229 



occurs, or the seat of it? We can on this question propose only 

 tJae three following alternatives: 1. the conversion is produced 

 by a function of the extreme arteries; 2. it is produced by one 

 belonging to the origins of the veins; or, 3. it is produced by a 

 function of the secerning extremities of the arteries. 



18. 1. That venous blood is not produced from the extreme 

 arteries appears sufficiently obvious from the facts, 1st, that the 

 arteries are no where observed to possess the properties of ac- 

 complishing the conversion in question; and, 2nd, that the ex- 

 treme arteries, so far as they can be traced, contain that blood 

 denominated arterial. 



19. 2. That the change is not accomplished in the beginning 

 of veins, it appears warrantable to infer, 1st, from the analogy of 

 the small to the large veins; and, 2nd, from the fact that veins 

 will circulate florid or arterial blood without changing its dis- 

 tinctive properties and appearance; as in the instance of the 

 pulmonary veins. 



20. 3. That venous blood is produced by a function of the 

 secerning extremities of the arteries, it appears warrantable to con- 

 clude, 1st, from those grounds of rejection of the only other al- 

 ternatives which suggest themselves; and, 2nd, from the agree- 

 ment which this supposed fact has with the process of nutrition 

 before expressed. This agreement will appear from the following 

 exhibition of particulars: 



21. The material of nutrition, separated from blood by the 

 secerning extremities of the arteries. This material, having many 

 constituents, appears to consist of coagulable lymph, possessing this 

 most obviously ; this lymph containing the matters of which the 

 structures are composed; this lymph containing also the oxygen 

 of arterial blood, the relation of which is with the Jiving spirit, or 

 with the process of assimilation; this lymph thus furnishing to the 

 living powers and constitution their elements, said to be contained 

 in, or derived from, earth and air: arterial blood thus deprived of 

 its oxygen which passes with a new medium to the places where 

 its nutrient relations exist,, is converted into venous blood. 



22. Venous blood also contains lymph, most probably in con- 

 siderable abundance, as we are informed by analysis, and as we 

 may guess from the appearance of venous blood abstracted in 

 fever or inflammation: but venous blood will not maintain life; 

 for although it might have lymph and all the other material con- 

 stituents, in being venous blood, it is deprived of its oxygen ; to say 

 nothing of other properties which, though less obvious, may never- 

 theless be supposed. 



23. The preceding theory of the conversion of arterial into 

 venous blood, is in agreement with all that is either perceptible or 

 inferred upon strong grounds in this stage of the circulation ; and, 

 in agreement with the theory itself, it is uecessnry to suppose the 

 arrangement of the circulating organs, &c. in the seat of this con- 

 version, to be such as may be briefly illustrated in the following way : 



