812 THE NERVOUS SYSTEM 



The brachium conjunctivum or superior cerebellar peduncle emerges from the 

 cerebellum on the medial side of the brachium pontis and also on the superior and 

 medial side of the course of the restiform body. It forms the lateral boundary of 

 the superior portion of the fourth ventricle and is the cerebello-cerebral peduncle. 

 Its transverse section appears semilunar in shape, with the concave side next to the 

 cavity of the ventricle. The medial border, which inclines toward the mid-line, 

 is connected with that of the corresponding peduncle of the opposite side by the 

 anterior medullary velum, which thus roofs over the superior part of the fourth 

 ventricle. The lateral border is distinguished from the pons by an open furrow 

 or lateral sulcus. 



The superior cerebellar peduncles are almost entirely efferent pathways as to the cerebellum 

 and form the chief connections between the cerebellum and the cerebrum. They arise almost 

 wholly from the dentate nuclei. As they course forward they slightly converge and disappear 

 under the inferior quadrigeminate bodies. Here, in the tegmentum of the mesencephalon) 

 they undergo an almost total decussation, and then the majority of the fibres of each peduncle, 

 having thus crossed the mid-line, terminate in the red nucleus of the opposite side. The red, 

 nucleus lies in the tegmentum of the mesencephalon, below the superior quadrigeminate bodies, 

 and therefore quite close to the decussation. The cells of the red nucleus, about which the 

 fibres of the peduncle terminate, in their turn send processes (axones) into (1) the rubro-spinal 

 tract of the spinal cord and (2) into the prosencephalon, most of which latter terminate in the 

 thalamus whose cell-bodies give fibres to the cerebral cortex by way of the internal capsule; 

 but some pass from the red nucleus under the thalamus to join the internal capsule. 



In addition to the fibres having the origin and course described above, and which constitute 

 the greater mass of the superior cerebellar peduncle, each peduncle is said to contain fibres 

 which — (1) arise in the cerebellar cortex of the same and opposite sides of the mid-line, instead 

 of from the dentate nucleus, and which join the peduncle at the side of the dentate nucleus, 

 between it and the restiform body; (2) fibres which do not cross the mid-line in the decussation, 

 but terminate in the red nucleus of the same side; (3) some fibres are not interrupted in the red 

 nucleus, but pass directly into the thalamus; (4) a small proportion of fibres afferent as to the 

 cerebellum, which arise in the structures of the cerebrum and pass in to the cerebellum; and 

 (5) the greater part, if not all, of the ascending fibres of the superficial ventro-lateral spino- 

 cerebellar fasciculus (Gowers' tract) of the spinal cord. The latter, instead of entering 

 the cerebellum by way of the restiform body, are deflected in the upper medulla and pass 

 in the lateral tegmentum of the pons to the anterior medullary velum, where they turn back- 

 ward to enter the cerebellum in its superior peduncle and pass to its cortex, probably from the 

 lateral side of the dentate nucleus (see fig. 656). 



The anatomy of the fourth ventricle. — The fourth ventricle is rhomboidal in 

 shape, being considerably widened at the level of the brachia pontis and pointed at 

 each end. Its floor consists of a shght depression in the brain-stem, the fossa 

 rhomboidea, and corresponds to the floor of the central canal. Its pointed inferior 

 end, the calamus scriptorius, is directly continuous with the central canal, and its 

 narrowed superior end is continued into the aquaeductus cerebri (Sylvii) of the 

 mesencephalon, which is nothing more than a resumption of the tubular form of 

 the canal. 



The entire cavity of the ventricle is fined with an epithelium which is continuous with the 

 epithelium, or ependyma, of the central canal below and the aqueduct above. The entire 

 ventricle involves the isthmus of the rhovihenccphalon, the metcncephalon and a portion of the 

 medulla oblongata. It is divided for study into an inferior, an intermediate and a superior 

 part. 



The roof of the superior poriion of the fourth ventricle is nervous, consisting of a 

 thin lamina of white substance, the anterior (superior) medullary velum, thickened 

 at the sides by the brachia conjunctiva. At its extreme mesencephalic end (in the 

 isthmus of the rhombencephalon) the anterior medullary velum is slightly thick- 

 ened by a continuation of the white substance of the inferior quadrigeminate 

 bodies, forming the frenulum veil. The inferior portion of the velum is contin- 

 uous with the white substance of the cerebellum, and is covered by the lingula 

 cerebelli, an extension of the cortical substance of the superior vermis (fig. 631). 



The roof of the intermediate portion of the fourth ventricle is formed by the 

 cerebellum proper, the vermis and the mesial portions of the hemispheres. The 

 nervous portion of the roof terminates with the posterior (inferior) medullary 

 velum, a thin, narrow band of white substance which is the continuation of the 

 peduncles of the floccular lobes, and which connects them at the mid-line with the 

 nodule of the inferior vermis. 



"■J'lio roof (f the inferior portion of the fourth ventricle is non-nervous. It is 

 the chorioid tela of the fourtli v(;ntriclo, a semilunar lamina consisting of the epi- 

 thelial lining of the ventrich;, reinforced by a continuation of the connective tissue 

 of the pia mater and the adjacent portion of the arachnoid. Along the line of its 



