HUMAN AND MAMMALIAN. 15 



of multipolar ceUs noticed above. The further course of the roots wiU be best 

 understood by reference to Plate XI, Fig. 40, representing a transverse section of the 

 hypoglossal nucleus, on a level with the vagus nucleus and juSt above the calamus 

 scriptorius. A portion of the fibres are connected with cells of the outer group (6), 

 thus becoming united secondarily with the great bundles of fibres proceeding from 

 the vagus, by which the hypoglossal nucleus is bordered anteriorly (Z>). By far 

 the greater number^ however, pass through these border fibres and cells, penetrating 

 the nucleus, untU they reach the groups lying in the central and posterior portion (7i). 

 A few fibres may be seen to leave the bundles, either just before or soon after they 

 cross the border, and pass along with the latter towards the raphe, or else branch 

 out into the anterior columns and join some of the numerous bundles passing towards 

 the raphe, where they decussate with those coming from the opposite side (c). It 

 is impossible to trace their further course. Numerous fibres, often forming consider- 

 able bundles, may be seen either just as they enter the nucleus, or not unfrequently 

 in its central region, sometimes forming quite a sharp curve (w, to), and bending 

 back towards deeper lying cells, or towards the nucleus of the vagus, which some 

 fibres from the hypoglossal certainly enter, forming a counterpart to the relation estab- 

 lished, as noticed by Clarke^ and confirmed by my own observations, between some 

 of the hypoglossal roots and those of the spinal accessory. 



The decussation of the hypoglossal roots, first pointed out by KoUiker, has 

 recently been denied by Schroder van der Kolk. KoUiker^ states that there is a 

 " total decussation of the roots of both sides, on the floor of the fourth ventricle, so 

 that those from one nucleus pass over into that of the opposite side." Lenhossek' also 

 makes a similar statement with regard to the inner nerve bundles. Clarke* states 

 that fibres from the hypoglossal " bend inwards and decussate through the raphe 

 with their opposite fellows." 



On the other hand, Schroder van der Kolk,® after many investigations on diff"erent 

 animals, as well as on the human meduUa, was able " to completely satisfy himself 

 that this nerve does not decussate, but is lost entirely in the hypoglossal nucleus, 

 being connected with multipolar cells by numerous fibres." He states, however, 

 that the two nuclei are brought into connection by means of commissiural fibres 

 crossing the raphe and derived from the cells on each side. 



The question is by no means an easy one to decide ; my first attempts at solving 

 it led me to think that Schroder van der Kolk was right in his opinion, but irj 

 going over the whole ground again with very great care, and examining specimens 

 from the medulla of man and various animals, prepared by dififerent methods,' I 

 could have no doubt that some of the hypoglossal roots certainly decussate directly 

 at the raphe, standing about in equal proportion to the main bundles as do those of 



« Philos. Transactions, 1858, 252, 253 ; pi. xvii, fig. 35. 

 " Mikroskopische Anatomie. II, 459. ^ Neue TJntersuchungen, 82. 



♦ Philos. Transactions, 1858, 253. * Medulla Spinalis and Oblongata, 1859, 97. 



^ Especially specimens hardened in chromic acid and made transparent by turpentine, this method 

 seeming to me decidedly the best for tracing the course of fibres. 



