304 ¥. H. ED(rEwoirrH. 



veuti'al position of the nucleus ambiguus is only found in 

 Mammal.';, where the importancG of the ventral tegmentum 

 is increased by the pyramidal tract, Avhilst a part keeps its 

 original position near the mid-dorsal Hue because not very 

 much influenced by the long descending tracts of the frontal 

 parts of the brain. 



It would result from a comparison of these researches that 

 the ventral position of the Vllth nucleus, and of the nucleus 

 ambiguus of the IXth and Xth, is a secondary one, the curved 

 path of their issuiug fibres representing a phylogenetic descent 

 of the whole or part of their nuclei ; whilst the motor nucleus 

 of the Vth has preserved its original position. This position 

 is a dorso-median one. The nucleus of the Xlth spinal 

 occupies a more or less lateral position in the cervical cord, 

 but, as shown by the development of the muscles it inner- 

 vates, the nerve is a specialised branch of the Xth, the 

 nucleus of which has extended backwards into the spinal cord. 



The following classification of the motor nuclei of the 

 cranial nerves is a repetition from a neurological point of 

 view of the theory which has been advanced above concern- 

 ing the morphology of the cranial muscles, and consequently 

 stands or falls with it. 



Somatic, innervating muscles derived from the myotomes 

 of the cerebral and three anterior body segments ; Ilird 

 (external ocular muscles), IVth (superior oblique), Vlth 

 (external rectus), Vth (temporal, masseter, pterygoids, tensor 

 tympani, anterior digastric), Vllth (posterior digastric, stylo- 

 hyoid, stapedius), IXth (iuterarcualis ventralis I s. branchio- 

 hyoidens, when present), Xth and Xlth medullary (iuter- 

 arcualis ventralis III s. interthyroideus, in Ornithodelphia), 

 Xlth spinal (sterno-mastoid and trapezius), Xllth (hypo- 

 branchial spinal muscles, and lingual muscles derived from 

 the genio-hyoid). Splanchnic, innervating muscles derived 

 directly or indirectly from the walls of the cephalic ccjelon, 

 i.e. part of motor nucleus of Vth, which innervates mylohyoid; 

 part of motor nucleus of Vllth, which innervates facial and 

 platysma muscles; part of motor nuclei of IXth, Xth, and 



