THE BLOOD. 61 



it does not acquire a crust or size. 1 May not this be explained 

 from what is observed in the case related in Experiment xxvn ? 

 that is, in such instances the vessels, either from a febrile or 

 from some other oppression, act more weakly than they do in 

 the ordinary cases of inflammation ; by which means the lymph 

 is not sufficiently attenuated to allow the red globules to sub- 

 side before the coagulation begins, and therefore the size does 

 not appear, as in other cases of inflammation where there is no 

 such oppression. 



As air is found to coagulate the blood and cold to thicken 

 it, an objection has thence been made to the conclusions from 

 some of the preceding experiments ; and it has been supposed 

 that the changes in the properties of the blood, that happen 

 during the time of bleeding (which I have attributed to a dif- 

 ference in the action of the vessels), might possibly be owing 

 merely to a difference in the exposition to the air or to cold (see 

 Notes xi and xn). For instance, since the blood that trickles 

 down the arm seems to be more cooled than that which flows in a 

 full stream, it has thence been supposed that its want of size, in 

 those cases, might be owing to the exposition to the air, which 

 made it coagulate sooner, and to the cooling which had thick- 

 ened it, and thereby prevented its red particles from subsiding, so 

 that the size should be formed. This objection is indeed plausible, 

 and to those who have not seen these experiments might at first 

 seem sufficient to explain the appearance ; but upon further ex- 

 amination it will not be found to do it satisfactorily. Thus, for 

 example, although it be true that air coagulates the blood, and 

 likewise cools it, yet there are changes remarked in the preceding 

 experiments that cannot be explained merely by a difference in 

 the exposition to the air : thus, in Experiment xxvn, the blood 



1 It may be necessary to observe, that it is not in every case where the blood 

 trickles down the arm that it is without a size ; on the contrary, it sometimes happens, 

 that even in such circumstances it has a very dense one ; an instance of which may 

 be seen below, in Experiment xxxi. In those cases the trickling down the arm may 

 perhaps be owing to some circumstance in the orifice preventing its flowing in a full 

 stream, or to a difference in the tightness of the ligature, rather than to a weak 

 action of the vessels. Or, although the size be occasioned by a strong (or some par- 

 ticular mode of) action of the vessels, and therefore is removed by weakening them, 

 yet it may not always be removed immediately on their being weakened. For it may 

 happen, that in some cases the lymph may not be so susceptible of changes as in 

 others ; or when it has been very much attenuated it may not again be thickened 

 immediately, on the vessels acting weakly. 



