CHAPTER XI 



SUPPORTING STRUCTURES 



The vertebrate supporting structures include the endodermal 

 notochord, and tlie mesodermal cartilage and bone. "With these 

 should be included the connective tissue sheaths of the latter and 

 the ligaments which tie them together. These are necessary for 

 proper function in protection and locomotion, and will be dis- 

 cussed under the leverage of muscles. 



The original function of the skeleton was leverage, the struc- 

 tures serving for the attachment of muscles. With the develop- 

 ment of the skull and vertebral column came a protective func- 

 tion as the bones surrounded the nervous system ; and in the land 

 vertebrates the marrow of the long bones forms most of the 

 corpuscles of the blood. 



The vertebrate has an internal skeleton. The external skeleton 

 of the insects and Crustacea is perhaps more efficient so far as 

 leverage is concerned, the muscular pull being more direct; but 

 what tlie vertebrate lost in leverage was more than counter- 

 balanced by flexibility of body parts. 



The skeleton of the vertebrate is divided into two regions: 

 (1) the axial skeleton including the skull and vertebral column, 

 both of which develop around the notochord; and (2) the appen- 

 dicular skeleton, including the skeletal girdles and the appen- 

 dages. These two regions are development ally independent, but 

 where both exist they are functionally dependent upon each 

 other. 



A. The Skull 



As was mentioned in the first part of the book, the vertebrate 

 skull has a double origin, (1) from the primitive chondro cranium 

 (cartilage skull) and (2) from the dermal plates or bones. The 



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