SUPPOllT, MOTION, AND SENSATION 329 



Endoskeletons 



Endoskeletons are characteristic of chordatc animals. An internal 

 supporting rod {notochord) is clearlj^ present in the larvae of the 

 tunicatcs and in the adult amphioxus, while a well-developed endo- 

 skeleton is found in all of the so-called higher forms from fishes to man. 



The skeleton of vertebrates is divided typically into three parts : 

 the axial skeleton, which includes the skull, thoracic basket, main 

 spinal column, and tail ; the appendicular skeleton, which pertains to 

 the appendages ; and the visceral skeleton, which is developed in con- 

 nection with the various modifications of the gill region. In adult 

 fish, the visceral skeleton forms the cartilaginous or bony bars (gill 

 arches). In other vertebrates, the visceral skeleton becomes con- 

 verted into various highly modified structures involved in the forma- 

 tion of the jaws, the hyoid support of the tongue, the larynx, accessory 

 parts of the skull, and even the bones in the middle ear. 



The Axial Skeleton 



Anteriorly, the axial skeleton of vertebrates is specialized into a 

 skull, a bony case covering the expanded anterior end of the spinal 

 cord, or brain. Incorporated into this skull are specialized protective 

 capsules for several of the major sense-organs, namely, the eyes, ears, 

 and nose. 



Many bones are fused to form the skull. These are of two sorts, 

 either memhranous or cartilaginous. The former are developed 

 directly from a connective tissue membrane, while the latter type 

 pass through a preliminary cartilaginous stage before becoming 

 bone. In primitive vertebrates, the brain is protected by cartilage 

 which later in the evolutionary picture becomes ossified. Still 

 later, this original cartilaginous cranium is further protected by 

 the addition of a group of thin, flat membrane bones, shingled over the 

 skull. In higher forms the number of embryonic bones in the skull 

 has been reduced. The skull of a dog, for example, contains fewer 

 bones than that of a codfish. A study of the earlier stages of develop- 

 ment in mammals shows, however, that representatives (or homo- 

 logues) of many of the bones present in the cod skull may be found. 

 These embryonic elements fuse in later development, making the 

 smaller number of skull bones found in the adult. In the skull of a 

 reptile, for example, there are four occipital bones surrounding the 

 point of exit of the spinal cord from the skull, which in most adult 



