OUR ANCIENT RELATIVES 



the formation of hair had already begun in them. 

 At any rate, there is evidence that the bony mask 

 of the earher reptiles was aheady beginning to 

 become leathery on its outer layer. 



Even in the most primitive of living mammals 

 the hard bony mask of the face has already begun 

 to sink beneath the surface and a more or less 

 pliable skin has been developed. But the most 

 remarkable fact is that as the bony mask sank 

 beneath the surface the "facial muscles," so char- 

 acteristic of mammals alone among vertebrates, 

 came into being. Where did they come from.? 

 In the reptiles the neck and throat are covered by 

 a thin wide band of muscle called the primitive 

 sphincter colli, which is activated by a branch of 

 the seventh cranial nerve. In mammals this 

 muscle, besides giving rise to the platysma muscle, 

 has grown forward between the bony mask and 

 the skin, along the sides and top of the face. As 

 it grew forward over the cheek it sent out various 

 subdivisions which either surrounded the eyes, or 

 covered the forehead and cheeks, or surrounded 

 the lips, or connected the lips with the cheeks, or 

 were attached to the ears. Whenever the muscle 

 mass sent forth a new branch it also sent into this 



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