Histology of the Frog 115 



thicker layer of sarcoplasm than is found between the fibrillae. The 

 cross striation is due to the fact that the fibrillae really consist of seg- 

 ments or sarcomeres ( )• The segments are 

 separated from each other by a very fine dark line, called Krause's 

 membrane. This membrane extends not only across the individual 

 fibrillae but across the entire sarcoplasm between the fibrillae of the 

 fiber. Krause's membrane is bordered on each side by a more or less 

 clear and lightly stained band formed by the ends of the two adjoining 

 segments. The middle portion of each segment forms a so-called dark 

 band, and across the center of this band there extends a second very 

 delicate membrane known as the line of Hensen. Should the muscle 

 fiber be cut transversely, the cut ends of the muscle columns present a 

 number of polygoneal areas, known as Cohnheim's fields. The spaces 

 between the fields are filled with sarcoplasm, and the dotted appearance 

 is due to the cut ends of the tiny individual fibrillae. 



The muscle fibers of the heart are different from either the striated 

 or unstriated fibers, although heart muscle does present cross striations, 

 and, as in ordinary striated muscle, each fiber also possesses more than 

 one nucleus. Further, every heart muscle cell has branches which con- 

 nect with other branches, thus forming a continuous network, called a 

 syncitium ( ). (A syncitium represents a group of 



cells whose separating walls or membranes have been lost, reabsorbed, 

 or failed to form.) 



4. Nerve tissue (Fig. 42) is made up of nerve fibers and ganglion 

 cells ( ). A nerve cell, together with all of its 



processes, is called a neuron. Each nerve is made up of a bundle of fibers 

 held together by connective tissue and surrounded by a common sheath. 

 The central strand of a nerve fiber is called the axis cylinder. About 

 this is found the medullary sheath ( ), (also called 



the white substance of Schwann), then a delicate external membrane 

 called the neurilemma or sheath of Schwann. 



There are various constrictions to be seen in any long nerve. These 

 are known as the nodes of Ranvier. It is at these nodes that the white 

 substance is interrupted although the axis cylinder and neurilemma 

 continue. 



The nuclei surrounded by a small amount of protoplasm are found 

 immediately beneath the neurilemma. There are also various oblique 

 markings across the medullary sheath between the nodes of Ranvier 

 known as incisures of Schmidt. The axis cylinder of a nerve is merely a 

 continuation of a ganglion cell, being made up of very fine fibrillae, with 

 an intervening fluid substance. The white or medullary substance con- 

 tains a large amount of fatty material, called myelin ( ). 

 This sheath is supposed to act somewhat as an insulator. 



Nerve fibers and muscle fibers develop differently. The former are 

 a composite structure formed of cellular elements which originate in 

 various ways. For example, the nerve sheaths, though coming in contact 



