64 



NERVOUS TISSUE. 



characterized by possessing a medullary sheath or white substance 

 of Schwann, which gives the white color to the nerve-fiber. This 

 is a protective covering to the essential part of a nerve, the axis- 

 cylinder. The space inside the medullary sheath is the axial space, 

 which is filled by the axial cord. This consists of axis-fibrils em- 

 bedded in the neuroplasm, a material of semi-fluid consistency, 

 both fibrils and neuroplasm being covered by a delicate mem- 

 brane, the axolemma. When nerve-fibers have been prepared for 

 microscopic examination the axial cord changes its appearance by 

 the coagulation of the neuroplasm, and the altered cord is what is 

 commonly called the axis-cylinder. The primitive sheath, nucleated 

 sheath of Schwann, or neurilemma, is a membrane which encloses 

 the white substance of the nerves, except- 

 ing those within the nerve-center. Neuri- 

 lemma (also written neurolemma) is a term 

 formerly applied to what is now called peri- 

 neurium. 



The medullary sheath is not continuous ; 

 at regular intervals it is absent, and only 

 the primitive sheath and axis-cylinder are 

 present. This gives to the nerve the ap- 

 pearance of constrictions, known also as 

 the nodes of Ranvier. The portion of 

 nerve between these constrictions is an 

 internode, in the middle of which is a nu- 

 cleus. 



Medullated fibers make up the white 

 part of the brain and spinal cord, and the 

 nerves that have their origin in these struct- 

 ures, the cerebrospinal nerves. In size they 

 vary from 2 fj. to 19 p.. This variety never 

 branches except near the termination. 

 Nonmedullated Nerve-fibers (Fig. 60). These are also known as 

 gray, gelatinous, and fibers of Remak. These have no white sub- 

 stance, but are composed of fibrillae, which are probably enclosed 

 in a sheath, the neurilemma, in which are nuclei. 



Nonmedullated fibers, unlike those that are medullated, fre- 

 quently branch. 



Nerve-fibers are associated together in bundles, funiculi (Fig. 

 61), each of which bundles is enclosed in a sheath of connective 

 tissue, perineurium. The funiculi are surrounded by a similar 

 sheath, the epineurium, which binds them together and in which 

 are the blood-vessels, lymphatics, and nerves of the nerves, the 

 last being the nervi nervorum. Within the funiculi is connective 

 tissue, embedded in which are the nerve-fibers. 



Modes of Termination of Nerve-fibers. The nerves which 

 supply striated muscle subdivide near their ends, and one of the 



- Nucleus. 



FIG. 60. Remak's fibers 

 (nonmedullated fibers) from 

 the pneumogastric nerve of 

 a rabbit ; X 360 (Bohm and 

 Davidoff). 



