THE VITAL FUNCTIONS OF ANIMALS. 19 
a cell that has become enclosed by a wall or envelope of later forma- 
tion. Muscles, according to SCHWANN, consist at first of nucleated 
cells which range themselves in a row; the nuclei adhere to the wall, 
and within the tube (of the primitive bundle) are formed the proper 
primitive fibres. According to VALENTIN and HENLE, on the other 
hand, the primitive fibres are arranged around the row of cells 
which occupies the middle of the primitive bundle, and the external 
covering of this bundle is a sheath formed afterwards. But these 
and other diverging views we cannot here develope more minutely. 
If once the fundamental truth of ScHWANN’s doctrine be ac- 
cepted, that cells are the original form of animal and vegetable 
tissues, then is it of subordinate importance whether this or that 
view in the case of particular tissues be adopted, and we may sup- 
pose, as, for example, in parts which are formed of plates in which 
there is no distinction of wall and cavity, that the cells have not 
been perfectly formed from the amorphous blastema, but were 
joined together before they possessed a cavity}. 
We must here add a word concerning the blood-corpuscles. They 
are flat vesicles, filled with the colouring matter of the blood: 
having in mammalia a round, in birds, reptiles, and most fishes, an 
oval outline. In man, the mean diameter is about = millim. In 
reptiles, especially in those without scales, they are larger. In the 
frog, for instance, they have the length of three and the breadth of 
two human blood-corpuscles. Here a nucleus is present, of which the 
existence In mammalia is doubted by some writers. The blood- 
corpuscles, therefore, are cells: and we may consider the fluid, so 
rich in albumen and fibrin, in which they swim and with which, 
during life, they circulate (liquor sanguinis), as a liquid intercellular 
substance of the blood-cells, 
The Vital Functions of Animals. 
In order to complete the general idea which we ought to form 
of the animal body, we must not stop at the membranes, but must 
also look at the structure of the principal organs. We unite organs 
' Henun, Allg, Anat. s. 188, 1809. 
