THE STRUCTURE OF VOLUNTARY MUSCLE 179 



dentally 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 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 



WTO 



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

 highly magnified. (SCHAFEE.) 



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 striae, arranged at right angles to its long 

 axis, and enclosed in a structureless sheath the sarcolemma. 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 fibrillse, or sarcostyles). Each 

 muscle prism is more transparent 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. 



The development of this regular cross and longitudinal stria tion is 

 closely connected with the evolution and specialisation of the muscular 

 function, i.e. contraction. Contractility is among others a function of all 

 undifferentiated protoplasm. Undifferentiated cells, such as the amoeba, 

 can effect only slow and weak contractions. Directly a specialisation of 

 function is necessary and some cell or part of a, cell has to contract rapidly 

 in response to some stimulus from within or without, we find a differentiation 

 both of form and of internal structure. In many cases, as in the developing 

 muscle of the embryo or the adult muscles of many invertebrates, this 

 differentiation affects only part of the cell, so that while one part presents 

 the ordinary granular appearance, the other half is finely and longitudinally 





