186 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



1. Corona radiata. 



2. Internal capsule. 



3. Cms cerebri (pes). 



4. Pons Varolii, in which they are broken into a number of 

 smaller bundles by the fibers of the middle peduncle of the 



cerebellum. In this region, also, some of the fibers cross the 

 mid-line, to end in the motor nuclei of the cranial nerves: 

 Third, fourth, fifth, sixth, and seventh. 



5. Anterior pyramids. 



6. Pyramidal decussation. 



7. Direct and crossed pyramidal tracts in the cord. 



After ending in the motor nuclei of the cranial or spinal nerves the 

 path is continued by a second neuron from these nuclei to the mus- 

 cles. The entire path involves, therefore, two neurons, and injury 

 to either will cause paralysis of the corresponding muscles. 



Difference in the Paralysis from Injury to the Spinal and the 

 Pyramidal Neuron. With regard to the musculature of the limbs 

 especially a difference has been observed in the paralysis caused by 

 injury to the spinal and pyramidal neurons, respectively. Lesions of 

 the anterior root cells in the cord or of the axons arising from them 

 cause complete paralysis of the corresponding muscles, since these 

 muscles are then removed not only from voluntary control, but also 

 from reflex effects. The muscles are entirely relaxed and in time ex- 

 hibit a more or less complete atrophy. When the pyramidal neurons 

 are affected, as in the familiar condition of hemiplegia resulting 

 from a unilateral lesion of the motor cortex, there is paralysis as 

 regards voluntary control, but, the spinal neuron being intact, the 

 muscles are still subject to reflex stimulation through the cord, 

 especially to the so-called tonic impulses. Under these conditions, 

 especially if the lesion is in the cord, it is frequently noticed that the 

 paralyzed muscles are thrown into a state of continuous contraction, 

 contracture, in which they exhibit a spastic rigidity. This fact, 

 therefore, may be used in diagnosing the general location of the 

 lesion. A satisfactory explanation of the cause of the contraction 

 has not been furnished. It may be due to uncontrolled reflex 

 excitation of the spinal neurons, or, as suggested by Van Gehuchten, 

 to the action of the indirect motor path by way of the rubrospinal 

 tract. 



Is the Pyramidal Tract the Only Means of Voluntary 

 (Cortical) Control of the Muscles? Much discussion has arisen 

 regarding this question. It is, in fact, one of those questions 

 of nervous mechanism in which experiments upon lower ani- 

 mals must be applied with caution to the conditions in man. 

 As we have seen, the entire cerebral cortex may be removed 

 from the frog, the pigeon, and the dog without causing permanent 



