182 THE NERVOUS SYSTEM ^ 



afferent impulses from the viscera may be vaguely represented in conscious- 

 ness, there must be a visceral afferent path to the thalamus; but concerning 

 the character and location of this path we are entirely ignorant. 1 The fibers 

 arising from the nucleus of the tractus solitarius enter the reticular formation, 

 and it is probable that a majority of them are distributed to the visceral motor 

 nuclei of the medulla oblongata, including the nucleus ambiguus and the dorsal 

 motor nucleus of the vagus. In this way arcs are established for a large and 

 important group of visceral reflexes. Some of these fibers descend to the spinal 

 cord and may play an important part in the reflex control of respiration and 

 in initiating reflex coughing and vomiting (Figs. 245, 246). 



THE GENERAL SOMATIC AFFERENT NUCLEI 



The general somatic afferent nuclei receive fibers from the skin and ecto- 

 dermal mucous membrane of the head by way of the trigeminal nerve. These 

 have their cells of origin in the semilunar ganglion, and within the pons the)' 

 divide into short ascending and long descending branches (Fig. 131). The as- 

 cending branches terminate in the main sensory nucleus; the descending branches 

 run through the spinal tract and terminate in the nucleus of the spinal tract of 

 the trigeminal nerve. Since these nuclei receive sensory fibers from the skin 

 and ectodermal mucous membrane of the head, they are exteroceptive in function. 

 The spinal tract and its nucleus also receives a few cutaneous afferent fibers 

 through the glossopharyngeal and vagus nerves from the skin of the external ear 

 (Fig. 120). 



The main sensory nucleus of the trigeminal nerve is located at the level of 

 the middle of the pons in the lateral part of the reticular formation some dis- 

 tance from the floor of the fourth ventricle (Figs. 110, 121, 130). The spinal 

 nucleus, with which it is continuous, at first lies deeply under cover of the resti- 

 form body; but when it is traced caudally it approaches the surface and, covered 

 by the spinal tract, forms the tuberculum cinereum (Figs. 99, 103). It finally 

 becomes continuous with the substantia gelatinosa Rolandi of the spinal cord. 

 Thus we have a continuous column of gray matter extending from the sacral por- 

 tion of the spinal cord into the brain stem and ending abruptly in an enlarge- 

 ment, the main sensory nucleus of the trigeminal nerve. This entire column 

 receives afferent fibers from the skin and belongs to the exteroceptive portion of 

 the somatic afferent division of the nervous system. 



1 Kohnstamm and Hindelang (1910) and von Monakow (1913) have described a secondary 

 visceral afferent path which arises from the gray matter in and around the tractus solitarius and 

 terminates in the thalamus. 



