96 COMPARATIVE ANATOMY 



according to the muscle from which they are taken, and each 

 consists of a number of fibres varying from 'i to *oi mm. in 

 diameter and 250 to 350 mm. in length in the human subject. 

 Each muscle-fibre is invested by a transparent, homogeneous, 

 elastic membrane, called the sarcolemma, which is particularly 

 thick and strong in fishes and Amphibia. The individual 

 fibres are not as long as the bundle which they compose, but 

 end with tapering extremities, which cohere with neighbouring 

 fibres, or at the ends of a muscle may be affixed to a tendon. 

 Muscle fibres rarely branch, but an exception is found in the 

 muscles of the frog's tongue, and also in the tongues and facial 

 muscles of mammals. 



The sarcolemma may be regarded as a tube whose contents 

 are the muscular substance. This substance is of soft, semi- 

 fluid consistency during life, but after death it becomes 

 coagulated and firm. The most characteristic thing in the 

 muscle-substance is its cross-striation. When viewed under 

 the microscope each fibre exhibits a number of parallel cross 

 stripes alternately light and dark. The fibre further exhibits 

 a fine longitudinal striation, and certain methods enable us to 

 break it up into a number of fine longitudinal strands, which 

 run from end to end of the fibre, and are called sarcostyles. 

 They are prismatic in section, and are separated from one 

 another by a more fluid substance known as sarcoplasm, so 

 that a transverse section of a fibre gives the appearance of a 

 number of polygonal areas separated by lines representing the 

 intervening sarcoplasm. Each isolated sarcostyle exhibits the 

 same cross-striations as the whole muscle-fibre, and this 

 appearance is due to the fact that the sarcostyle itself is made 

 up of a number of segments, called sarcomeres, separated from 

 one another by fine membranes, called the membranes of 

 Krause, which in optical section appear as fine lines running 

 across the middle of every bright band. An individual 

 sarcomere consists of a median darker portion, which is called 

 the sarcous element, and hyaline extremities abutting on the 

 membranes of Krause. When the muscle-fibre is fully 

 extended, the sarcous element divides into two portions, leav- 

 ing a clear cross stria between them known as the line of 

 Hensen. W T hen the muscle-fibre is contracted the lines of 

 Hensen become obliterated by the apposition of the halves of 

 the sarcous elements, and the whole sarcomere being shortened, 



