount of magnesium in milk is in general relatively small. 

 Its function in the animal economy is not yet definitely known. 5 Appar- 



1 Arch, exper. Path. Pharm. 29, 212 (1891), and Arch. ital. biol. 21, 1 (1894). 



2 Arch, exper. Path. Pharm. 33, 102 (1894); cf. Filippi: ibid. 34, 462 (1895). 



3 Cf. Harless: Miiller's Arch. 1846, 148. Schlossberger: Ann. 102, 86 (1857). 

 Fre'de'ricq: Arch. Zool. expe>. 7, 535 (1878); Compt. rend. 87, 996 (1878). Dhere*: 

 Compt. rend. soc. biol. 52, 458 (1900). Griffiths: Proc. Roy. Soc. Edinburgh, 18, 288 

 (1890-91) ; 19, 127 (1892) ; Compt. rend. 114, 496 (1892). Cf. Otto v. Fiirth: Vergleich- 

 ende chemische Physiologic der niederen Tiere, p. 74, Jena, 1903. 



4 Griffiths: Compt. rend. 114, 840 (1892); 115, 259 and 474 (1892); 116, 1206 (1893), 

 and Respiratory Proteids, London, 1897. 



s Cf. Lecture XVI, p. 354 et. seq. 



