CHAPTER I. 



INTRODUCTORY ACCOUNT OF THE SKELETON IN 

 GENERAL. 



BY the term skeleton is meant the hard structures whose 

 function is to support or to protect the softer tissues of the 

 animal body. 



The skeleton is divisible into 



A. The EXOSKELETON, which is external ; 



B. The ENDOSKELETON, which is as a rule internal ; 

 though in some cases, e.g. the antlers of deer, endoskeletal 

 structures become, as development proceeds, external. 



In Invertebrates the hard, supporting structures of the 

 body are mainly exoskeletal, in Vertebrates they are mainly 

 endoskeletal ; but the endoskeleton includes, especially in 

 the skull, a number of elements, the dermal or membrane 

 bones, which are shown by development to have been originally 

 of external origin. These membrane bones are so intimately 

 related to the true endoskeleton that they will be described 

 with it. The simplest and lowest types of both vertebrate 

 and invertebrate animals have unsegmented skeletons ; with 

 the need for flexibility however segmentation arose both in 

 the case of the invertebrate exoskeleton and the vertebrate 

 endoskeleton. The exoskeleton in vertebrates is phylogeneti- 

 cally older than the endoskeleton, as is indicated by both 

 R. 1 



