SECRETION. 263 



business ; and vessels may as well be presumed to have also 

 carried the secreted fluid from the blood. Indeed, that the se- 

 cretion takes place in vessels, is proved, if an author is correct 

 in asserting that the formation of the new substance within 

 the vessels may be demonstrated " by forcing coloured injections 

 into the arteries of growing bones, when the lime is seen to issue 

 from their orifices in the form of a white powder, and deposit 

 itself, like the farina of a flower, for the office of consolidation. 

 In a similar way, the injected arteries of the common domestic 

 hen, while her eggs are incomplete, will show the deposition of 

 lime from their exhalant branches upon the membrane which af- 

 terwards becomes the shell." 



Some have thought no organic opening necessary, because the 

 changes in the blood of the lungs take place through membrane. 

 But the separation of carbonic acid is a mere physical or chemical 

 occurrence, not a secretion, and takes place equally in dead 

 blood, through dead membrane. The entrance of oxygen into 

 the blood might be equally alleged as an argument against the 

 existence of absorbent vessels on surfaces. 



Just as solids are not originally firm, nor exactly of the 

 nature they are when their texture is perfect, but the soft 

 substance, which is their elementary portion, hardens by subse- 

 quent changes ; so the fluids which pass from the blood-vessels 

 are probably more and more changed, till they ooze perfect 

 into the secreting canal. Even after this they become more con- 

 sistent, as may be seen in mucus which has lain upon a mucous 

 membrane, or the bile which has been in the gall-bladder. 



The difference between nutrition and secretion is, that, in the 

 former, the fluid does not pass away, but remains and coheres 

 to the solids, and, undergoing further changes, solidifies, and 

 becomes part of them. 



The secreting surface of a gland must be very extensive. The 

 blind extremities are of endless forms. Not only are some mere 

 cups or crypts; some longer, so as to be pouches; some longer 

 still, so as to resemble a portion of blind intestine ; some, again, 

 extremely long canals, of the same diameter throughout their 

 course 11 : but any of these may be single or aggregated, and 



B A View of the Structure, $c. of the Stomach, $c. By Thomas Hare, F.L.S. 

 London, 1821. p. 77. 



h " Malpighi, in works repeatedly quoted, and also in his Diss. de glandulis con- 

 globatis, Lond. 1689, 4to, (but consult especially his Opera Posthuma, ib. 1697, 



