872 CENTRAL NERVOUS SYSTEM 



nated. They have been conducted along fibers of primary afferent neurons 

 which terminate in the nucleus gracilis and nucleus cuneatiis, from which 

 arise the secondary afferent neurons which carry them to the optic thal- 

 amus. These fibers cross the medulla immediately and take up a position 

 in the medial lemniscus close to those which are conducting impulses for 

 the sensations of pain, temperature, and touch. In their passage through 

 the brain stem impulses for all qualities of sensation follow tracts which 

 are grouped together closely in the opposite side from that which they 

 have originated. Lesions in the brain stem consequently tend to produce 

 a complete, contralateral anesthesia. Impulses for any one quality of 

 sensation are still grouped together in tracts which are distinct from 

 those carrying impulses for other qualities of sensation. Consequently 

 lesions of limited extent may abolish sensation of any one quality with- 

 out disturbing the other qualities of sensation. Observations on cases 

 of lesions of this sort indicate that impulses involved in the recognition 

 of posture and movement, which have been associated with those for the 

 two dimensional localization in their passage up the spinal cord, become 

 separated in the brain stem, so that the power of recognizing posture and 

 passive movement can be effected independently of the discrimination 

 of two points applied simultaneously to the skin. 



Afferent Impulses Which Fail to Produce Sensation 



Those impulses which are destined to give rise to sensation will be 

 traced in their course beyond the termination of the secondary afferent 

 neurons in the nuclei of the optic thalamus, but we shall first consider 

 the courses followed by those afferent impulses which do not reach the 

 level of consciousness. These are of three types: (1) the afferent im- 

 pulses of spinal reflexes, (2) the afferent impulses of visceral reflexes, 

 (3) the afferent impulses of cerebellar reflexes. 



The Afferent Paths of Spinal Reflexes. The connections within the 

 cord for spinal reflexes are undoubtedly very primitive. They do not, 

 however, appear to have been worked out in many cases or in great de- 

 tail. The simplest reflexes probably involve neurons lying in a single 

 segment and on one side of the cord. The flexion reflexes which result 

 from noxious stimuli applied to the limbs are of this type. In other cases 

 the reflex pathway may extend through many segments, but lie totally 

 within one half of the cord. An example is seen in the scratch reflex ol ; 

 the brainless dog. The method by which its course was worked out by 

 Sherrington was described on page 849. It was found to consist of an 

 afferent neuron connecting the skin of the shoulder with the grey matter 

 of the cord in the upper thoracic region, a propriospinal neuron having 

 its cell body in this part of the cord and its axon descending in the lateral 

 column of the same side to connect with the motor neurons of the limb 



