THE VITAL FUNCTIONS OF ANIMAL8. 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 1 . 



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 



1 MKM/K, All;/. An fit. s. 188, 189. 



2 ~2 



