THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 



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7. The second or hyoid gill arch gives rise to a cartilage bone, the hyomandibular, and 

 the remainder of it takes part in the formation of the hyoid apparatus. 



8. All other bones of the skull and jaws are membrane bones. 



9. The number of membrane bones of the skull is greatest in the lower vertebrates, and 

 the membrane bones present the most typical arrangement in extinct and primitive groups 

 of Amphibia and reptiles. The number of membrane bones diminishes in the higher verte- 

 brates, chiefly by loss but also by fusion. Thus, we may note that several membrane bones 

 present in the alligator are missing in the cat and rabbit. This loss of membrane bones is 

 illustrated in Figure 36, page 106. 



10. The number of cartilage bones is likewise greatly reduced in the higher forms by a 

 process of fusion of originally separate bones. Thus, the occipital bone of mammals consists 

 of four originally separate bones; the sphenoids of mammals are more or less fused and in 

 man unite to a single sphenoid bone which consists of six originally separate cartilage bones 



Fig. 40. — Some bones of the human skull at an early age, showing their compound nature. A, the 

 occipital bone at birth, showing the five elements of which it is composed; a, interparietal; b, supra- 

 occipital; c, exoccipital; d, is the foramen magnum; e, basioccipital. B, the sphenoid bone in an 

 embryo of four months, showing its components; /, center for the presphenoid; g, center for the orbito- 

 sphenoid; h, alisphenoid; i, center for the basisphenoid; j, center for the lingula; k, center for the 

 pterygoid. C, the temporal bone at birth, showing its three components; I, squamosal; m, tympanic; 

 n, petromastoid or periotic. Membrane bones blank; cartilage bones open stippling; cartilage, close 

 stippling. The subsequent ossification of the cartilage obliterates the boundaries between the com- 

 ponents. (A and C from specimens loaned by the anatomy department; B after McMurrich's Develop- 

 ment of the Human Body, copyright by P. Blakiston's Son and Company.) 



and includes some membrane bones in addition; the ethmoid of mammals is composed of 

 three ethmoid bones; the petromastoid of several otic bones, etc. (Fig. 40). 



11. The lower jaw undergoes reduction to the mammalian condition, where it consists 

 of but a single pair of bones. In most vertebrates the articulation of the lower jaw is between 

 the quadrate and the articular bones, while in mammals it is between the dentary and the 

 squamosal bones. 



12. The quadrate, the articular, and the hyomandibular are believed to be represented 

 in mammals in the tiny bones found in the middle ear. The remnant of the quadrate forms 

 the incus, of the articular, the malleus, and of the hyomandibular, the stapes. 



13. The hyoid and other gill arches are gradually reduced in land vertebrates but persist 

 in part in the hyoid apparatus and the cartilages of the larynx. 



