IN THE ANIMAL KINGDOM. 23 



the corpuscle and pigment have been morphologically, chemically, and 

 functionally specialized. 



The erythrocyte, in common with all living structures, must be con- 

 ceded to be a respiratory structure of the first type by virtue of its proto- 

 plasm, but in addition to this there exists the third type of exchange which is 

 manifested in the continual alternating give-and-take in external and internal 

 respiration, respectively, which is rendered possible through the feeble com- 

 binations of O and C0 2 in the erythrocyte in association with the differences 

 in partial pressures and tensions of these gases in the proximal and distal 

 portions of the vascular system, and with mass actions, in which operations 

 the corpuscles act as a carrier and store-house for both gases. This respira- 

 tory phenomenon is to be attributed essentially to the agency of hemoglobin, 

 and it will be noticed that it differs materially from the first and second types 

 of exchange, which involve intrinsic changes, and which are manifestations of 

 the activity of energy-transforming mechanisms. While, therefore, as has 

 been shown in previous pages, hemoglobin and chlorophyl are intimately 

 related chemically, and are the most important bodies in plant and animal 

 life, respectively, in the exchange of O and CO 2 , it is obvious that they have 

 become so specialized in the character of their work that the mechanisms 

 concerned in the exchange are totally different in character and object: we 

 observe a phenomenon which in the first instance is manifested essentially 

 through a passive vehicle; in the second, through the operations of an 

 energy transformer. 



The third type of respiratory activity, which is the preeminent prop- 

 erty of the erythrocyte, is a property that belongs to the cell as a whole as 

 an individual vital mechanism. Hemoglobin in solution in the plasma of 

 the vertebrate blood has been shown to be in the nature of a foreign body; 

 as a component of the erythrocyte it is an energetic respiratory substance, 

 under which condition its dissociable O is more readily removed than when 

 it is in solution ; in the erythrocyte it behaves as though it were in colloidal 

 form. Isolated hematin is absolutely inert in relation to both O and C0 2 , 

 and isolated stromata and isolated globin have not been found to have 

 respiratory energy in and C0 2 absorption and elimination greater than 

 protein substances generally under comparable conditions. It seems there- 

 fore obvious that it is not the hemin, globin, or stroma, or the hemoglobin 

 per se, that is the normal functionating substance, but a hemoglobin-stroma 

 combination; and that from analogy, when the hemoglobin is normally in 

 non-corpuscular form, as in certain of the invertebrates, it is probably 

 in a primitive hemoglobin-protein combination similar to the assumed 

 primitive non-corpuscular chlorophyl-protein combination noted in certain 

 phanerogams. 



What chemical and functional relationships the hemoglobin bears to 

 the stroma, and hematin to the globin, are not known. But from the facts 

 that the exchange of O and C0 2 goes on quite rapidly in all forms of active 

 protoplasm, that isolated hematin, like chlorophyl, is absolutely inert in 

 relation to these gases, and that chlorophyl behaves in the nature of an 

 energizer in relation to the cytoplasm, it seems likely that hematin is of a 



