ANATOMY AND PHYSIOLOGY. . 147 
become smaller and are more closely united to each 
other. And the transverse markings so characteristic 
of voluntary muscle fibre become much less distinct. 
The intention of these triangular straps is to com- 
press the vessel at successive intervals, and thus, 
by the aid of the valves, to cause the blood to flow 
in one direction. The long string which we have 
withdrawn from the thorax is simply a tube, with no 
further means of propelling the blood, or of directmg 
the current, than what the muscles between which it 
passes may supply. It carries the blood through the 
thorax into the head, and there it divides into two or 
more terminal branches. 
Further than this we shall not be able to follow this 
system of vessels; and the channels by which the 
blood is returned to the heart are quite beyond the 
range of ordinary observation. For the investiga- 
tion is one of exceeding difficulty. The arrange- 
ment of the blood-vessels in insects, and indeed the 
fact of insects having any regular circulation at all, 
have eluded discovery till quite recent times, Hunter,* 
indeed, described the insect circulation in language 
which needs only extending, not correcting, to bring 
the description up to the level of the knowledge of 
the present day. But the evidence of the fact, appa- 
rent to him, was otherwise explained by Cuvier, and 
not till 1827 was the arrangement of the blood- 
vessels in insects fairly demonstrated by Carus.t 
It is reserved for only the most patient and de- 
voted students of insect anatomy to trace in detail 
all the channels by which the blood comes and goes. 
* Hunter. ‘ Posthumous Works.’ Vol. I, p. 107. 
+ See Owen, ‘ Lectures on the Invertebrate Animals,’ p. 222. 
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