THE AFFERENT PATHS OF SENSORY IMPULSES 873 



muscles which are involved in the scratching movements. Other reflexes 

 are executed through paths which cross the cord. In addition to pro- 

 ducing a flexion of the foot to which it is applied, a strong noxious stim- 

 ulus causes an extension of the opposite limb. This is known as the 

 crossed-extension reflex. Crossed reflexes also involve paths which trav- 

 erse considerable lengths of the cord. Thus a flexion of the hind limb, 

 produced by a noxious stimulus applied directly to it, is frequently ac- 

 companied by a flexion of the fore limb on the opposite side of the body. 



The Afferent Paths of Visceral Reflexes. The visceral reflexes are con- 

 cerned chiefly with the regulation of the circulation, respiration, and the 

 motor activities of the internal organs. Such reflexes are set up by af- 

 ferent neurons extending to the visceral organs themselves as well as 

 to the skin and muscles. 



Visceral afferent fibers are found in the ninth and tenth cranial nerves 

 and in the spinal nerves. Their cell bodies lie in the ganglia of the 

 medulla and in the dorsal root ganglia of the spinal nerves. The fibers 

 reach the viscera by following the course of the pre- and postganglionic 

 fibers of the autonomic nervous system, passing through the ganglia and 

 plexes without interruption. In spite of this close anatomical associa- 

 tion with the autonomic nervous system, the visceral afferents are analo- 

 gous in function and homologous in origin and structure with the affer- 

 ent neurons from the skin and muscles. It is logical to classify the vis- 

 ceral afferents with the latter, rather than with the autonomic system 

 which is wholly motor in function. While certain impulses from the 

 visceral afferents reach the sensory centers of the brain and give rise to 

 visceral pain and the other sensory symptoms of visceral disease, the 

 greater number never affect consciousness. They take part rather in the 

 execution of visceral reflexes, which modify the activity of the muscles 

 of the internal organs and regulate the blood flow through them in ac- 

 cordance with the varying demands of their functional activity. The 

 most important visceral afferents are those which modify the action of 

 the cardiovascular and respiratory centers of the medulla. The depres- 

 sor nerve and the afferent fibers of the vagus which extend to the lung 

 are examples with which we are already familiar (pages 243 and 227). 

 The centers regulating blood pressure and respiration are also influenced 

 by impulses which have their normal origin in the receptors of the skin 

 and which may be initiated by stimulating the nerve trunks in the limbs. 

 Ranson and von Hess have studied the location of the afferent pathways 

 in the cord over which such impulses travel. 



Two kinds of vascular reflexes were studied, pressor and depressor, 

 the former being elicited by strong and the latter by very feeble stimu- 

 lation of the central end of the sciatic and brachial nerves. They found 

 that the pathways for the pressor and depressor afferent impulses were 



