96 HAND-BOOK OF PHYSIOLOGY. 



muscle except that of the rectum. It has also been found in Mollusca in 

 certain muscles which are specially active, viz., those which work the rasp- 

 like tongue. 



In the muscles of Fish it has hitherto only been met with in the very 

 active muscle which moves the dorsal fin of the Hippocampus (Ray Lan- 

 kester). 



The Carbon Dioxide Gas in the Blood. Of this gas in the 

 blood, part exists in a state of simple solution in the serum, and the rest 

 in a state of weak chemical combination. It is believed that the latter 

 is combined with the sodium carbonate in a condition of bicarbonate. 

 Some observers consider that part of the gas is associated with the cor- 

 puscles. 



The Nitrogen in the Blood. It is believed that the whole of the 

 small quantity of the nitrogen contained in the blood is simply dissolved 

 in the fluid plasma. 



DEVELOPMENT OF THE BLOOD. 



The first formed blood-corpuscles of the human embryo differ much 

 in their general characters from those which belong to the later periods 



Fift. 86. Part of the network of developing blood-vessels in the vascular area of a guinea-pig. 

 U, blood corpuscles becoming free in an enlarged and hollowed out part of the network; or, process 

 of protoplasm. (E. A. Schafer.) 



of intra-uterine, and to all periods of extra-uterine life. Their manner of 

 origin is at first very simple. 



Surrounding the early embryo is a circular area, called the vascular 

 area, in which the first rudiments of the blood-vessels and blood-corpuscles 

 are developed. Here the nucleated embryonal cells of the mesoblast, from 

 which the blood-vessels and corpuscles are to be formed, send out processes 

 in various directions, and these joining together, form an irregular 

 meshwork. The nuclei increase in number, and collect chiefly in the 

 larger masses of protoplasm, but partly also in the processes. These 

 nuclei gather around them a certain amount of the protoplasm, and be- 



