196 



They increase in size ; their outline becomes indistinct ; a bright 

 border appears immediately within their margin ; their contents be- 

 come decidedly granular ; their outline is broken and interrupted ; 

 and presently an irregular cluster of granules is all that remains, and 

 these soon disappear. 



It sometimes happens that the nuclei perish while in contact, be- 

 fore the fibre elongates ; but the subsequent changes are the same. 



The striae generally first become visible at this period, imme- 

 diately within the margin of the fibre. 



The fibre is subsequently increased in size, and its development is 

 continued by means of the surrounding cytoblasts. These attach 

 themselves to its exterior, and then become invested by a layer of the 

 surrounding blastema. Thus, as it were, nodes are formed at inter- 

 vals on the surface of the fibre. These invested nuclei are at first 

 readily detached, but they soon become intimately connected and in- 

 definitely blended with the exterior of the fibre. All its characters 

 are soon acquired, the nuclei at the same time gradually sink into 

 its substance, and an ill-defined elevation, which soon disappears, is all 

 that remains. 



Lastly, the substance of the fibre becomes contracted and con- 

 densed. The diameter of a fibre towards, or at the close of intra- 

 uterine life, is considerably less than at a much earlier period. 



At the period of birth muscular fibres vary much in size. 



The several stages in the development of muscular fibre, above 

 mentioned, do not succeed each other as a simple consecutive series ; 

 on the contrary, two, or more, are generally progressing at the same 

 time. Nor does each commence at the same period in all cases. 



II. " On the General Integrals of the Equations of the Internal 

 Equilibrium of an Elastic Solid." By WILLIAM JOHN 

 MACQUORN RANKINE, Civil Engineer, F.R.SS. Lond. & 

 Edinb., &c. Received December 7, 1854. 



The First Section of this paper is introductory, containing a sum- 

 mary of principles already known respecting the elasticity of solids. 

 Those principles are treated as the consequences of the following 



