46 MEMOIR OF BARON HALLER. 
duced by the action of the heart : this net-work 
commences as it were by points ; these points soon 
become threads, which ere long are coloured, and 
turn out to be arteries and veins, dividing at very 
small angles. These angles enlarge ; whitish coloured 
spaces appear between the vessels, which, with time, 
dilate exactly like the spaces betwixt the fibres of 
leaves. In retracing the successive changes of this 
membrane, it would evidently appear that it had 
always existed, as also its vessels ; that it had ex- 
panded upon itself ; that the impulse of hlood had 
prolonged the arteries, or divided its folds ; that it 
had elongated the vessels from each other, and given 
to the membrane its length and breadth, its colour- 
less spaces, and even its solidity. I regard this 
example as instructive, and calculated to exhibit 
the shades by which a soft and semi-fluid substance 
can pass into a state wholly different from its first 
condition, by simple evolution. 
Regarding solidity , we have only to trace the 
successive increase of the lungs and other internal 
viscera, of the flesh, bones, &c. to perceive the steps 
by which a true fluid may become viscid, may then 
harden by insensible degrees, and this without the 
mixture of any new parts. All these portions of 
the young animal are produced from a fluid, appa- 
rently organized, they then become consistent, and 
gradually acquire well defined limits. We need 
not here dwell on the causes of these changes. We 
may simply remark, that a simple diminution of 
the fluid parts, the effect of the dilatation of the 
