154 
ANATOMY AND PHYSIOLOGY 
case without a parallel in the animal economy, for in 
the membranes of a developing embryo, the blood 
originally flows without vessels. 
With regard to the blood itself, it generally appears 
quite colourless, or slightly tinged with green. Its 
motion can be discerned only by means of the globules 
which it contains, although these, also, are more or 
less transparent. The globules are very minute, but 
in this respect they vary much. Those of the cater- 
pillar of the goat-moth are described by Lyonnet as 
three millions of times smaller than a grain of sand ; 
while those of Agrion Puella , according to Carus, are 
larger than the globules of the human blood. Their 
general dimensions may be stated at from the 200th 
to the 250th part of a line in diameter. 
The mechanism of the dorsal vessel, and the man- 
ner in which it works, were first rightly understood 
by Dr. Carus of Dresden, who made his discoveries 
known to the public in 1826. Much additional light 
was shortly afterwards thrown on the subject by M. 
Strauss, in a work published at Paris in 1 828.* Tho 
earlier anatomists had observed its pulsations, and 
investigated its structure with much care, but were 
unable to come to any satisfactory conclusion re- 
garding its functions, chiefly from the circumstance of 
supposing it to have no opening in any part. Lyonnet 
conceived it to be designed for the purpose of secret- 
ing a substance for forming the nerves. Cuvier, also, 
after a pains-taking investigation, concluded that it 
* Considerations generates sur l ’ anatomie des animaux arti - 
cules, $c. 
