THE MESENCEPHALON. 



1 107 



Pulvinar 



Superior colliculus. 



Inferior colliculus 



Superior medullary 



velum 



Superior cerebellar 



peduncle 



Lingula 



Middle cerebellar 

 peduncle, cut 



Fig. 958. 



Superior brachium 



Median geniculate body 



Lateral geniculate body 



Tractus transversus 

 Cerebral peduncle 

 / Optic tract 



The elevations forming the upper pair of quadrigeminal bodies, the colliculi 

 superiores, are the larger and more conspicuous, and measure from 7-8 mm. in 

 length, about 10 mm. in breadth, and 6 mm. in height. Laterally each superior col- 

 liculus is continued into an arm, the superior brachium (brachium quadrigeminum 

 superius) which is defined by a groove above and below, and passes upward and 

 outward, between the optic thalamus and the median geniculate body, to be lost 

 within an indistinctly circumscribed oval eminence, the lateral geniculate body 

 (corpus geniculatum laterale), which lies beneath the pulvinar. In like manner, each 

 of the smaller lower pair of quadrigeminal bodies, the colliculi inferiores, (about 

 6 mm. in length by 8 mm. in breadth and 5 mm. in height) is prolonged laterally 

 into the inferior brachium (brachium quadrigeminum inferius), which in turn ends 

 in the sharply defined median geniculate body (corpus geniculatum mediale), an 

 oval elevation about 10 mm. in length. Ventrally the quadrigeminal plate becomes 

 directly continuous with the adjacent part of the cerebral peduncles. 



The cerebral peduncles (pedunculi cerebri), also called the cerebral crura, 

 constitute the bulky ventral part of the mid-brain. Dorsally, the two peduncles are 

 fused into a continuous tract, the tegmentum, which contributes the side-walls and 

 floor of the Sylvian aqueduct and blends on each side with the overlying quadri- 

 geminal plate. Ventrally the peduncles are unfused and appear on the inferior sur- 

 face of the brain as two robust stalks (Fig. 993). These emerge from the upper 

 border of the pons and pass, diverging at an angle of from 70-85°, upward and out- 

 ward to enter, one on each 

 side, the cerebral hemi- 

 spheres just where the 

 peduncles are crossed by 

 the outwardly winding 

 optic tracts. At the pons 

 each peduncle possesses 

 a breadth of from 12-15 

 mm. , which increases to 

 from 18-20 mm. at the 

 upper end of the stalk ; the 

 borders of each peduncle 

 are, therefore, not quite 

 parallel, but slightly di- 

 verging. Neither are the 

 mesial margins of the pe- 

 duncles in contact as they 

 issue from the pons, but 

 separated by an interval 

 of about 3 mm. This 

 distance increases until at their upper ends the peduncles are about 13 mm. apart. 

 Superficially each peduncle is formed by strands of fibres which do not pursue a 

 strictly longitudinal course, but wind spirally from within outward ; in consequence 

 of this arrangement the surface of the peduncle presents a characteristic twisted or 

 rope-like striation. The regularity of this marking is sometimes disturbed by a 

 faintly defined strand of fibres (tractus peduncularis transversus), that winds over the 

 median border and ventral surface of the peduncle, passes upward and outward across 

 the lateral surface of the mid-brain, to be lost in the vicinity of the medial geniculate 

 body. The depressed triangular area included between the diverging peduncles is the 

 interpeduncular fossa, the floor of which is pierced by numerous minute openings 

 that transmit small blood-vessels, and hence is known as the posterior perforated 

 substance. The blunted inferior angle of the fossa, immediately above the pons, 

 corresponds with a depression, the recessus posterior; another, but less marked 

 depression, the recessus anterior, is bounded by the postero-median surfaces of 

 the mammillary bodies. A shallow lateral groove (sulcus mesencephali lateralis) 

 extends along the outer surface of the peduncle, whilst along its inner aspect, and 

 therefore looking into the interpeduncular fossa, runs the median or oculomotor 

 groove (sulcus nervi oculomotorii), that is more distinct than the lateral furrow and 



:> 



Pons 



Dorso-lateral aspect of mid-brain. 



