552 THE NERVOUS SYSTEM. 



the sterno-mastoid and trapezius muscles and the striated muscles of the larynx, 

 pharynx and face and those concerned with mastication ; and (3) a splanchnic 

 column of nuclei, giving efferent fibres which pass out in the vagus, glosso- 

 pharyngeal and facial nerves, to be widely distributed to unstriped muscle, glands 

 and other tissues in the head, neck, thorax and abdomen. 



Further, the terminal nuclei of the sensory nerves which are developed in the 

 alar lamina of the rhombencephalon do not unite to form a definite posterior 

 column, as happens in the spinal medulla, but form discrete masses ; and as 

 these act as receptive organs for a much greater variety of sensory nerves than 

 are represented in the spinal nerves there is a much greater number of nuclei than 

 would be formed if the various components of the posterior column in the spinal 

 medulla were dissociated. Thus, there are terminal nuclei in the medulla oblongata 

 not only for the ordinary cutaneous nerves, but also for nerves coming from the 

 mucous membranes of the alimentary and respiratory organs, as well as from 

 other visceral structures ; and there are also special nerves of taste (nervus 

 intermedius and glossopharyngeal), of hearing (cochlear part of the acoustic) and 

 of equilibration (vestibular part of the acoustic). But this does not exhaust the 

 peculiar features of the terminal sensory nuclei of the rhombencephalon. In the 

 description of the spinal medulla attention was called to the fact that certain of 

 the fibres of the posterior nerve-root did not end in the gray matter of the spinal 

 medulla, but passed upwards throughout the whole length (above their points of 

 entry) of the funiculus posterior to reach the medulla oblongata. Special terminal 

 nuclei are developed from the alar lamina to receive these fibres. They are the 

 nucleus gracilis and nucleus cuneatus. 



In addition, part of the terminal vestibular nucleus receives accessions of fibre? 

 from these (gracile and cuneate) nuclei as well as from other sensory terminal 

 nuclei in the spinal medulla and develops into that great mass of tissue, the 

 cerebellum, to which vast numbers of other fibres come and go, adding considerably 

 to the complexity of the region of the pons and medulla oblongata. Moreover 

 there is developed from the alar lamina a whole series of other masses of gra) 

 matter the nucleus olivaris inferior, nuclei arcuati, nucleus pontobulbaris and nude 

 pontis as links in the complex chains that bind all parts of the central nervous 

 system to the great co-ordinating mechanism of the cerebellum. 



Thus it comes about that, instead of having, as in the spinal medulla, a definite 

 column of gray matter ensheathed in a thick mass of white substance, the rhomb 

 encephalon is composed of many scattered masses of gray matter; and its white sub 

 stance is represented partly by great longitudinal strands, but also by man? 

 great systems of fibres passing transversely through its substance, or upon it 

 surface, e.g., the superficial fibres of the pons and many of the arcuate fibres. 



From what has already been said concerning the external form of the rnedull 

 oblongata and pons it will be apparent that the distortion of the neura 

 tube which occurred as the result of the pontine flexure has also been large! 

 responsible for the distinctive features of this region of the brain.i 



As the pontine flexure develops, a strain is thrown upon the thin roof-plat< 

 which yields and becomes stretched so as to permit the thick lateral walls of th 

 neural tube to fall laterally (Figs. 483 and 484). One result of this process is th 

 great lateral expansion of the cavity of the hind-brain, which assumes the charac 

 teristic rhomboid form. If the thin and greatly attenuated epithelial roof is tor 

 away from the rhombencephalon of an embryo of the third month the fourt 

 ventricle will present the appearance (viewed from behind) shown in Fig. 48 

 The ventricle is seen to be prolonged laterally/on each side, to form a little rece; 

 upon the lateral aspect of the rhombencephalon. This is called the recessus lateral] 



This thin epithelial roof becomes invaginated towards the cavity of the fourt 

 ventricle, on each side of the median plane, in the whole length of the epitheli 

 roof, i.e. from the cerebellar attachment, above, almost as far as the closed part 

 the medulla oblongata below (Fig. 519). The upper end of this invaginated fo 

 becomes prolonged laterally ag far as the extremity of the recessus lateralis (F 

 527). Pia mater and blood-vessels extend into these folds, which are then knov 

 as the chorioid plexuses of the fourth ventricle. At the extremities of these to 



