44 



PHYSIOLOGY OF MUSCLE AND NERVE 



tained. This connective-tissue reticulum serves as the highway for 

 the local blood-vessels and nerves. If one of these fibers is examined 

 in cross-section, it appears as a rounded area possessing a rather dark 

 granular center and a lighter non-differentiated outer zone, or sarco- 

 plasm. In longitudinal section, these fibers are 

 cylindrical in shape and rounded at their ends, 

 where they are joined with neighboring ones by 

 means of connective tissue. They do not branch 

 as a rule, but those of the tongue and skin divide 

 into finer filaments which are finally inserted in the 

 mucous membrane. 



Each fiber consists of a thin, hyaline sheath, or sarco- 

 lemma which fulfills the purpose of a cell membrane, and 

 should not be confounded with the more external connec- 

 tive-tissue envelope. These saccules are filled with soft 

 contractile protoplasm arranged in alternate discs of dark 

 and light substance. The former which is doubly refract- 

 ing, or anisotropic, forms the so-called transverse discs, and 

 the latter which is singly refracting, or isotropic, the so- 

 called lateral discs. In the middle of the clear band is seen 

 a very delicate dark line which has been regarded by Krause 

 as a dividing membrane to mark off definite segments, 

 called sarcomeres. In accordance with the preceding termi- 

 nology, these lines may be referred to as the intermediate 

 discs. Each fiber is provided with a number of nuclei which, 

 in mammals and birds, are situated directly below the sarco- 

 lemma and are embedded in a mass of sarcoplasm. Owing 

 largely to the transverse bands which recur in numbers of 

 close to 10,000 for each 1 cm. of distance, these muscle fibers 

 present a distinct striated appearance.^ 



These fibers also display a delicate longitudinal stria- 

 tion, for the reason that each fiber is really made up of a 

 number of extremely fine contractile filaments which are 

 arranged parallel to one another. They are known as the 

 primitive fibrillce or sarcostyles. These fibrillae are closely 

 packed together in sarcoplasm which unites them in turn 

 with the fibrillae of neighboring fibers. Hence, each striated 

 muscle fiber consists of fibrillae, sarcoplasm and sarcolemma. 

 A large number of fibers (2000) are bound together into 

 muscle-bundles which are separated from one another by 

 the epimysium, and many bundles into a muscle which is 

 enveloped externally by the perimysium. This arrange- 

 ment may be brought out most clearly in a muscle which 

 is copiously supplied with sarcoplasm, by hardening it in 

 alcohol. Naturally, each fibrilla presents alternate discs of 

 dark and light substance, the different fibrillae of the fibers 

 being arranged in such a way that their cross-bands come 

 to lie in practically the same horizontal plane. In this con- 

 nection it should be remembered that some of the higher vertebrates are in 

 possession of two types of striated muscle tissue which is either rich or poor in sarco- 

 plasm. In fact, certain animals, such as the rabbit and different fish, possess certain, 

 muscles which are composed of only one type of fibers and thus present either a dark 

 or a light appearance. The former are commonly designated as red (semitendi- 



^ Gutherz, Archiv fiir mikr. Anat., Ixxv, 1910. 



Fig. 14. — Mus- 

 cular Fibers of the 

 Adductor Magnus 

 OF A Dog. 



M, muscular fiber ; 

 n, nuclei; s, sarco- 

 lemma; ee, spaces left 

 by the retraction of 

 the muscular sub- 

 stance from the in- 

 terior of the sarco- 

 lemma. (Ranvier.) 



