352 THE SOLIDS. 



In the first, isolated cells, arranged in linear series, unite to 

 form the unstriped fibrilla, the nuclei remaining. 



This stage appertains to all unstriped muscular fibre. 



In the second, the transverse stria3 or markings appear 

 upon the fibrillse, the nuclei still remaining. 



This stage is permanently exemplified in the muscles of the 

 heart, temporally in the voluntary muscles of the foetus, and 

 probably also in some few other muscles. 



In the third period, the fibrilla become very slender, the 

 transverse markings more defined, and the nuclei altogether 

 disappear. 



This condition is represented in all the fully developed 

 striped muscles of animal life. 



But the striped voluntary muscular fibre, even of the adult, 

 constantly exhibits and is constantly passing through the 

 three stages just described ; a fact not generally known. It 

 has been ascertained, indeed, since the time of Valentin, that 

 the striped muscular fibre of the foetus originates in cells, 

 and also that cell nuclei are contained in each fibre of the 

 adult ; but it has not been perceived that the fibrillse also pro- 

 ceed from cells, and that the stage of muscular development, 

 that of unstriped muscular fibre likewise exists in the vo- 

 luntary muscles of both the foetus and the adult, 



It has been supposed, as we have already seen, that the 

 nuclei which are met with in the striped muscular fibre are 

 scattered throughout its entire thickness : this has been shown 

 to be erroneous, and also that the nuclei are situated only on 

 the exterior of each fibre. 



Now in every adult striped muscular fibre, we find the 

 nuclei under the following circumstances : some few of them 

 are in a free state, others more numerous are contained in 

 fibrilla, both striped and unstriped : of these fibrillas, the 

 majority are on the inside of the sarcolemma ; but some of 

 them are also on its exterior, adhering to its surface, and con- 

 stituting a considerable part of its substance : lastly, internal 

 to these nucleated fibrilla other fibrilla?, which form the chief 

 bulk of each fibre exist destitute of nuclei. 



With a good defining object-glass many of these unstriped 



