PAIN AND PLEASURE 251 



visceral functions) and all pleasurable feelings are devoid of this 

 external projicience and are experienced merely as a non-local- 

 ized awareness of malaise or well-being (see p. 259). They are 

 also more variable in relation to habit, mental attitude, fatigue, 

 and general health. This latter group of affective processes is so 

 different from the ordinary sensations as to make it desirable to 

 consider them separately, and, as will appear beyond, they prob- 

 ably involve a quite different series of nervous processes. 



There has been much controversy regarding the pathway 

 taken by painful impulses through the spinal cord and brain 

 stem, and it is probable that this pathway is very complex. 

 All painful impulses carried by the spinal nerves, no matter 

 what the peripheral source, are discharged immediately upon 

 entering the spinal cord into its gray matter, and after a synapse 

 here the nerve-fibers of the second order seem to take several 

 courses. The most recent experiments (Karplus and Kreidl, 

 1914) go to show that the ascending impulses of painful sensi- 

 bility in the spinal cord of cats follow a chain of short neurons, 

 some of whose axons immediately cross to the opposite side of 

 the cord and some ascend on the same side. These short fibers 

 belong to the fasciculus proprius system (p. 127), and the nervous 

 impulse is at frequent intervals returned to the gray matter to 

 pass from one neuron to another, and it may cross the midplane 

 repeatedly. This diffuse method of conduction appears to be 

 the primitive arrangement. In the human spinal cord it is 

 probably present to a limited extent, but has been largely sup- 

 planted by a more direct pathway in the spinal lemniscus, whose 

 precise localization has been determined by the clinical studies of 

 Henry Head and others (pp. 139, 173) . This direct path for fibers 

 of painful sensibility includes axons of neurons of the dorsal gray 

 column, which immediately cross to the opposite side of the cord 

 and ascend directly to the thalamus. Injury to this path in the 

 human body may cause complete insensitivity to both superficial 

 and deep pain on the opposite side of the body below the site 

 of the injury, without loss of general tactile sensibility. The 

 two methods of transmission of impulses of painful sensibility 

 are shown diagrammatically in Fig. 117. 



It may be assumed that pain and an avoiding reaction and pleasure and a 

 seeking reaction have come to be instinctively associated by natural selec- 



