114 GENERAL BIOLOGY 



ments or sarcomeres ( ). The segments are sepa- 



rated 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 trans- 

 versely, the cut ends of the muscle columns present a number of polygo- 

 neal 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. Heart muscle presents cross striations, although, 

 contrary to the ordinary striated muscle, each fiber 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, resorbed, 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 pro- 



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

 held together by connective nerve 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 mem- 

 brane called the neurilemma or sheath of Schwann. 



There are various constrictions to be seen in any long nerve known 

 as the nodes of Ranvier. This is where the white substance is inter- 

 rupted 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 being 

 a composite structure formed of cellular elements that originate in vari- 

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

 with, and surrounding the axis cylinder, have a totally different origin 

 from. the cylinder. It is interesting to know that in its development the 



