532 PHYSIOLOGY. 



common breadth in human muscle, yet they are much narrower in 

 different parts; so that there may be twice as many bands existing 

 in the space just mentioned. This striation is found in all muscles 

 attached to the skeleton, in the heart, pharynx, upper oesophagus, 

 diaphragm, urethral sphincter, external anal sphincter, as well as 

 in the muscles of the middle ear. 



When a muscle is deeply focused, the appearance of the striae 

 is somewhat altered; a finely dotted line is seen to pass across the 

 middle of each light band. This is supposed to represent Krause's 

 membrane stretching across the fiber and attached to the surface of 

 the sarcolemma. However, there is reason to believe that the ap- 

 pearance of a dotted line in this position in the fresh fiber is due to 

 the peculiar optical condition of the tissue. 



A fine, clear line is sometimes seen in the middle of each dark 

 band, and is known as the line, or disc of Hensen. 



Since there seems to be such variance as to muscle-structure and 

 so many different names are met with in text-books, it might be well 

 to call the student's attention to the fact that Dobie's line, Amici's 

 line, and Krause's membrane are terms used to describe the same 

 condition. They designate the dark line bisecting the white band. 

 Hensen's band occurs in the dark bands. 



In addition to the cross-striping, the fiber of the muscle has 

 longitudinal striation. When a muscle has been very carefully teased 

 with fine needles after having been previously hardened in spirits, 

 an interesting result follows. The muscle-fibers break up into fine, 

 longitudinal elements of a rounded or angular section which run from 

 end to end of the fiber. These have been very aptly termed muscle- 

 columns., or sarcostyles. 



Each sarcostyle appears to consist of a row of elongated pris- 

 matic particles with clear intervals. These particles are termed 

 sarcous elements. The sarcostyles in some muscles are striated longi- 

 tudinally. This appearance has led some authors to believe that they 

 are composed of still finer elements, or fibrils. 



Under some conditions, the fibers show a tendency to cleave 

 across in a direction parallel to the bands, and even to break up into 

 transverse plates, or. discs. The latter are made up by the lateral 

 cohesion of the sarcous elements of adjacent sarcostyles. To the for- 

 mation of such discs, therefore, every sarcostyle furnishes a particle, 

 which coheres with its neighbors on each side, and this with perfect 

 regularity. 



Sarcoplasm is the intercolumnar substance by which the sarco- 



