VII 



THE MEDULLA OBLONGATA 



389 



str 



to. 



their origin and mode of development that it seems advisable to 

 study them separately, in discussing the olfactory and visual 

 senses. Embryologically, they are not, like the other cranial 

 nerves, mere prolongations from the walls of the primitive neural 

 tube, but vesicles that have budded out from that tube, the lumen 

 being subsequently obliterated. 



The apparent origin of the ten remaining pairs from the 3rd 

 to the 12th is readily seen from a glance at the base of the 

 brain (Fig. 201). Their real origins lie in more or less elongated 

 .nuclei, which extend from the caudal end of the bulb to the 

 cranial end of the ventral wall of the Sylvian aqueduct. Fig. 

 209 gives an approximate idea of their positions. 



II. The nucleus of origin of the Hypoglossus consists in a long 

 column of grey matter in the im- 

 mediate vicinity of the median line. 

 It begins at the level of the striae 

 acusticae, and ends a few millimetres 

 below the tip of the calamus scrip- 

 torius ; its total length is approxi- 

 mately that of the olive. Below, it 

 occupies a ventral position in respect 

 of the spinal canal (Fig. 205, n.XII) ; 

 above, where the spinal canal opens to 

 form the rhomboid sinus, it assumes a 

 dorsal position (Fig. 206, ?^.XII). 



The nucleus of the hypoglossus 

 consists of a group of large ganglion 

 cells, enclosed in a line nervous net- 

 work (Fig. 210); their axis-cylinders 

 run ventralwards through the formatio 

 reticularis and emerge, as a series 

 of little bundles, between the olive 

 and the pyramids. Eoller's nucleus of small cells, as shown in 

 the figure, is not an accessory nucleus of the hypoglossus, but 

 belongs to the diffuse nucleus of the formatio reticularis. 



Morphologically speaking, the hypoglossal is not a simple 

 nerve, but a compound one, formed by the union of at least three 

 ventral roots fused into a single 'trunk, as may be seen from a 

 study of its root filaments. In all probability it originally had 

 a corresponding dorsal root on the type of the spinal nerves, and 

 this has in fact been described as an anatomical variation in the 

 ox, dog, pig (C. Mayer), in the cat (Vulpian), and also in man 

 (Vulpian, Chiarugi). Complete or partial disappearance of dorsal 

 roots during phylogenesis can also be seen in the first spinal 

 nerves in reptiles and birds. In adult man, more often than in 

 other mammals, the dorsal roots of the 1st cervical nerve are 

 rudimentary, which, as Chiarugi rightly remarks, is a proof of the 



2 C i 



Flu. 20S. Aiitei inr boundary (floor) of 

 fourth ve utricle. (Schafer.) Natural 

 size, in.s., median sulcus ; .-'/, striae 

 acusticae. marking Limit between 

 pontine part of ventricle ami medul- 

 lary part of calamus sr-riptorius ; 

 Lr., lateral recess ; i.f., inferior 

 (posterior) fovea ; a.c., ala cinerea ; 

 t.a., tri.u'onuin acustici ; s.f., superior 

 (anterior) fovea, close to lateial 

 margin of superior part of ventricle. 



