THE MID-BRAIN 



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THE MID-BRAIN. 



The mid-brain includes the crura cerebri, the corpora quadrigemina, and 

 the geniculate bodies. 



The Crura Cerebri. The crura diverge from the anterior edge of the 

 pons Varolii and pass upward on either side toward the cerebral hemispheres. 

 At their anterior termination each of them appears to have upon its dorsal 

 surface, to the inner and outer sides respectively, two large masses of gray 

 matter which have been already spoken of, viz., the optic thalamus and the 

 corpus striatum. The crus is made up of two principal parts. Thecrusta 



FIG. 382. Diagram of the Motor Tract as Shown in a Diagrammatic Horizontal Section through 

 the Cerebral Hemispheres, Crura, Pons, and Medulla. Fr., Frontal lobe; Oc., 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 converging 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.) 



or pes is in the ventral position, and the tegmentum in the dorsal position. The 

 two are separated by the substantia nigra. 



The pes consists of .longitudinal fibers which pass anteriorly between the 

 optic thalamus and the posterior part (lenticular nucleus) of the corpus stria- 

 tum, and also more anteriorly. In this situation the fibers form a compact 

 mass which 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 hemi- 

 sphere. 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 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 

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