THE MID-BRAIN 895 



an arborescence around the cells of the granular layer. Other fibres derived 

 from the medullary centre can be traced into the molecular layer, where their 

 branches cling around the dendrites of Purkinje's cells, and hence they have been 

 named the clinging or tendril fibres (Fig. 658). 



The cerebellum is an important senso motor organ, transmuting sensor im- 

 pressions into motor impulses under the dominance of the cerebral centres. 

 Its connections with other brain portions and the spinal cord are established by 

 the peduncular fibres. It is essentially an apparatus for the coordination of 

 movements and the space-sense perceptions or orientation of the body and its 

 parts in space. These functions depend principally upon the reception of sensor 

 impulses from (1) the vestibular nerve (from the semicircular canals) and (2) 

 the spinocerebellar (ascending) tracts conveying sensor impulses from the skin, 

 muscles and joints. Motor impulses pass along (1) the cerebellospinal (descend- 

 ing) tracts to the ventral horn nuclei of the cervical cord; (2) the tractus rubro- 

 spinalis, which arises in the red nucleus an intercalated ganglionic mass con- 

 nected with the cerebellar cortex by the superior peduncles, or tractus cerebello- 

 tegmentalis. The tractus rubrospinalis is a tract for voluntary motor impulses 

 next in importance to the pyramidal tract. 



Weight of the Cerebellum. Its average weight in the male is 165 grams (5.8 

 ounces avoirdupois) and 155 grams (5.4 ounces avoirdupois) in the female. 

 It attains its maximum between the twenty-fifth and thirty-fifth years, its increase 

 in weight after the fourteenth year being relatively greater in the female than in 

 the male. The proportion between the cerebellum and the cerebrum is as 1 to 

 7.5; among eminent men it is 1 to 8.5. In the newborn the ratio is as 1 to 20. 



The Mid-brain (Mesencephalon). 



The mid-brain is the short and constricted portion of the brain which lies in 

 the opening of the tentorium cerebelli (incisura tentorii) and which connects the 

 pons with the inter-brain and hemispheres, and hence it is frequently called the 

 isthmus cerebri. It is developed from the second brain vesicle, the cavity of 

 which becomes the aqueduct. It comprises the crura cerebri, the corpora quadri- 

 gemina, the internal geniculate bodies, and the aqueduct. Its two surfaces are ven- 

 tral and dorsal. They are free, but concealed; the ventral surface by the apices of 

 the temporal lobes which overlap it; the dorsal, by the overhanging cerebral hemi- 

 spheres. The ventral surface, when exposed by drawing aside the temporal lobes, 

 is seen to consist of two cylindrical bundles of white substance, which emerge 

 from the pons and diverge as they pass forward and outward, to enter the inner 

 and under part of either hemisphere. They are the crura cerebri, and between 

 them is a triangular area, the intercrural space j 1 near the point of divergence of the 

 crura cerebri the roots of the third nerve are seen to emerge in several bundles 

 from a groove, the sulcus oculomotorius (sulcus nervi oculomotorii) (Fig. 619). 

 The dorsal surface is not visible until a considerable portion of the cerebral hemi- 

 spheres and other overlying structures have been removed. It then presents 

 four rounded eminences placed in pairs, two cephalad and two caudad, and sepa- 

 rated from one another by a crucial depression. These are termed the corpora 

 quadrigemina (tubercula quadrigemina) (Fig. 659). The ventral and dorsal sur- 

 faces meet on the side of the mid-brain, and are separated from each other by a 

 furrow, the lateral groove (sulcus lateralis mesencephali), which runs caudocephalad 

 (Fig. 659). 



External Morphology. Dorsal Surface. The corpora quadrigemina are four 

 rounded eminences placed in pairs separated by a shallow median groove and a 



1 Also called interpeduncular space, or posterior perforated space. 



