CHEMICAL BASIS OF THE ANIMAL BODY. 231 



accepted, is derived from observations of the amount of oxygen which 

 can be pumped out from a mixture of methsemoglobin and oxy-haemo- 

 globin of known composition 1 , and from the amount of oxygen which is 

 displaced from a given weight of methaemoglobin when it is treated 

 with nitric oxide 2 . We may probably say therefore that under certain 

 conditions, without our being able to state exactly what has taken 

 place, the oxygen loosely united to haemoglobin as oxy-hsemoglobin 

 becomes more stably combined, and is now not removable by either 

 a vacuum, or carbon-monoxide, or a current of hydrogen, and further 

 that the resulting substance (methsemoglobin) has the same composition 

 and crystalline forms as oxy-hsemoglobin, and may be reconverted into 

 the latter body by suitable means, such as reduction by ammonium 

 sulphide and subsequent oxidation. 



7. Hsemocyanin 3 . As previously stated (p. 218) the blood-plasma 

 of many invertebrates contains haemoglobin in solution ; in a few cases 

 this is united to special corpuscles in the blood. But in the case of other 

 invertebrates this respiratory pigment is replaced by another to which, 

 since it turns blue on exposure to air (oxygen), the name haemocyanin 

 has been given. Hence the arterial blood of those animals in which 

 it occurs is blue, while the venous is colourless. 



Hsemocyanin is a proteid of the globulin class; it is therefore 

 partially precipitated by a current of carbon-dioxide, by saturation of 

 its solutions with sodium chloride and completely by saturation with 

 magnesium sulphate 4 . Unlike haemoglobin it has not yet been crystal- 

 lised and contains copper, presumably as a constituent of its molecule, 

 in place of the iron characteristic of haemoglobin. It exhibits no 

 absorption bands when examined spectroscopically. 



Another animal pigment is known, into whose composition copper (5 8 p.c.) 

 enters ; this is the substance called turacin 5 . It gives the characteristic colour to 

 the plumage of certain African birds known as Touracos or Plantain-eaters, whence 

 the name turacin. It differs entirely from haemocyanin in its general properties, 

 and is only mentioned here because it contains copper, as does the former pigment. 

 It is slightly soluble in water, readily soluble in dilute alkalis, the solutions in 



1 The literature of the dispute is fully quoted and abstracted down to 1883 by 

 Otto, Pfliiger's Arch. Bd. xxxi. Sn. 245 255. The remaining literature to date 

 (1892) has been given passim in the above account of this substance. 



2 Hiifner u. Kiilz, Zt. f. physiol. Chem. Bd. vn. (1883), S. 366. 



3 For literature see Halliburton, Chem. Physiol. and Pathol. 1891, p. 321. 

 Details of previous work to date (1880) are given by Krukenberg, Vergleich-physiol. 

 Studien, in. Abth. (1881), S. 66. 



4 Halliburton, Jl. of Physiol. Vol. vi. (1884), p. 319. 



5 Church, Phil. Trans. Vol. CLIX. (1870), p. 627. Cf. Ber. d. d. chem. Gesell. Bd. n. 

 (1869), S. 314; in. (1870), S. 459. See later Krukenberg, Vergl.-physiol. Stud. 

 v. Abth. (1881), S. 72 ; 2 Keihe, i. Abth. (1881), S. 151. The same work (2 Eeihe, 

 Abth. n. u. in. 1882, Sn. 1 u. 128) contains elaborate observations on other pigments 

 from feathers. 



