210 USES OF RED 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 
the 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. 
Eut if the blood be withdrawn from the living body, it soon 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 huffy coat, being composed almost exclu- 
