vii THE MEDULLA OBLONGrATA 389 



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, w.XII) ; 

 above, where the spinal canal opens to 

 form the rhomboid sinus, it assumes a 

 dorsal position (Fig. 206, w.XII). 



_,. r i r> 11 i Fl - 208. Anterior boundary (floor) of 



Ihe nUCleUS OI the hypOglOSSUS fourth ventricle- (Schafer.) Natural 



consists of a group of large ganglion ^'5^1ffi^tn 

 cells, enclosed in a fine nervous net- ^J^ySSSf^SSSSl 



WOrk (Fig. 210); their axis-Cylinders ? -'-. lateral recess; i.f., inferior 

 , i n ,-, i .-, .. . (posterior) fovea ; a.c., ala cinerea ; 



run ventralwards through the lormatio .o.,tri g onumacustici; ./., superior 



rptipnlariq flnH pmpro-p ftv. n CSPTIPCJ (anterior) fovea, close to lateral 

 erge, as a i .IBS margin of superior part of ventricle. 



of little bundles, between the olive 



and the pyramids. Boiler'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 



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