i 5 8 



MUSCULAR SYSTEM. 



[sect. 86. 



 



Fie .27. 



ing to my observation on certain poisons, the rigor mortis is quite independent 

 of the nerves of the muscles, and is owing to a peculiar alteration of the 

 molecules of the muscles, which may be produced by different causes (Virch., 

 Archiv., 1856). 



§ 86. Development of Muscles and Tendons. — In man. the muscles 

 first become distinct at the end of the second month of intra- 

 uterine life, but are then recognisable only with the aid of the 

 microscope ; and being soft, pale, and gelatinous, cannot be distin- 

 guished from their tendons. In the tenth or eleventh week, they 

 can be recognised more distinctly, particularly in specimens pre- 

 served in alcohol ; and the tendons also make their appearance as 

 somewhat lighter, but likewise transparent "streaks. In the fourth 

 month, the muscles and tendons are still more distinct; the 

 former, on the trunk, being slightly reddish; the 

 latter, greyish, and less transparent, but both still 

 soft. From this period onwards, both structures 

 more and more assume the condition which they are 

 afterwards destined to have; so that, in the mature 

 foetus, they present no difference worthy of note, 

 except that the muscles are softer and paler, and the 

 tendons are vascular, and not so white. With regard to 

 their intimate conditions, the fibres in the embryo, at 

 the end of the second month, are elongated bands 

 (fig. 72), from 0001'" to o - oo2'" broad, with nodular 

 enlargements at different points, in which places 

 they are provided with elongated nuclei. These 

 bands appear either homogeneous, or finely-granu- 

 lated, and only seldom present a very slight indica- 

 tion of transverse striation. These muscular fibres, 

 according to my observation, arise each from a 

 single cell, which increases considerably in length. 

 In the further progress of development, they become 

 broader and longer, and their contents — the origi- 

 nal cell-contents — are transformed into the muscular 

 fibrils. In the fourth month (fig. 73), the majority 

 measure croo28'" to 0005'" in diameter, some even 

 0006"'; whilst others do not exceed o - ooi6"' to 

 0002'". The larger fibres are still flattened, but uniformly broad, 

 and considerably thicker than before, and, for the most part, 

 with distinct longitudinal and transverse striae ; or they even 

 contain fibrils, capable of being isolated. Even in longitudinal, 

 but still better in transverse sections, it may be seen that 



Muscular fibres 

 of a human em- 

 bryo of the 8th 

 to Oth week, 350 

 times magnified ; 

 without transvers e 

 striae ; a. nuclei. 



