356 



THE MUSCLES AND FASCIAE 



Structure of the Muscle Fibre. A muscle fibre is a long cylindrical cell varying from 

 one to five inches (2.5 to 12.5 cm.) in length. It is surrounded 'by a delicate sheath^called 

 the sarcolemma, within which lie the nuclei and muscle substance. The muscle substance 

 consists of tAvo elements the fibrillae, or contractile portion of the fibre, and the sarcoplasm, 

 or undifferentiated portion. The fibrillae are arranged parallel to one another and are sep- 

 arated by the sarcoplasm; and as the former respond well to the protoplasmic stains, and the 

 latter practically not at all, the alternation of such stained and unstained stripes produces the 



Isotrapic layer \', 



Nucleus 



Primitive fltril 



ji ii>i fiff .'tiriU jjj{) 

 /HnmmnwjJ! 1 .! 

 {'lijiiiitniiii.'-:'. 





Krn 'ise's membrane 



Uwmtum")! 



FIG. 281. Two human muscle fibres. 

 X 350. In the one the bundle of fibrillse 

 (6) is torn, and the sarcolemma (a) is 

 seen as an empty tube. 



FIG. 282. A bit of a cross-striated muscle of a frog, show- 

 ing the nucleus and the ease of its division both transversely 

 and longitudinally. X 650. (Szymonowicz and MacCallum.) 



longitudinal striations so typical of the appearance of a longitudinal section of muscle under 

 the microscope. In a cross-section the fibrillse are arranged more or less in groups called 

 Cohnheim's fields. 



The fibrilla?. or sarcostyle, are not unbroken threads, but all are interrupted at intervals into 

 short segments called sarcous elements. As a result of this segmentation a fibre exhibits alter- 

 nating transverse dark and light bands (cross-striations). If a muscle fibre be examined under 



high magnification, a light line is seen traversing each 

 dark band (anisotropic) ; this is the membrane of Hensen. 

 The light band (isotropic) is traversed by a dark line, 

 Dobie's line, or the membrane of Krause, that divides 

 the band into two parts, the lateral disks. A sarcomere 

 is that part of a fibril between two rnembranes of 

 Krause, and consists of a sarcous element and a lateral 

 disk at each end of the sarcous element. 



The nuclei are numerous and are situated peripherally, 

 i. e., just beneath the sarcolemma. They are narrow 

 and elongated in form, and respond readily to the usual 

 stains. Branched fibres are occasionally seen in the 

 tongue. There are tw r o kinds of fibres, red and 'icltifr; 

 the former are rich in sarcoplasm and the nuclei are 

 deeply placed and are intermediate in development be- 

 tween myoplasm and the white fibres. The white fibres are poor in sarcoplasm and predominate 

 in human muscles. The Trapezius muscle contains both red and white fibres. 



The arteries of voluntary muscles are numerous. They pierce the epimysiurn, pass along- 

 the septa from the epimysium, and divide into small branches, which enter between the 

 fasciculi. These small branches pass into capillaries which run parallel to the fibres. At 

 intervals dilatable connections (ampulla 1 ) are established between the capillaries, and it is into 



FIG. 283. Section of a muscular fibre, 

 showing areas of Cohnheim. Three nuclei 

 are seen lying close to the sarcolemma. 

 (Schafer.) 



