74 THE AMERICAN MONTHLY [March 



connective tissue to some firm part ui)ou wliicli they may 

 act (Fig. 1, 2), The wliole muscle is enclosed within a 

 connective tissue sheath. Kacli muscle is divided into 

 bundles called fasicles, which are also surrounded by 

 sheaths of connective tissue. The fasicles again are 

 divided into individual fibres, which are separated hy 

 very similar connective tissue sheaths from each other. 



These fibres, the structural elements, are elongated 

 transversely striated cells, or rather fibres, composed of 

 two distinct parts, the sarcolemna and the sarcous sub- 

 stance. The sarcolemna is a thin transparent and elastic 

 sheath. The sarcous substance is a semi-fluid with the 

 appearance of alternate dark and light l)and.-> and also 

 of longitudinal striations. This substance is the essen- 

 tial part of a muscle fibre. It is not yet certain whether 

 the covering or sarcolemna fits over this sarcous sub- 

 stance like a glove finger or whether it is connected 

 structurally with it. 



Very complex theories have been proposed concerning 

 the ultimate structure of the muscle fibre; the simplest 

 and that most in harmony with the probable structure of 

 other cells is as follows: The protoplasm of the fibre is 

 composed of a network of threads. This network, in- 

 stead of being arranged irregularly as in most cells, is 

 arranged regularly in ktngitudinal and transverse threads 

 (Fig. 3). These threads cross each other at right angles 

 and at points of crossing, enlargements like heads are 

 formed. The enlargements optically run together across 

 the fibre, making the dark bands, the thin parts between 

 appearing to form light bands. Owing to the fact that 

 the longitudinal threads are stronger than the transverse 

 threads, there is a tendency for the fibres to break up 

 into longitudinal elements known as fibriles. 



In a transverse section each muscle fibre shows a divi- 

 sion into a number of small polygonal areas, known as 

 Cohnheim's areas. These are composed of bundles of 



