THE MANDIBULAE AND HYOID MUSCLES OF I\[AMMALS. 605 
The division of the muscles and nerve-branches into medial 
and lateral groups is further evidenced, as I have recently 
shown, by the differing paths of the nerve-fibres — both motor 
and sensory — into the rami. The fibres of the ramus lateralis 
have a simple direct path, whilst the fibres of the ramus 
medialis are for a space split into two parts by the ramus 
posterior.^ Both rami contain motor fibres from the motor 
root, and muscle-sensory fibres from the Gasserian ganglion. 
The division is also corrobatedby the grouping of the cells 
in the motor nucleus of the trigeminal nerve. Willems found 
three groups of cells in this nucleus^ in the rabbit — a dorsal, 
a ventro-median, and a ventro-lateral. The occurrence of 
chromolytic changes after avulsion of individual motor 
branches showed that the external pterygoid, the temporal 
and sphenoidal,^ and the masseter, are innervated by the 
dorsal group, the internal pterygoid by the ventro-lateral 
group, the mylohyoid and the anterior digastric by the ventro- 
median group. No chromolytic changes Avere found after 
avulsion of the tensor tympani, and no operations were 
performed on the tensor veli palatini. 
Willems Avas of the opinion that ces groupeinents cel- 
lulaires repondent relativement bien a des functions differ- 
entes."” The internal pterygoid, hoAvever, is more closely 
associated in function Avith the temporal and masseter than is 
the external pterygoid. The grouping agrees Avith the 
division of their Anlage into lateral and medial masticatory 
muscles or rauscle-Anlagen during development, and is a 
morphological one — probably dating from a period antecedent 
1 I showed this to he the case in Man, Macacus, dog, and rabbit. I 
can now also add Dasynrns, Ornithorhynchns, and Echidna. 
2 AVillems applied the term “ masticatory nucleus ” to the whole 
of the motor nucleus of the trigeminus. In this paper the term 
“ masticatory ” is used — as is usual in English text-books — to denote 
those muscles innervated by the fifth cranial nerve which are situated 
dorsal to the lower jaw, i. e. to the exclusion of the mylohyoid and 
anterior digastric, Avith a similar restriction in the case of the parts of 
the motor nucleus. 
3 The part of the temporal innervated by the anterior deep temporal 
nerve. 
