210 USES OF EED CORPUSCLES LIQUOR SANGUINIS. 



carbonic acid which, it gives-off by the same channel, vary, 

 therefore, with the muscular exertion it makes. This variation 

 is most easily observed and measured in Insects ; and it is found 

 in them to be enormous ( 308). As, however, the blood of 

 the Invertebrata does not contain these red particles, to which 

 so important a function has been assigned, it may be asked, 

 how the conveyance of oxygen to their tissues is provided 

 for. The reply is very simple. In Insects, and other ARTI- 

 CULATA which have active powers of motion, the air is con- 

 veyed to the tissues, not through the medium of the blood, 

 but directly through air-tubes which convey it to every part 

 of the body ( 321). And in the MOLLUSCOUS classes, as 

 among the Crustacea also, the nervo-muscular system forms 

 so subordinate a part of the general mass of the body, and its 

 movements are so sluggish, that the quantity of oxygen which 

 $he fluid part of the blood conveys to them, is sufficient for their 

 need. 



236. Of the properties of the Liquor Sanguinis, whilst it 

 is circulating in the vessels, the microscope tells us nothing ; 

 since it constantly remains in the state of a transparent fluid. 

 But if the blood be withdrawn from the living body, it soon 

 undergoes a very curious and important change. A large 

 portion of it passes into the solid state, forming the crassa- 

 mentum or clot ; whilst there remains a transparent liquid of 

 a yellowish hue, which is termed the serum. It is evident 

 that the clot contains all the red particles ; but it is easily 

 proved that its coagulation is not due to them. For the blood 

 of a Frog, or of any other animal having blood-discs suffi- 

 ciently large, may be caused to pass through filtering-paper, 

 which will retain and collect its blood-discs, allowing the 

 liquor sanguinis to flow through it ; and this fluid will coagu- 

 late just as completely as if these particles were retained in 

 it. Again, in certain conditions of the blood (generally result- 

 ing from disease), even when the coagulation is allowed, to 

 take place in the ordinary manner, the fibrin and the red 

 particles separate from one another, the latter gradually 

 subsiding, whilst the former are left at the surface ; and the 

 upper part of the clot is then nearly colourless, exhibiting 

 what is commonly known as the huffy coat or crust ; whilst 

 the lower part of it includes the red particles, and has a very 

 deep colour. The buffy coat, being composed almost exclu- 



