THE GREAT AFFERENT SYSTEMS 



307 



fibers of painful sensibility, and possibly those of temperature sensations as 

 well, are unmyelinated and enter the cord through the lateral division of the 

 dorsal root to end in the substantia gelatinosa Rolandi. 



Neuron II. From these dorsal root fibers the impulses are transmitted 

 (perhaps through the intermediation of one or more intercalated neurons) to the 

 neurons of the second order. These have their cell bodies located in the pos- 

 terior gray column, and their axons cross the median plane and ascend in the 

 lateral spinothalamic tract to end in the ventral part of the lateral nucleus of 

 the thalamus. In addition to this long uninterrupted path, there probably 

 also exists a chain of short neurons with frequent interruptions in the gray 

 matter of the spinal cord, which serves as an accessory path to the same end 

 station. In the medulla oblongata the spinothalamic tract lies dorsolateral to 

 the inferior olivary nucleus. In the pons it joins the medial lemniscus and 

 runs in the lateral part of this fillet through the pons and mesencephalon to the 

 thalamus (Figs. 231, 234). 



Neuron III. Fibers, arising from nerve-cells located in the lateral thalamic 

 nucleus, convey thermal and possibly also painful impulses to the somesthetic 

 area of the cerebral cortex in the posterior central gyrus by-way of the thalamic 

 radiation, and the posterior limb of the interal capsule. It is important to 

 note that it is not necessary for painful afferent impulses to reach the cerebral 

 cortex before they make themselves felt in consciousness, the thalamus being 

 in itself sufficient for the perception of pain (Head and Holmes, 1911 ; Head, 1918). 



The Exteroceptive Paths Associated with the Trigeminal Nerve. The tri- 

 geminal nerve mediates tactile, thermal, and painful sensations from a large part 

 of the cutaneous and mucous surfaces of the head. While there is reason to be- 

 lieve that the tactile impulses mediated by this nerve follow a central course 

 distinct from that of thermal and painful sensibility, we cannot as yet assign 

 definite paths to either group, and shall consider the exteroceptive connections 

 of this nerve as a unit. 



Neuron I. The axon of a unipolar cell in the semilunar ganglion divides 

 into a peripheral branch, distributed to the skin or mucous membrane of the 

 head, and a central branch, which runs through the sensory root (pars major) 

 of the trigeminal nerve into the pons. Here it divides into a short ascending 

 and a long descending branch. The former terminates in the main sensory 

 nucleus, and the latter in the spinal nucleus of that nerve (Fig. 232). 



Neuron II. The fibers of the second order in the sensory paths of the tri- 

 geminal nerve arise from cells located in the main sensory and the spinal nucleus 



