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THE NERVOUS SYSTEM 



turn. In this situation the fibers form a compact mass. This constitutes 

 the internal capsule, and that portion of it which forms the angle at which 

 the fibers are bent is called the genu of the capsule. The internal capsule 

 spreads out dorsally in the corona radiata. The fibers thus have the form 

 of a fan bent upon itself as they rise to pass into the cerebral hemisphere. 

 The fibers of the internal capsule are connected with different districts of the 

 cerebral cortex. Briefly the connections are, a, the fronto-pontine fibers are 

 in the anterior limb of the capsule; b, the pyramidal fibers in the genu and 

 the anterior part of the posterior limb; c, the temporo-pontine fibers in the 



FIG. 389. Diagram of the Motor Tract as Shown in a Diagrammatic Horizontal 

 Section through the Cerebral Hemispheres, Crura, Pons, and Medulla. Fr., Frontal lobe; 

 Oc. t occipital lobe; AF., ascending frontal, AP., ascending parietal convolutions; PCF., 

 pre-central fissure, in front of the ascending frontal convolution; FR., fissure of Rolando; 

 IFF., inter-parietal fissure, a section of crus is lettered on the left side; SN., substantia 

 nigra; Py., pyramidal motor fiber which on the right is shown as continuous lines con- 

 verging to pass through the posterior limb of 1C., internal capsule (the knee or elbow of 

 which is shown thus), upward into the hemisphere and downward through the pons to 

 cross the medulla in the anterior pyramids. (Gowers.) 



posterior part of the posterior limb. Fibers connecting the optic thalami 

 and corpora striata with the cerebral cortex also run in the capsule. The 

 pes, internal capsule, and the corona radiata form the great sensory and motor 

 highway to and from the cerebral cortex. 



The tegmentum is the continuation anteriorly of the reticular formation of 

 the medulla. It ends for the most part in the neighborhood of the optic 

 thalamus and in the parts beneath. The tegmentum of either side is sup- 

 posed to be concerned chiefly with afferent impulses. It is made up to a very 

 considerable extent of collections of gray matter, the most important of which 

 are the substantia nigra, separating the pes and tegmentum, and the nucleus 

 ruber, which is a rounded mass situated near the aqueduct of Sylvius. The 



