EXISTING ORGANISMS— ANATOMY 



73 



of adjacent sacral vertebrae is the most complete in birds and in 

 man, and for the same reason, namely the employment of the 

 hind limbs alone for the support of the body, although in the two 

 cases the number and arrangement of the associated parts differ 

 very considerably." He adds that "variation in the sacral region 

 is not confined to the lower forms, although it is more frequent 

 in these latter (e.g., Necturus) and becomes relatively stable in 

 the higher and more specialized classes." 



The Visceral Skeleton. In the visceral skeleton and associated 

 membrane bones relationships are in some cases more obscure, 

 but as worked out by comparative anatomists and checked by 



Fig. 48. — Variations in the composition of the human sacrum. (From Wilder's 

 History of the Human Body, after Gegenbaur, with the permission of Mrs. 

 H. H. Wilder, and Henry Holt and Company.) 



the facts of embryology, they are well established and remarkable 

 (Fig. 49). 



The Jaws. The upper and lower jaw cartilages (pterygo- 

 quadrate and Meckel's, respectively) have already been mentioned. 

 These belong to the visceral skeleton, and in the elasmobranch 

 fishes are the only skeletal structures about the mouth. In higher 

 groups each is associated with membrane bones which finally 

 supplant it entirely as the skeleton of a jaw. The membrane 

 bones of the upper jaw include the anterior premaxillaries, which 

 lie at the tip of the skull, just below the anterior nares. Behind 

 them are the larger maxillary bones, followed along the outside 

 of each cartilage by a zygomatic (malar or jugal), a quadratojugal 

 and a squamosal bone. On its mesial surface it is associated with 

 an anterior palatine and a posterior pterygoid which form part 

 of the roof of the mouth. The posterior part of the cartilage 

 itself gives rise to the quadrate bone which sometimes intervenes 



