THE SULCI AND GYEI OF THE CEREBRAL HEMISPHERES. 653 



cells of Betz in the posterior part of the precentral cortex (p. 663) in the district 

 immediately in front of the sulcus centralis. The fibres descend through the 

 corona radiata into the posterior limb of the internal capsule. From this point the 

 further course of the pyramidal tract has been traced, viz., through the central part 

 of the basal region of the cerebral peduncle and pons, and the pyramid of the 

 medulla oblongata. At the level of the foramen magnum it decussates in the 

 manner already described, and enters the spinal medulla as the lateral cerebro- 

 spinal and anterior cerebro-spinal tracts. The fibres composing these end in 

 connexion with the ventral or motor column of cells, from which the fibres of the 

 anterior roots of the spinal nerves arise. 



Similar fibres arise from the inferior part of the precentral area and proceed 

 through the internal capsule and cerebral peduncle to all the motor nuclei upon 

 the opposite side of the brain stem (fasciculi cerebronucleares). Hence the cerebral 

 cortex of one hemisphere can control all the muscles of the opposite side of the body. 



The fronto-pontine strand is composed of fibres which arise as the axons of 

 the cells in the cortex which covers the frontal region that lies in front of the 

 precentral furrows. It descends in the anterior limb of the internal capsule, 

 enters the medial part of the base of the cerebral peduncle, through which it 

 gains the basilar part of the pons. In this its fibres end amongst the cells of the 

 nuclei pontis, from which axons arise and establish relations with the cortex of 

 the opposite cerebellar hemisphere. 



The temporo-pontine tract consists of fibres which spring from the cells, of that 

 part of the cortex which covers the middle portions of the lower two temporal 

 gyri. The temporo-pontine tract passes medially under the nucleus lenti- 

 formis, enters the retrolenticular part of the posterior limb of the internal capsule, 

 and thus gains the lateral part of the cerebral peduncle. From this it descends 

 into the basilar part of the pons, in which it ends in the nuclei pontis. 



Cortico-striate and other Descending Fibres. From the fibres of the internal 

 capsule numerous collateral branches are given off to the nucleus caudatus and 

 nucleus lentiformis, and from these basal ganglia fibres arise which enter the 

 cerebral peduncle as constituent elements of the great cerebro-spinal tract. 



Some of the fibres from the corpus striatum, especially the nucleus lentiformis, 

 as well as others descending from the frontal cortex, pass into the red nucleus 

 (Fig. 571), which also receives afferent tracts from the tectum mesencephali and 

 from the cerebellum : it emits an important efferent tract (fasciculus rubrospinalis), 

 which crosses the median plane and descends in the brain stem and spinal medulla 

 to the various motor nuclei (see Figs. 454 and 475). 



THE SULCI AND GYRI OF THE CEREBRAL HEMISPHERES. 



Fissura Cerebri Lateralis (O.T. Fissure of Sylvius). This is the most con- 

 spicuous furrow on the surface of the cerebral hemisphere. In reality it is formed, 

 not as a furrow upon the surface of the hemisphere, but as a great fossa, the 

 margins of which develop into large lip-like folds that bulge over the fossa and 

 meet to form the superficial pattern of the lateral fissure. It is composed of a 

 short main stem, from the lateral extremity of which two or three branches or 

 limbs radiate. The stem of the lateral fissure is placed on the inferior surface of 

 the hemisphere. It begins at the substantia perforata anterior and passes 

 laterally, forming a deep cleft between the temporal pole and the orbital surface of 

 the frontal region. Appearing on the lateral surface of the hemisphere, the fissure 

 immediately divides into two or three radiating rami. These are : (1) the ramus 

 posterior ; (2) the ramus anterior horizontalis ; (3) the ramus anterior ascendens, 

 of which the last is inconstant. 



The posterior ramus is the longest and most constant of the three limbs. It 

 extends backwards, with a slight inclination upwards, on the lateral surface of the 

 hemisphere for a distance which may vary from about two to three inches. It 

 intervenes between the frontal and parietal regions, which lie above it, and the 

 temporal region which lies below it ; and it finally ends in the region subjacent to 



