390 



PHYSIOLOGY 



CHAP. 



tendency of the first segment of the trunk to become modified 

 according to the type of the occipital segments. 



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 be 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 ; Vd, descending root of 5th nerve ; Vs, so-called sensory nucleus of 

 5th ; Va, ascending root of 5th ; Vm, motor nucleus of 5th ; VI, nucleus of abducens ; 

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

 dorsal nucleus of auditory ; VIII', outer or ventral nucleus of auditory ; IX, X, vago-glosso- 

 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 



