390 



PHYSIOLOGY 



CHAP. 



tendency of the first segment of the trunk to become modified 

 according to the type of the occipital segments. 



B 



FIG. 200. Diagram to show situation of chief nerve-nuclei and terminations of cranial nerves in 

 medulla oblongata and pons near floor of fourth ventricle. Twice the natural size. A, from 

 behind ; B, profile view of right half, the medulla and pons being supposed to lie transparent. 

 The efferent or motor nuclei are coloured red, the afferent or sensory nuclei, blue. In A the 

 motor nuclei are represented on right side only, the sensory on the left. Ill, IV, oculomotor 

 and trochlear nucleus; I'd, descending root of 5th nerve; 1's, so-railed sensory nucleus of 

 5th; l'a, ascending root of 5th; I'm, motor nucleus of 5th; VI, nucleus of abdncens; 

 VII, nucleus of facial ; ?tVII, root of facial curving round abducens nucleus ; VIII, inner or 

 dorsal nucleus of auditory ; VIII', outer or ventral nucleus of audit' ay ; IX. X. vm;n-<_;losso- 

 pharyngeal nucleus ; na, nucleus ambiguus. accessory or efferent vago-glosso-pharyngeal 

 nucleus; XI, nucleus of spinal accessory; XII, nucleus of hypoglossal ; XII', issuing roots 

 of hypoglossal. 



At its origin the hypoglossal is an exclusively motor nerve. 

 This was recognised by Galen, who in Book VIII. cap. v. " de 



