PHYSIOLOGY OF NERVE TISSUE. 67 



activity. It is quite probable that one and the same nerve may subserve 

 both sensational and reflex action, owing to the collateral branches which are 

 given off from the posterior roots as they ascend the posterior column of the 

 cord. 



3. Inhibitor nerves, those which are capable reflexly of retarding or inhibiting 

 the activity of either nerve centers or peripheral organs. 



The Terminal Endings of Nerves. The efferent nerves, as they approach 

 their ultimate terminations, lose both the neurilemma and medullary sheath. 

 The axis-cylinder then divides into a number of tufts or branches, which 

 become directly and intimately connected with the tissue cells. The particular 

 mode of termination varies in different situations. These terminations are 

 generally spoken of as "end organs." 



In the skeletal muscles the nerve-fiber loses both neurilemma and myelin 

 sheath at the point where it comes in contact with the muscle-fiber. After 

 penetrating the sarcolemma, the axis-cylinder breaks up into small branches 

 with bulbous extremities, forming the so-called "motor-plate," which rests 

 directly on a disc of granular material containing oval, vesicular nuclei. 

 Each muscle-fiber possesses an individual end-plate. 



In the visceral muscles the terminal nerve-fibers form a plexus around the 

 muscle-fibers, and become organically connected with them. In the glands 

 the nerve fibers have been traced directly to their secreting cells. The exact 

 mode of their termination and connection with the cells has not been clearly 

 determined. 



The afferent nerves, as they approach their peripheral terminations, become 

 connected in like manner with end organs, which, in some distances, are 

 extremely complex, such as those found in the eye (retina), the internal ear, 

 the nose, and the tongue. (A consideration of these end organs will be found 

 in the chapters devoted to the organs of which they form a part.) The end 

 organs of the skin and mucous membranes present a variety of forms, and 

 may be classified as follows: 



1. Free endings in the epithelium of the skin, mucous membrane, and 

 cornea. 



2. Tactile cells of Merkel in the epidermis. 



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



4. Pacinian corpuscles found attached to the nerves of the hands and feet, 

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



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



The end organs of the afferent nerves are specialized, highly irritable 

 structures placed between the nerve-fibers and the surface of the body. 

 They are especially adapted for the reception of those external forces tech- 



