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.* The 

 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 P anatomic des animaux arti* 

 cults, c. 



