THE SPECIAL ANATOMY OF THE FROG 51 



H. THE SKELETON 



For this work dried mounted skeletons will be provided. (The student may 

 prepare a skeleton as follows: Remove all possible flesh and organs from a 

 fre*shly killed frog, and dip what remains from time to time in hot water, brush- 

 ing away the remaining flesh* until the bones are cleaned. Too liberal use of hot 

 water or boiling will cause the bones to fall apart.) 



1. General considerations on the skeleton. The skeleton, or hard parts 

 of the body, is generally of two kinds: the external skeleton, or exoskeleton, and 

 the internal skeleton, or endoskeleton. The exoskeleton covers and protects 

 the body and is derived from the skin. Examples are scales, feathers, hair. The 

 frog has no exoskeleton except such as has become fused to the endoskeleton 

 (see below). The endoskeleton is the internal framework of the body, and is 

 composed of cartilage in the embryo which becomes partially converted into 

 bone in the adult frog. Such bone formed in cartilage is known as cartilage 

 bone. Investigation shows that not all the bones of the frog's skeleton have 

 arisen in this way but some have appeared in connective tissue without passing 

 through a cartilage stage. Such bones are called membrane bones, dermal bones t 

 or investing bones. They are produced from the dermis of the skin, and are there- 

 fore not really endoskeleton, but part of the exoskeleton. In the course of 

 evolution these bones, originally scales, sank into the body and attached them- 

 selves so closely to the endoskeleton that they are usually treated in textbooks 

 as parts of the endoskeleton. The clavicle and the superficial bones of \ the skull 

 and jaws of the frog are membrane bones; all other bones of the skeleton are 

 cartilage bones. 



The skeleton may be divided into an axial part including the skull, the 

 visceral skeleton, and the vertebral column; and an appendicular part consisting 

 of the bones of the limbs, the girdles, and the sternum or breastbone (Holmes, 

 chap, xiii, pp. 229-45). 



2. The vertebral column (Holmes, pp. 237-38). The anterior half of the 

 vertebral column of the frog consists of nine bony rings, the vertebrae; the 

 posterior half is an elongated slender , bone, the urostyle. Obtain some isolated 

 vertebrae and identify the following parts: 



a) The neural canal is the cavity in the vertebra. 



b) The centrum or body is the thickened part of the vertebra ventral to the 

 neural canal. It articulates with the centra before and behind it by a ball and 

 socket arrangement. 



c) The neural arch is that portion which arches dorsally over the neural canal. 



d) The neural spine is a sharp dorsal projection from the center of the neural 

 arch. 



e) The transverse processes are the conspicuous lateral projections extending 

 horizontally outward from the place of junction of centrum and neural arch. 



