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fi 
506 Remarks on the Morphology of Arteries, [May, 
their contour or by means of processes which they throw out, so 
that there results a net-work, at first indistinct but gradually in- 
creasing in clearness and color as the cells enlarge. 
Each of these blood-islands then undergoes vacuolation, a por 
tion of the protoplasmic contents becoming liquified and leaving 
a cavity. By a budding process new cells are formed in the in- 
terior of the mother-cells and becoming detached float free in the 
fluid which fills the vacuole. At this period, therefore, the blood- 
islands present an outside cell-wall with a contained fluid in 
which float free cells or corpuscles, the whole arranged in a close 
mesh-work. 
Next the cell-walls wherever in contact thin away and disap- 
pear, there resulting a tube the walls of which are the original 
cell-walls of the blood-islands, the contents a fluid, plasma, m 
which swim free blood-corpuscles. There is at first no special 
difference in size among the vessels thus formed, nor is there any 
structural difference by which we can distinguish arteries from 
veins. No trunks or branches can as yet be made out, itisim 
fact a capillary plexus that appears, all vessels lying on the same 
plane and communicating equally with each other. ni 
But a difference soon begins to be manifest. The rapidity of 
growth varies greatly. Along certain lines the vessels begin s 
increase in size so that soon there is visible distinction of capil- 
laries, branches and trunks. This process of capillary and trun 
formation extends from without inward, attains the PIOnsy b 
of the embryo, finally reaching the rudimentary vesicle whi 
represents the heart. (Plate VIII.) g Ak 
It should be noted that the development is centripetal. No 
ing is more natural than to look upon the arteries as a system 
proceeding centrifugally from the heart outward. 
However convenient this may be to the physio 
surgeon, to the anatomist it embodies a fallacy. $ 
are the first formed, next the arterioles, then the branches a 
larger size, finally the trunks. It is owing to the subtle p ` 
of this fallacy that the study of the arterial system has a 
no farther. along 
I have mentioned that the rapidity of growth is gee aa 
certain lines, thus leading to the formation of trunks. It ge 
ceivable that these trunk-lines should be intermediate g a 
_ tion, but in fact they usually become established in certain his? 
~ _ situations. What can be ascertained as to the causes for u7 
