THE MID-BRAIN 897 



brachium likewise appears to run into this body; as a matter of fact, so far as is 

 known, the internal geniculate body is (1) a way-station for auditory impulses 

 in their course toward the cerebrum; (2) the origin and terminus for the arched 

 commissure of Gud&en(infracommissure; commissura inferior [Guddeni]), by means 

 of which circuitous path, through the optic chiasm, and along the mesal root of 

 the optic tract, the internal geniculate bodies of the two sides are connected. 



The quadrigeminal lamina is continuous caudad with the superior peduncles 

 of the cerebellum and the intervening superior medullary velum. A slight, median 

 ridge-like projection, the frenulum valvulae, descends from between the inferior 

 quadrigeminal bodies onto the superior medullary velum; on either side of the 

 frenulum emerge the slender trochlear nerves. 



The crura cerebri constitute the bulk of this portion of the brain stem. Upon 

 the ventral aspect of the brain they appear as two large, white, rope-like strands 

 emerging from the pons and diverging to either cerebral hemisphere, becoming 

 embraced by the optic tracts. Each crus is composed of a dorsal tegmental 

 part a continuation of the tegmentum of the hind-brain and a ventral crusta 

 or pes. These parts are demarcated from each other on the external surface 

 by the oculomotor sulcus ventrad (which looks into the intercrural space) and the 

 sulcus lateralis mesencephali on the lateral aspect. The lateral surface shows dor- 

 sally the superior cerebellar peduncle dipping into the substance of the mid-brain, 

 while between it and the crusta is a small triangular field of oblique fibre strands, 

 not always well defined, called the trigonum lemnisci because the lateral lemniscus 

 tends to reach the surface of the brain stem at this situation (Fig. 659). 



The surface of the crura cerebri shows a rope-like twist in the course of its 

 fibre bundles. Oblique or transverse fasciculi are sometimes seen upon the sur- 

 face, two of which are fairly constant. They are (1) the taenia pontis, and (2) 

 Gudden's tractus peduncularis transversus (cimbia). 



The taenia pontis, as Horsley has shown, takes origin contralaterally in the 

 gray substance continuous with the " interpeduncular ganglion," but ventral 

 to it. The taenia then passes over the lateral lemniscus and superior cerebellar 

 peduncle to the dentate nucleus and nucleus fastigii. 



The tractus peduncularis transversus, or cimbia, 1 may be traced from the supe- 

 rior quadrigeminal body and internal geniculate body over the surface of the 

 crus cerebri to near the ventromeson, disappearing from view in the oculomotor 

 sulcus. 



Internal Structures of the Mid-brain. If a cross-section be made through 

 the mesencephalon it will be seen that each lateral half is divided into two unequal 

 portions by a lamina of deeply pigmented gray substance, named the substantia 

 nigra (inter calatum; ganglion of Soemmering^). The postero-superior portion of 

 the crus is named the tegmentum, and the antero-inferior the crusta or pes. The 

 substantia nigra is curved on section with its concavity upward, and extends from 

 the lateral groove externally to the oculomotor sulcus internally. The two crustse 

 are in contact in front of the pons, from which point they diverge from each 

 other, but the tw r o halves of the tegmentum are joined to each other in the mesal 

 plane by a forward prolongation of the raphe or median septum of the pons. 

 Laterally the tegmenta are free, but dorsally they blend with the corpora quadri- 

 gemina. 



Traversing the mid-brain in the median plane and nearer the dorsal surface is 

 the aqueduct, surrounded by the central tubular gray, which in this brain segment 

 has retained the comparatively primitive arrangement of the embryonic brain 

 tube. 



The Aqueduct (mesocele) and Central Aqueduct Gray. The aqueduct is a nar- 

 row canal connecting the third with the fourth ventricle, and demarcating the 



1 In architecture, a band or fillet about a column. Also called fasciculus arciformis pedis. 



57 



