THE MEDULLA OBLONGATA. 701 



idol or sensory decussation. The caput cornu is displaced outward, so as almost to 

 reach the surface, where it forms a projection, the funiculus Rolando, which 

 enlarges above into a distinct prominence, the tubercle of Rolando. Above the 

 level of the tubercle of Rolando the caput cornu is separated from the surface by 

 a band of fibres, termed the ascending root of the fifth nerve, and by the external 

 arcuate fibres. The neck of the cornu becomes broken up into a reticular forma- 

 tion by the decussation of the columns of Goll and Burdach, and by this means 

 the caput is separated from the rest of the gray matter. The base of the cornu 

 increases in size, and, as the central canal expands into the fourth ventricle, 

 becomes pushed outward, and portions of it extend into the funiculi graciles and 

 cuneati, and produce externally the eminences of the clava and cuneate tubercle. 

 A third portion of the base becomes separated from the rest, and is placed outside 

 the nucleus of the funiculus cuneatus. This is called the accessory cuneate nucleus, 

 and is supposed to be a continuation upward of Clarke's vesicular column of the 

 cord. 



The Upper Part of the Medulla. The upper part of the medulla comprises the 

 portion which enters into the formation of the floor of the fourth ventricle, where, 

 in fact, the upper end of the central canal has opened out into this cavity. In 

 this region the formatio reticularis is confined chiefly to the anterior and lateral 

 areas. In the ventral portion of the posterior area there is only a small amount 

 of reticular formation, but in addition to this there are individual masses of cells 

 scattered among the longitudinal fibres. 



The formatio reticularis is situated in the medulla, behind the pyramid and 

 olivary body, extending laterally as far as the restiform bodies, and dorsally to 

 within a short distance of the floor of the fourth ventricle. It presents a peculiar 

 reticulated appearance, from which it derives its name, and which is due to the 

 intersection of bundles of fibres running at right angles to each other, some being 

 longitudinal, others transverse. The formatio reticularis presents a different 

 appearance in the anterior area from what it does in the lateral area. In the 

 former there is almost an entire absence of nerve-cells in the reticulated network, 

 and hence it is known as the formatio reticularis alba ; whereas, in the lateral 

 area, the nerve-cells are numerous, and, as a consequence, this part is known as 

 the formatio reticularis grisea. In the substance of the formatio reticularis is a 

 small nucleus of gray matter. It is situated near the dorsal aspect of the hilum 

 of the olivary nucleus, and has been named the inferior central nucleus. The 

 fibres of the formatio reticularis are longitudinal and transverse. In the anterior 

 area the longitudinal fibres may be arranged in two well-defined sets : (1) one set 

 lies immediately behind the pyramid, and is named the fillet or lemniscus. The 

 fibres of the fillet are chiefly derived from the cells of the gracile and cuneate 

 nuclei, and may therefore be regarded as relay fibres of the columns of Goll and 

 Burdach of the spinal cord, which terminate in synapses around the cells of the 

 gracile and cuneate nuclei. They are prolonged inward and forward across the 

 middle line forming the superior pyramidal or sensory decussation (decussation of 

 the fillet) ; (2) the other set is continued from the antero-lateral ground bundle 

 of the cord, and a portion of these fibres forms the posterior longitudinal 

 bundle already referred to (page 695). Both these sets of fibres are continued 

 upward into the pons and mid-brain. The longitudinal fibres of the reticular 

 formation in the lateral area are not arranged in distinct bundles. They 

 are derived from the lateral column of the cord, after the crossed pyramidal 

 tract has passed over to the opposite side. The longitudinal fibres of the posterior 

 area are merely indeterminate fibres of the formatio reticularis, with the exception 

 of two distinct bundles, which may be regarded as ascending roots of the fifth and 

 vago-glosso-pharyngeal nerves ; the latter is termed the funiculus solitarius. 



The transverse fibres of the reticular formation are the arched or arcuate fibres. 

 The external arciform fibres have already been described. The internal arciform 

 fibres are more numerous than the superficial set ; they traverse nearly the whole 

 area of the upper half of the medulla, except the pyramid. They pass from the 



