262 SECRETION. 



substance of the sides of secreting canals that secretes, still it must 

 secrete from fluid poured into it from blood-vessels. The contents 

 of these vessels is blood. But. it is not likely that blood oozes 

 through the sides of its vessels, nor that the substance of a secret- 

 ing canal has blood diffused in it. It is more likely that minute twigs 

 arise from blood-vessels, and, being of a peculiar nature, admit 

 only certain parts of the blood, which they change and transmit 

 through open mouths into the secretory substance of the canal ; or, 

 that living openings exist in their sides that will permit the exit 

 of only these certain portions of the blood, and these portions are 

 poured forth into the substance of the sides of the secreting canal 

 to be farther elaborated. But, on either supposition, the opening of 

 the blood-vessel into its twig, or, if there be no twig, into the 

 substance of the sides of the secreting canal, must have a living 

 margin. Dr. Mueller, however, contends that the minutest streams 

 of blood are contained in solids scarcely more dense than the 

 blood itself, in boundaries which are not vessels, but mere fur- 

 rows, and so slightly different from the fluid blood, that this freely 

 mingles with them and is changed into them or various new pro- 

 ducts. Unquestionably the minutest parts, and those which are 

 the fundamental portion of the rest, are, like the embryo frame, 

 of exquisite delicacy and softness. If we remark that the 

 smallest artery of fins and webs may, under the microscope, be 

 seen terminating in veins, and giving off minute vessels which run 

 to veins, not colliquescing into a pulpy substance ; we receive 

 this reply, that the minutest arteries must be pulpy enough 

 to allow the blood to mingle with their substance. Yet the 

 effect of emotions and certain articles is more explicable on 

 the idea of organic openings and canals. 



If Dr. Mueller is correct in supposing that the substance of 

 the tubes secretes from Hood poured into them, still I would 

 contend that the blood passes into them through organic open- 

 ings in the blood-vessels ; and the fluid produced cannot pass from 

 the substance through inorganic pores, because, being mixed with 

 blood, or what is left of the blood, inorganic pores would transmit 

 both. Again, what is left of the blood, after the separation of the 

 new fluid from it, must be taken back, and we cannot suppose it to 

 pass again through inorganic pores into the blood-vessels. Open- 

 ings in them must have a power of selection, or the secreted fluid 

 would equally pass back ; and, if absorbents take up what is left, 

 not mere organic openings but vessels are brought into play in the 



