i ORGANISATION AND STRUCTURE OF MUSCLE 27 



reversed ; the fibrils form the chief bulk of the fully-developed 

 muscle-fibre, while the remains of the formative plasma, together 

 with many nuclei, is interspersed between the mass of fibrils, in 

 varying bulk and disposition. Each fibre thus presents a long, 

 cylindrical, prismatic figure, with conical or blunted ends. The 

 fibres are usually undivided, but may branch more or less freely 

 with many nuclei, and even form an anastomosis. 



The multinuclear, striated muscle-fibres must be reckoned 

 among the largest cells with which we are acquainted. Accord- 

 ing to Ivolliker the uninuclear, spindle-shaped myoblasts of 

 the human embryo, seven to eight weeks old, are already 132 to 

 176 p. in length, and over 300 fi a little later. 



In adult muscle-fibres, Felix finds some that certainly exceed 

 12 cm. in length, to which we must add that the fibres, even 

 thus, were not at their greatest extension. The bulk is relatively 

 very small : it is greatest 

 at an early stage of develop- 

 ment. According to Felix, 

 human muscle-fibres at the 

 third month attain the con- 

 siderable diameter of 1 3 to 1 9 

 H : these dimensions are rare 



ill Older embryOS, and Only re- F IG- IT. Embryonic muscle-fibres. , Man ; b, Frog. 



appear in the new-born infant. 



The length of the single primitive fibril is in no regular ratio 

 with the length of the muscle developed from it. Whereas 

 formerly it was supposed that the muscle-fibres, generally speak- 

 ing, corresponded in length with the coarser muscle-bundles, 

 it is now known that numerous fibres, particularly in the 

 longer muscles, terminate freely, are shorter, i.e., than the whole 

 muscle. Both free ends accordingly may be pointed, and the 

 whole fibre spindle-shaped, or one end only may be free, and the 

 other blunt and connected with the tendon. In small muscles, 

 on the contrary, according to Ivolliker, all the fibres run the 

 length of the entire muscle, and are generally rounded off at both 

 ends. 



Nearly all cross -striated, uninuclear muscle-fibres (except- 

 ing only in certain Arthropoda) possess a distinct sheath, the 

 sarcolemma, which consists of a fine, transparent, structureless 

 membrane, lying next to, and closely investing the contents of, 



