TISSUES. 



89 



of muscle. In tough meat the connective-tissue 

 element is extensively developed and the fasciculi 

 are large and coarse. 



Each muscle fiber has a delicate, transparent, 

 smooth cell wall called the sarcolemma. The oval 

 distinct nuclei lie immediately beneath the sarco- 

 lemma in higher vertebrates, but in lower forms and 

 in all embryos the nuclei lie deeper in the muscle 

 protoplasm. These nuclei have the same structure 

 as the nuclei of any other tissue, but the cytoplasm 

 shows a distinct and regular cross and longitudinal 

 striation, characteristic of only one other tissue 

 the cardiac muscle. The 

 longitudinal striation is 

 due to the presence of 

 delicate fibrillae called 

 sarcostyles, which is an- 

 alogous to the spongio- 

 plasm of other cells. A 

 more homogeneous and 

 fluid substance intervenes 

 between the sarcostyles, 

 called sar co plasm, and is 

 in turn analogous to the hyaloplasm of other cells. 

 The sarcostyles are not uniformly or evenly dis- 

 tributed in each muscle fiber, but are grouped into 

 bundles. In cross sections the fiber has therefore 

 a honeycomb structure, the minute areas being 

 known as Cohnheim's fields. A single Cohnheim 

 field represents the cut ends of a single bundle of 

 fibrils or sarcostyles. 



The cross striation is intricate and therefore more 

 difficult to explain. This striation consists of alter- 



Cohnheim's 

 area, a bundle 

 of sarcostyles. 



Sarcoplasm. 



Muscle nucleus. 



Fig. 56. Cross section of three 

 voluntary muscle fibers. 



