592 THE NERVOUS SYSTEM. 



medulla oblongata it sends fibres across the median plane to the various moto: 

 nuclei on the opposite side of those sections of the brain-stem. 



DEVELOPMENT OF THE MESENCEPHALON. 



Even in the early embryo the mesencephalon constitutes the smallest section of th< 

 brain-tube, although the disproportion in size between it and the other primitive sub 

 divisions of the brain is not nearly so marked as in the adult. Owing to the cephalii 

 flexure, the mid-brain for a time occupies the summit of the head. Later it become! 

 completely covered over by the expanding cerebral hemispheres. 



The corpora quadrigemina are derived from the alar laminae of the side walls of th< 

 brain-tube, whilst the basal laminae thicken and ultimately form the tegmenta. Th< 

 original cavity of the mid-brain is retained as the aqueduct. 



For a considerable time the cavity of the mesencephalon remains relatively large, anc 

 the lower part of its dorsal wall is carried downwards in the form of a diverticulum 0] 

 recess, which overlaps the cerebellar plate. About this time, also, the dorsal wall shows i 

 median fold or ridge. Both of these conditions are transitory. As the corpora quadri 

 gemina take shape, the median ridge disappears and is replaced by the median longitudina 

 groove, which separates the quadrigeminal bodies. Only its inferior part is retained, and thii 

 is represented by the frenulum veli of the adult brain. The diverticulum of the cavitj 

 gradually becomes reduced, and finally disappears as the aqueduct assumes form. 



The precise mode of origin of the red nucleus is not known. 



Later in this account reasons will be given for the belief that the representatives o 

 the neural crests in the region of the mesencephalon become absorbed and assimilatec 

 in the walls of the neural tube as it closes in. 



THE DEEP CONNEXIONS OF THE CEREBRAL NERVES ATTACHED TO 

 THE MEDULLA OBLONGATA, PONS, AND MESENCEPHALON. 



There are twelve pairs of cerebral nerves, of which the inferior eight are attached 

 to the medulla oblongata and pons. From above downwards these are named the 

 trigeminal (fifth), the abducens (sixth), the facial (seventh), the acoustic (eighth), the 

 glossopharyngeal (ninth), the vagus (tenth),. the accessory (eleventh), and the hypo- 

 glossal (twelfth). Two others, the trochlear (fourth) and oculomotor (third) spring 

 from the mesencephalon. The hypoglossal, the accessory, the greater part of the 

 facial, the abducens, the motor root of the trigeminal, the trochlear and the oculo- 

 motor are efferent nerves; the acoustic, the nervus intermedius (sensory root oi 

 the facial) and the sensory root of the trigeminal are purely afferent nerves ; whilst 

 the vagus and the glossopharyngeal are composed of both efferent and afferent fibres. 

 In all these cases (with a possible reservation in the case of part of the trigeminal) 

 afferent fibres arise from ganglionic cells placed outside the brain and penetrate the 

 brain-stem, to end in connexion with the cells of certain nuclei of termination. 

 Efferent fibres, on the other hand, take origin within the brain as the axons of 

 cells which are grouped together in certain places in the form of nuclei of origin. 



Nuclei of Origin, or Motor Nuclei. In the spinal medulla the nuclei of 

 origin are represented by elongated columns of cells which run more or less con- 

 tinuously in the anterior column of gray matter of successive spinal segments, and 

 from these the series of efferent anterior nerve-roots take origin. In the medulla 

 oblongata, pons, and mesencephalon the nuclei of origin, or, in other words, the motor 

 nuclei of the individual nerves, become, for the most part, discontinuous, and are 

 represented by certain isolated clumps of compact gray matter, in which are placed 

 the clusters of cells from which the fibres of the efferent nerves arise. The nucleus 

 ambiguus, however, which consists of a column of cells from which root- fibres of the 

 bulbar part of the accessory, of the vagus, and also of the glossopharyngeal are 

 derived, is an exception to this rule. At the decussation of the pyramids, the anterioi 

 column of gray matter of the spinal medulla is broken up by the intercrossing bundle,' 

 into a detached head and a basal part which remains in relation with the ventro 

 lateral aspect of the central canal. Certain of the efferent nuclei of the medulk 

 oblongata, pons, and mesencephalon lie in the line of the basal portion of the anterio: 

 column of gray matter of the spinal medulla, and, thus, close to the median plane 

 These are termed medial somatic nuclei, and are met with at different levels in th< 



