MOTOR AREAS OF THE CEREBRUM 889 



are given off which cross to the nucleus of the facial nerve 011 the op- 

 posite side of the body. Similarly fibers leave the main tract in the 

 upper part of the medulla to cross and connect with the hypoglossal 

 nucleus. The main decussation of the pyramidal tract occurs in the 

 lower part of the medulla. A small bundle of fibers continue beyond this 

 point uncrossed and descend the cord in the direct pyramidal tract, 

 (tract us cortico-spinalis ventralis) in the ventral column of the cord. The 

 fibers which enter the decussation in the medulla descend the cord in 

 the crossed or lateral pyramidal tract (tractus cortico-spinalis lateralis) 

 in the lateral column of the cord. In these tracts the pyramidal fibers 

 descend to the level of the motor neurons of the peripheral nerves. The 

 connection between cortex and muscle is consequently probably con- 

 summated by no more than two nerve cells, the pyramidal and the lower 

 motor neuron. It is quite conceivable that other descending tracts in 

 the cord may be involved in voluntary acts, but their relationships are not 

 clearly enough understood to enable us to base our explanations of the 

 motor symptoms of nervous disease upon their action. 



The Paralysis Resulting from Injuries to the Pyramidal Tract. The 

 arrangement of the pyramidal fibers in their descent through the cord 

 imposes certain characteristics on the distribution of the paralysis which 

 results from lesions in different parts of their course. We have seen 

 that lesions in the cortex rarely result in a complete destruction of the 

 motor area, so that the resulting paralysis is usually limited to a few 

 groups of muscles on the opposite side of the body. Such monoplegia 

 would be accompanied by no loss of sensation, or by impairment in the 

 special aspects of sensation in a limited region. In deeper parts of the 

 brain the pyramidal tracts converge with the result that a lesion the 

 size of a pea in the internal capsule will result in a complete destruction 

 of the tract. Monoplegia is consequently rare at this level, the paralysis 

 usually involving the entire opposite side of the body. Such a condi- 

 tion is known as hemiplegia. If lesions in this region affect sensibility, 

 the discriminative aspects of it will be destroyed, but those sensations 

 which make a thalamic appeal will persist. Lesions in the brain stem 

 as far back as the pons will similarly produce complete contralateral 

 hemiplegia, accompanied usually, if they affect the afferent paths of 

 sensation, by complete anesthesia on the paralyzed half of the body. At 

 the pons the first group of pyramidal fibers cross to the opposite side to 

 connect with the facial nerve. Lesions in the lower part of the pons 

 consequently do not involve these fibers and the facial muscles of the 

 opposite side do not share in the paralysis. This lesion, however, will 

 interrupt the tracts from the opposite cerebral hemisphere which have 

 crossed to join the facial nucleus on the side of the lesion. Consequently 

 the face becomes paralyzed on the same side as the lesion. The decussa- 



