6 5 6 



RODENTS AND RABBITS 



xxv. 3 



the rodents is that the upper tooth-rows are set closer together than the 

 lower and bite inside the latter, often giving an oblique grinding surface. 

 The milk teeth are shed very early and are not functional. 



The lower jaw and its muscles show many modifications. The 

 articulation is very long, and the lower jaw moves backwards and 

 forwards on the upper; indeed the lower incisors are thrust so far 

 forward that while gnawing the molar surfaces no longer occlude. A 



Fig. 423. The masseter of rodents. 



A, Aplodontia, primitive sciuromorph : the muscle takes origin from the zygomatic arch ; 



B, myomorph: the deep masseter passing through the orbit is also attached to the face; 

 c, advanced sciuromorph: the superficial masseter is attached to the skull in front of the 

 orbit; D, hystricomorph: superficial masseter unspecialized, large foramen for deep portion. 



(After Romer, Vertebrate Paleontology, University of Chicago Press.) 



curious feature is that the mandibles are not united in a symphysis but 

 are freely movable, with a joint cavity between them. A special portion 

 of the mylohyoid muscle, known as the m. transversus mandibulae, 

 draws the two mandibles together, causing the lower incisors to 

 separate (Fig. 422). The action of the lateral portions of the masseters 

 then brings the two teeth together again with a scissor action. 



The jaw-muscles are very large and modified to produce the back- 

 ward and forward motion of the jaw (Fig. 423). In the more primitive 

 condition, found in squirrels (Sciuromorpha), the masseter is attached 

 to the zygomatic arch as it is in other mammals, but is divided into 

 a more lateral portion with simple up and down action, and a medial 

 part that pulls the jaw forward. In more advanced rodents both of 

 these parts of the muscle obtain extra insertions. In rats and mice 

 (Myomorpha) the lateral part extends forward on to the face and 



