322 THE NERVOUS SYSTEM 



and go chiefly to the nuclei of the opposite side. The decussating fibers are grouped in very 

 small bundles, those for a given nucleus crossing at the level of that nucleus. There is great 

 variation in the course of the bundles of aberrant pyramidal fibers in different brains. 



The chief aberrant bundles which can be traced dorsalward into the reticular formation 

 (indicated in solid red in Fig. 234) are as follows: 



1. The aberrant fibers of the peduncle (Fig. 240, F. A. Pd.) form two bundles, which 

 have been called by some authors the median and lateral corticobulbar tracts. These 

 descend in the territory of the medial lemniscus (Figs. 234, 240) and give off fibers to the 

 nuclei of the third, sixth, and eleventh cranial nerves. With these two bundles run some 

 fibers destined for the upper cervical segments of the spinal cord. This group of aberrant 

 fibers therefore controls the movements of the eyes and the associated movements of the head. 



2. The aberrant fibers of the pons (Fig. 240, F. A. P.) which join the preceding in the 

 medial lemniscus run to the motor nuclei of the trigeminal and hypoglossal nerves and to the 

 nucleus ambiguus. 



3. The bulbopontine aberrant fibers (Fig. 240, F. A. B. P.) leave the main trunk of the 

 pyramidal system near the level of the sulcus between the pons and medulla. They reinforce 

 the preceding groups, supply the motor nucleus of the facial nerve, and send fibers to the 

 nucleus ambiguus and to that of the hypoglossal nerve. 



These facts are of the greatest importance for the clinical neurologist. Lesions re- 

 stricted to the basilar portion of the pons are likely to destroy at the same time the cortico- 

 spinal fibers and those of the corticobulbar tract which end in the facial nucleus. A lesion 

 confined to the reticular formation and involving the medial lemniscus may, according to its 

 level, sever the corticobulbar fibers for the motor nuclei of the eye-muscle nerves or those 

 for the motor nuclei of the trigeminal, accessory, and hypoglossal nerves without involve- 

 ment of the corticospinal tracts. Conjugate deviation of the head and eyes, not often seen 

 as a result of damage to the basilar portion of the pons, may result from tegmental lesions 

 involving the aberrant fibers of the peduncle. 



The physiologic and clinical significance of the course of the corticospinal and 

 corticobulbar tracts is obvious. It is because of the decussation of these fibers 

 that the muscular contractions produced by cortical stimulation occur chiefly 

 on the opposite side of the body, and that the paralyses resulting from lesions 

 in the pyramidal system above the decussation are contralateral. If the lower 

 motor neuron is injured, the associated muscle atrophies and a flaccid paralysis 

 results. Injury to the upper motor neuron, on the other hand, leads to a loss 

 of function without atrophy, but rather with an increased tonicity of the affected 

 muscle, i. e., to a spastic paralysis. By means of such differential characteristics 

 as these it is possible to tell which of the two links in the motor chain has been 

 broken. 



In order to understand the combination of symptoms, which result from 

 damage to the motor path at different levels, it is necessary to have in mind the 

 topography of its constituent parts. Some of these relations are indicated in 

 Fig. 241. Since the motor cortex is spread out over a rather extensive area, 

 it is usually not entirely destroyed by injury or disease. A restricted cortical 



