THE STRUCTURE OF VOLUNTARY MUSCLE 199 



muscles of cold-blooded animals, such as the frog. The choice of 

 skeletal muscles for this purpose is justified by the fact that a function 

 is most easily investigated in the organs in which it is most highly 

 developed. The choice of cold-blooded animals is guided by the 

 fact that it is possible to isolate the muscle from the rest of the bodv 

 and to study its reactions during a considerable time without the 

 research being interfered with by the death of the tissue. We may 

 therefore deal at length with the properties of the skeletal muscles, 

 pointing out incidentally in what respects the heart muscle and 

 involuntary muscle differ from the skeletal muscle. 



The voluntary or striated muscles form a large part of the body, 

 and are known as the flesh or meat. Each muscle is embedded in a 

 layer of connective tissue, and is made up of an aggregation of muscular 



FIG. 34. Muscular fibre of a mammal, examined fresh in serum, 

 highly magnified. (SCHAFEB.) 



fibres, which are united into bundles by means of areolar connective 

 tissue. The individual fibres vary much in length, and may be as 

 long as 4 or 5 cm. At each end of the muscle the fibres are firmly 

 united to tough bundles of white fibres, which form the tendon of 

 the muscle, and are attached as a rule to bones. Running in the 

 connective tissue framework of the muscle we find a number of blood- 

 vessels, capillaries and nerves. 



On examination of a living muscle, each fibre is seen to consist 

 of a series of alternate light and dark strive, arranged at right angles 

 to its long axis, and enclosed in a structureless sheath the sarco- 

 lernma. Lying under the sarcolemma are a number of oval nuclei 

 embedded in a small amount of granular protoplasm. In some animals 

 these nuclei occupy a central position in the fibre. Each band may 

 be considered to be made up of a number of prisms (sarcomeres) side 

 by side, with interstitial substance (sarcoplasm) between them. The 

 muscle prisms of adjacent discs are connected to form long columns 

 (primitive fibrillae, or sarcostyles). Each muscle prism is more trans- 

 parent at the two ends than in the middle, thus giving rise to the 

 appearance of light and dark striae. In the middle of the light band 

 is a line or row of dots (often appearing double), called Krause's 

 membrane. 



