Motor System — Muscles and Skeleton 



115 



(Fig. 112F). In reptiles, birds, and mammals, a varying number of 

 thoracic ribs are attached to the sternum (Fig. 112) so that the thorax 

 is completely enclosed by a skeletal frame — vertebral column above, 

 sternum below, and pairs of ribs connecting them. Also the skeleton of 

 the forelegs usually has a connection with the anterior end of the ster- 

 num. But legless animals such as snakes have no sternum. The so-called 

 "sternum " of amphibians has no connection with the very short dorsal 

 ribs — a fact which throws doubt on the correspondence of the am- 

 phibian "sternum" with that of other vertebrates. Fishes have no 

 sternum. The sternum is evidently primarily related to terrestrial loco- 

 motion and to air-breathing. 



Skull 



The skull is usually thought of as being the skeleton of the head. 

 Strictly defined, however, it must be made to include a series of skeletal 

 rings or arches which develop in close relation to the pharynx and lie 

 rather more in the territory of the neck than in that of the head. De- 

 fined thus, the skull consists of the cranium (already referred to as 

 being the anterior terminal division of the skeleton of the notochordal 

 axis, and enclosing the brain and organs of smell, sight, and hearing) 

 and the visceral arches which primarily surround (partially) the di- 

 gestive tube. In a shark (Fig. 113) the cranium and the visceral skeleton 

 have so little connection with one another that it would seem more 

 reasonable to regard them as two quite distinct skeletal structures 

 rather than as together constituting a "skull." In contrast to a shark, 

 an adult reptile or mammal (Fig. 114) has a skull whose many parts 

 are so completely unified into a compact whole that there is no obvious 

 reason for regarding it as a duplex thing. But its embryonic history re- 

 veals its double origin. 



Fig. 113. Skull of a shark, Squalina. (h) Hyale; (hm) hyomandibula; (hr) 

 hyomandibular rays; (I 1 , I 2 ) labial cartilages; (m) mandibular (Meckel's) cartilage; 

 {pq) pterygoquadrate; (r) rostrum. (After Gegenbaur. Courtesy, Kingsley: "Com- 

 parative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



