76 HUMAN PHYSIOLOGY. 



The afferent nerves, as they approach their peripheral termina- 

 tions, become connected in like manner with end organs, which, in 

 some instances, are extremely complex, such as those found in the 

 eye (retina), the internal ear, the nose, and the tongue. (A con- 

 sideration 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 papilla 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 technically known as stimuli, and for the liberation of 

 energy capable of exciting the nerve-fiber to activity. 



Relation of Spinal Nerves to the Spinal Cord. The nerves in 

 connection with the spinal cord are thirty-one in number, on each 

 side and have two roots of origin, an anterior and a posterior, which 

 arise from the anterior and posterior surfaces of the cord respectively. 

 They are more properly termed ventral and dorsal roots. The dorsal 

 roots present, near their entrance into the cord, an enlargement 

 termed a ganglion. Beyond the spinal canal these two roots unite 

 to form the ordinary spinal nerve. Some of the nerves in con- 

 nection with the base of the brain also present a ganglionic enlarge- 

 ment, and may, therefore, be regarded physiologically as dorsal nerves, 

 while others may be regarded as ventral nerves. 



Experimentally, it has been determined that the anterior or ventral 

 roots contain all the efferent fibers, the posterior or dorsal roots all 

 the afferent fibers. The proofs in support of this view are as 

 follows : 



Stimulation of the ventral roots produces : 



1. Convulsive movements of muscles. 



2. The formation of a secretion in glands. 



3. Changes in the caliber of blood-vessels. 



