2l6 



COMPARATIVE ANATOMY 



If an opinion were to be based upon the relation of the sternum in 

 Necturus and of the ontogenesis of the mammalian sternum alone, we 

 should have to conclude that the sternum arose from paired segmented 

 cartilages formed near the mid-ventral line independently both of the 

 girdle and of the ribs. Under the circumstances, and until more decisive 

 evidence is discovered, suspension of judgment is necessary. 



In the reptiles the sternum is converted into a metameric structure 

 composed of a series of sternebrae and connected with the ribs as in 

 mammals. 



Fig. 173. — Diagrams showing the development of the primordial skull. Since this 

 organ develops primarily beneath the brain as a support the figures represent the ventral 

 aspect. (A) Early stage, before the appearance of cartilage. The notochord is seen 

 lying along the nerve cord as far forward as the hypophysis. The three sense-organs, 

 nose, eye, and ear, have already appeared. (B) This stage shows the trabeculae (l), 

 the parachordals (p), and the capsules around the sense-organs. (C) In this the 

 trabeculae, the parachordals, and the nasal and otic capsules have fused into a single 

 mass, the primordial skull, or chondrocranium. The anterior end of the notochord is 

 imbedded in this. The cartilaginous capsule of the eye remains free to allow the 

 necessary movements of the eyeball. (From Wilder's "History of the Human Body," 

 Courtesy of Henry Holt and Co.) 



The mammalian sternum differs little from that of reptiles. It is 

 divided into the same three elements as those of reptiles and man, pro- 

 meso-, and meta-sternum. 



The Skull. There are two chief parts to the skull, which have different 

 origins and a different history. One of these is the cranium or brain-case, 

 together with the bones of the face except the two jaws. The other is 

 the visceral skeleton, that is to say, the two jaws, the hyoid bone, the 

 ear bones, and the cartilages of the larynx. 



The Evolution of the Cranium. In the early part of the nineteenth 

 century it was generally assumed by morphologists that the skull consists 

 of four or five enlarged vertebrae. Originally suggested by the poet 



