NERVOUS TISSUE. 107 



seen in cross-section in Fig. 79, where they have a semilunar 

 form ; while the Schwann's sheath is continuous around the 

 whole fibre. In higher animals only one nucleus is found in 

 each segment of Ranvier. 



Many authors believe that the Schwann's sheath is inter- 

 rupted at each node of Ranvier, and joins on each side of the 

 node with the axis cylinder, instead of being so connected by 

 cement substance. Others go still farther, and claim that the 

 sheath of Schwann is continuous at the nodes of Ranvier with 



FIG. 79. 



i's corpuscle 



Axis cylinder 



Nucleus of connective 

 tissue cell 



Medullary sheath 

 Sheath of Schwann 



From a cross-section through the human median nerve, treated with Miiller's fluid and 



safranin. X 380. 



the so-called Mauthner's membrane or inner neurilemma, which 

 is inside the medullary sheath and next to the axis cylinder 

 (Fig. 80). 



The significance of the various coats of the nerve fibre is 

 not clear. It is generally recognized from embryological 

 studies that the axis cylinder is a process from a ganglion cell 

 which has grown very much in length and possesses at its free 

 end a globular thickening i. <?., the growing point. The sur- 

 rounding coats are of entirely different origin, and arise from 

 the connective tissue . 



Concerning the nature of the medullary sheath there are 

 many views. According to some, each segment of Ranvier has 

 the value of a cell, the sheaths of Schwann and Mauthner 

 being regarded as parts of the cell membrane. The whole is 

 considered as a- connective-tissue cell, annular in shape, a part 



