198 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



1. Corona radiata. 

 2 Internal capsule. 



3. Peduncle of cerebrum. 



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



smaller bundles by the fibers of the middle peduncle of 

 the cerebellum (brachium pontis). 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. Anterior and lateral pyramidal fasciculi 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, at a minimum, two neu- 

 rons, 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 (cerebrospinal) 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 re- 

 moved not only from voluntary control, but also from reflex 

 effects. The muscles are entirely relaxed and in time exhibit 

 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 tonic con- 

 traction 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 (fasciculus intermediolateralis). 



Is the Pyramidal System the Only Means of Voluntary (Cor- 

 tical) 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 animals 



