GENERAL PHYSIOLOGY OF NERVE-TISSUE. 



in 



In the skin and mucous membranes the mode of termination 

 varies considerably. The following are some of the principal modes: 



1. Free endings in the epithelium. 



2. Tactile cells of Merkel. 



3. Tactile corpuscles in the papillae of the true skin. 



4. Pacinian corpuscles found attached to the nerves of the hand and 



feet, to the intercostal nerves, and to nerves in other situations. 



5. End-bulbs of Krause in the conjunctiva, clitoris, penis, etc. 



(A consideration of these end-organs will be found in the chapters 

 devoted to the organs of which they form a part.) 



In the skeletal muscles afferent fibers become associated with small 

 spindle-shaped structures known as muscle-spindles or neuromuscle 

 end-organs. These spindles vary in length from i mm. to 4 mm. 

 They consist of a capsule of fibrous tissue containing from five to 

 twenty muscle-fibers. After penetrating the several layers of the 

 capsule, the nerve-fibers lose the neurilemma and myelin sheaths. 

 The axons or axis-cylinders then divide into several long narrow 

 branches which wind themselves in a spiral manner around the con- 

 tained muscle-fiber and terminate in small oval-shaped discs. Similar 

 endings have been observed in the tendons of muscles. 



Development and Nutrition of Nerves. The efferent nerve- 

 fibers, which constitute some 

 of the cranial nerves and all 

 the vejfljral roots of the spinal 

 nerves, have their origin in 

 cells located in the gray -matter 

 beneath the aqueduct of .Syl- 

 vius, beneath the floor of the 

 fourth ventricle, and in the 

 anterior horns of the gray mat- 

 ter of the spinal cord. These 

 cells are "trie" m6dified descend- 

 ants of independent, oval, 

 pear-shaped cells the neuro- 

 blasts which migrate from 

 the medullary tube. As they 

 approach the surface of the 

 cord their axons are directed 

 toward the ventral surface, 

 which eventually they pierce. 

 Emerging from the cord, the 



axons continue to grow, and become invested with the myelin sheath 

 and neurilemma, thus constituting the ventral roots. 



The afferent nerve-fibers, which constitute some of the cranial 

 nerves and all the dorsal roots of the spinal nerves, develop outside 



FIG. 47. DIAGRAM SHOWING THE MODE OF 

 ORIGIN OF THE VENTRAL AND DORSAL 

 ROOTS. 



