674 THE NERVOUS SYSTEM. 



to each other in the mesial plane by a prolongation forward of the raphe" or median 

 septum of the pons. Laterally the tegmenta are free, but dorsally they blend 

 with the corpora quadrigemina. 



Crust*. The crustse, which are two in number, separated by the interpeduncu- 

 lar space, are semilunar in section, and consist of longitudinal bundles of white 

 fibres, which may be divided into three principal sets : (1) Those occupying the 

 outer third of the crusta are believed to arise from the cells of the nuclei pontis, 

 gray nuclei in the pons Varolii, and pass through the posterior part of the 

 internal capsule to the cerebral cortex of the occipital and temporal lobes. (2) The 

 fibres occupying the middle third of the crusta take their origin in the cells of 

 the Rolandic area of the cortex, and, converging to the internal capsule, pass down 

 through its genu and through the anterior two-thirds of its posterior limb to the 

 crusta, from which they are prolonged through the pons into the anterior pyramid 

 of the medulla oblongata. (3) The origin, from below, of the fibres occupying the 

 inner third of the crusta is uncertain, though by some they are believed to arise in 

 the crusta itself, from the cells of the locus niger. Above, they pass through the 

 anterior part of the internal capsule to the cerebral cortex of the frontal lobe. In 

 addition to these three sets of longitudinal fibres, a w ell-marked bundle, defined 

 by having an oblique direction, must be noted. This is named the mesial fillet. 

 It arises from the fillet (see below), and at the lower part of the crusta is situated 

 at its mesial border ; as it ascends it courses obliquely outward to reach the lateral 

 border of the middle group of fibres (pyramidal tract) and becomes lost in the 

 subthalamic region. 



The tegmentum, or that portion of the mid-brain which is superior to the sub- 

 stantia nigra, consists of longitudinally directed strands of white fibres, which are 

 separated from each other by transversely arched fibres. There is also a consider- 

 able quantity of gray matter. It thus forms a peculiar reticulated structure, which 

 has been named formatio reticularis, and is similar to a like structure- in the pons 

 and medulla, with which it is continuous. In some parts of the tegmentum the 

 longitudinal fibres are arranged in fairly well-defined tracts, which are as follows : 

 1. The posterior longitudinal bundle, which is composed of large nerve-fibres, and 

 lies on either side of the median line, just below the aqueduct. These fibres are 

 continued upward from the anterior column of the cord, in which they probably 

 form short longitudinal commissures between its different segments. They pass 

 through the pyramid of the medulla, then form the posterior longitudinal bundle 

 of the pons, and enter the tegmentum ; here they give off fibres to the nuclei of the 

 third and fourth cranial nerves. At the front of the mid-brain some of the fibres 

 of the posterior longitudinal bundle enter the posterior commissure and there 

 decussate (see page 671) ; others pass upward to the subthalamic region. 2. Fibres 

 from the superior cerebellar peduncle. These lie on either side of the middle line 

 of the tegmentum, and, as they pass through it, decussate with each other, so that 

 the fibres of one half of the cerebellum pass to the opposite half of the cerebrum. 

 Having crossed to the opposite side, the bundle of fibres passes upward and for- 

 ward, enclosing a mass of gray matter, the red nucleus, or nucleus of the tegmen- 

 tum, from which it probably receives fibres, and eventually passes into the optic 

 thalamus. 3. The fillet. This takes its chief origin in the medulla, and passes 

 through the pons to the mid-brain, as will be described in the sequel. It forms a 

 considerable bundle of longitudinal fibres in the ventral part of the tegmentum, and 

 divides into three parts the upper, mesial, and lower fillet. The upper fillet 

 passes to the upper pair of corpora quadrigemina and the occipital region of the 

 cerebral hemisphere. The mesial fillet has already been alluded to in the descrip- 

 tion of the crusta. After separating from the rest of the fillet its fibres assume an 

 oblique direction, and are eventually lost in the subthalamic region. The lower 

 fillet, also called lemniscus, is situated in the ventral part of the tegmentum, 

 through which it passes obliquely and emerges at its side, and after crossing the 

 superior peduncle of the cerebellum, passes to the inferior quadrigeminal bodies. 

 It is reinforced by some fibres from the superior medullary velum. 4. Fibres from 



