724 THE NERVOUS SYSTEM. 



the upper and lower angles. From the fancied resemblance in the combined 

 lower end of this groove and lower angle, to the nib of a writing pen, this lower 

 angle has been named the calamus scriptorius. 



On jeach side of the median fissure are two spindle-shaped longitudinal emi- 

 nences, the fasciculi or funiculi teretes; they extend the entire length of the Moor. 

 Each eminence consists of Avhite fibres, and is due to a portion of the base of the 

 anterior cornu of gray matter of the cord which comes to the surface of the floor 

 of the fourth ventricle after the central canal of the spinal cord has opened out 

 into this cavity, This gray matter of the "base of the anterior cornu" now 

 constitutes the nuclei of origin of the hypoglossal and sixth cranial nerves. The 

 white fibres of the funiculus teres are partially those of the " ascending part " of 

 *the root of the seventh nerve (see page 722) and those of the formatio reticularis 

 of the posterior " area " of the medulla. 



The widest part of the floor of the ventricle is crossed by several white trans- 

 verse lines, linece transverse?, auditory strice, or striae acusticse ; they emerge from 

 the posterior median fissure, and, passing over the fasciculus teres of the same 

 side, some of the fibres enter the lateral root of origin of the auditory nerve, 

 while others may be traced to the flocculus of the cerebellum. Ventrally. through 

 the posterior median fissure, these fibres are traceable to the raphe. 



Below these striae, on each side, and external to the fasciculus teres, is a little 

 fossa, called the fovea inferior ; while above, similarly placed, is a fossa, called 

 the fovea superior. Extending upward to the top of the ventricle from each 

 superior fovea is a shallow groove ; this groove is called the locus coeruleus, which 

 presents a bluish tint through the thin stratum covering it. This tint is due to 

 an underlying stratum of pigmented nerve-cells (substantia ferruginea). The 

 locus coeruieus lies along the extreme lateral limit of the upper half of the floor of 

 the ventricle, and hence converges upward toward its fellow of the opposite side. 

 It is slightly overhung by the inner margin of the cerebellar superior peduncle. 

 Just ventral to the locus coeruleus in the substance of the pons is the motor 

 nucleus of the fifth nerve (see page 722). 



The fovea inferior is the depressed apex, which is directed upward, of a tri- 

 angular area. The floor of this triangular area is darker in color than the rest 

 of the floor of the ventricle; hence it is called the ala cinerea. The base, being 

 elevated in consequence of the depression of the apex, is knoAvn as the eminentia, 

 cinerea. The triangular area itself, as a whole, including inferior fovea (apex), 

 ala cinerea (floor), and eminentia cinerea (base), is known as the trigonum vagi. 

 Immediately ventral to this trigonum is the nucleus of origin of the vagus, and 

 at the apex is that of the glossopharyngeal nerve. 



Between the trigonum vagi and the mesial groove is the lower half of the 

 funiculus teres. This is triangular in shape, its base turned upward toward the 

 striae acusticae. This lower half of the funiculus teres is the trigonum hypoglossi. 

 Ventral to it is the nucleus of origin of the hypoglossal nerve. 



Between the trigonum vagi and the restiform body is another triangular area, 

 whose base is also directed upward, and across which the striae acusticae pass. 

 This area is the trigonum acustici. On its base is a slight eminence, the tuber- 

 culum acusticum. Ventral to this trigonum and tubercle is the dorsal nucleus of 

 the auditory nerv^. 



Between the superior fovea (above the striae acusticae) and the middle groove is 

 the upper half of the funiculus teres. Just ventral to this, but not close to the 

 middle line, is the nucleus of origin of the sixth rferve, while the superior fovea 

 itself may be taken as indicating the position of the nucleus of the seventh nerve> 

 which, however, is quite deeply situated in the pons (see page 722). 



THE CEREBELLUM. 



The cerebellum, together with the pons Varolii, forms the hind-brain. It is, 

 morphologically, the enormously thickened and hjpertrophied middle portion of 

 the brain-matter forming the r.oof of that part of the brain-cavity known as the 



