THE MANDIBULAR AND HAOID MUSCLES OF MAMMALS. 61 1 
veli palatini, though in Ornithorhynchus and some Edentates 
its ancient insertion into the tympanic bone is still preserved. 
The proximal end of the tensor tympani disappears and the 
muscle gains a new origin. The disappearance, in the ontogeny 
of Dasyurus, pig and rabbit of the detrahens mandibuhe may 
also be related to the change of jaw-joint. Two slightly 
different views as to this muscle have been put forward. 
Toldt held that the muscle bei der Xeubildung- des Kiefero e- 
lenkes, als ein infolge des Schwindens des M. depressor 
mandibulge notwendig gewordener Factor fiir die Kieferbe- 
wegung, von dem M. masseter abgespalten hat^^; whilst 
Graupp was of the opinion that it sich zugleich mit dem 
Masseter selbst aus einer gemeinsatnen Mnskelmasse der 
Reptilieh (dem sogennanten M. capiti-mandibularis) bei der 
Nenbildung des Kiefergelenkes herandifferenzierte.^^ Ac- 
cording to both theories the muscle was developed with the 
new squamoso-mandibular joint. The above-recorded observa- ■' 
tions, however, suggest a converse explanation — that it is an 
old muscle possessed by Mammalian ancestors with an incudo- 
meckelian jaw-joint, which has dropped out, except in 
Monotremes, owing to its being inserted behind the new joint. 
The phylogenetic history would thus present an interesting 
contrast to that of the pterygo-tympanicus and tensor 
tympani. 
The theory above advocated differs in some particulars 
from that advanced by Gfaupp. In his treatise on Die 
Reichertsche Theorie ” it is assumed that the substitution of 
an incudo-meckelian jaw-joint by a squamoso-mandibular one 
was accompanied by a change in the masticatory muscles — 
from a Reptilian to a Mammalian type. The developmental 
phenomena occurring in the muscles, however, suggest that 
this substitution occurred subsequent to changes in the 
masticatory muscles from what may be called a pre-amphibian 
type, i. e. one characterised by median and lateral masticatory 
muscles and a levator hyoidei, to a Mammalian one. 
The amount of such change is very doubtful. Possibly 
Mammalian ancestors with an incudo-meckelian jaw-joint 
