742 THE NERVOUS SYSTEM, 



third nerve pass. The two crustce are often spoken of as the crura or pedunculi 

 cerebri. 



Fibres of the Crusta. These are (1) the upward continuations of the fibres 

 of the pyramids, pyramidal tract, which in passing through the pons are known 

 as its ventral longitudinal fibres : this pyramidal tract forms about the middle 

 third of the crusta. Superiorly, it is continued into the middle part of the internal 

 capsule of the hemisphere, and thence to the fronto-parietal or Rolandic region 

 of the cortex. (2) The direct sensory tract : these fibres occupy the outer third 

 of the crusta ; below, they probably come from the nuclei pontis ; above, they 

 pass into the posterior part of the internal capsule, and thence to the cortex of 

 the occipito-ternporal regions of the hemisphere. (3) Fibres of the inner third 

 of the crusta : the lower origin of most of these is not well defined. They pos- 

 sibly arise from the cells of the substantia nigra. Above, they pass through the 

 anterior part of the internal capsule to the cortex of the prefrontal region of the 

 hemisphere. One bundle, however, is distinct. It is the mesial fillet (see below). 

 As it passes upward it crosses obliquely outward through the other fibres of the 

 crusta, and its fibres are finally lost in the subthalamic region, where they prob- 

 ably become continuous with the ansa lenticularis (see page 747). Below, it is 

 one of the three divisions of the fillet. 



The substantia nigra or locus niger is, as already mentioned, a lamina of gray 

 matter situated between the crustse and tegmenturn and projecting here and there 

 between the bundles of the former. It is thicker at the inner than at the outer 

 side, and is traversed in its mesial part by the fibres of origin of the third nerve. 

 It contains irregular nerve-cells, in which are lodged numerous dark pigment- 

 granules. The portion between the crustae, together with the crustse, form the 

 ventral surface of the mid-brain, which, on each side, is limited by the sulcus 

 lateralis. 



The Tegmentum. This comprises all that part of the mid-brain dorsal to and 

 superior to the substantia nigra. Its longitudinal fibres run up through the 

 "dorsal" part, and then bend forward in the "superior" part, from which they 

 pass to their destination. There are really two tegmenta, but each is united with 

 its fellow of the opposite side by a prolongation of the median septum or raphe 

 of the pons. 



The tegmentuin consists of longitudinal bundles of white fibres interlaced by 

 transverse fibres, together with a quantity of gray matter with scattered nerve- 

 cells. It forms a well-marked formatio reticularis similar to that found in the 

 pons and medulla, with which it is continuous, receiving, however, a bundle of 

 fibres (superior peduncle] from the cerebellum. 



The most distinct of the longitudinal fibres comprise the following bundles, 

 whose course in medulla arid pons has already been described: (1) The posterior 

 longitudinal bundle: this lies on each side of the median line and just below the 

 aqueduct of Sylvius. In the mid-brain it gives off fibres to the nuclei of the 

 third and fourth cranial nerves, and receives fibres, in the pons, from the nucleus 

 of the sixth. At the junction between the dorsal and superior surface of the 

 mid-brain each posterior longitudinal bundle passes dorsal to the aqueduct and 

 across to the opposite side in the posterior commissure; some fibres, however, pass 

 to the subthalamic region of the same side ; there are also decussating fibres 

 between the two "bundles " in the raphe. (2) The fillet : its mode of origin and 

 course in the medulla and pons have been described. Arrived at the mid-brain, it 

 divides into three subdivisions the mesial fillet, the upper fillet, the lower or 

 lateral fillet. The first has already been described with the crusta. The upper 

 fillet passes to the superior corpus quadrigeminum. The lower fillet appears on 

 the dorsal surface (see below). Above, its fibres pass to the inferior corpus quad- 

 rigeminum, being reinforced by some fibres from the superior medullary velum. 

 It also receives, through the trapezium of the pons, some fibres from the ventral 

 auditory nucleus of the opposite side. (3) Fibres from the superior peduncle of 

 the cerebellum : these on leaving the pons dip ventrally, and beneath the corpora 



