THE j\IANDlBULAK AND HYOID MUSCLES OE MAMMALS. 579 
masticatory muscles consist of long striated muscle-cells, in 
two groups — a medial and lateral. The anterjor fibres of the 
medial muscle descend vertically from just below the ala 
temporalis towards Meckel’s cartilage, those next behind pass 
downwards and backwards, and the succeeding ones more and 
more backwards; the hindmost reach as far back as the hind 
end of Meckel’s cartilage; there is no break in the muscle 
mass. The lateral muscle consists of fibres which, anterioi’ly, 
pass towards the upper edge and outer side of the anlage of 
the mandible; the upper edge of the innermost fibres is close 
to the ala temporalis, whilst that of the moi'e external fibres 
is outside the lateral surface of the Gasserian ganglion. The 
muscle-fibres which dorsally lie behind the hind end of the 
Anlage of the mandible pass downward and forwards towards 
its outer surface, and the lower end of this portion of the 
muscle is not continuous with that of the anterior portion; 
dorsally, the two portions of the muscle are continuous. 
The third division of the fifth nerve passes downwards 
between the medial and lateral muscles, giving off the lateral 
and medial rami to the muscles. The buccal nerve penetrates 
the inner fibres of the lateral muscle. 
The intermandibnlaris forms a ventrally curved sheet, 
attached on each side to the inner surface of Meckel’s 
cartilages. It does not at this stage extend further back. 
The depressor mandibulas anterior is being proliferated trans- 
versely outwards from the under surface of the interman- 
dibularis. 
In a 25 mm. specimen of Echidna ( = Stage 50 of Semon), 
the masticatory muscles consist of pterygo-t^onpauicus, tensor 
tympani, temporal, external pterygoid, zygomatico-mandi- 
bularis, masseter, and detrahens mandibulte (figs. 34-36). 
The pterygo-tympanicus arises from the (Mammalian) 
pterygoid bone and is inserted into Meckel’s cartilage. The 
tensor tympani arises fioin the (Mammalian) pterygoid bone 
and is inserted into .the malleus poi tiou of Meckel’s cartiluge. 
The ramus medialis of the mandibular division of the fifth 
nerve innervates these two muscles. The detrahens mandi- 
