110 ESSENTIALS OF ZOOLOGY 



reproductive organs) just ventral to the anus, and below this is the 

 smaller opening from the excretory system at the distal end of a 

 prominent elevation or papilla. 



Skeleton 



Characteristic of verebrates the skeleton of the rat is principally 

 an endoskeleton which is completely enclosed by the soft parts. It is 

 composed of 223 bones (exclusive of the teeth, chevron bones and 

 sesamoid bones) and a number of cartilages. There are present 

 both cartilage hones, which are preformed in cartilage and then re- 

 placed by bone, and memhrane hones, which are formations of bone 

 in the dermis in certain portions of the skin. Chevron hones are 

 paired and extend ventrally from the anterior ends of several of 

 the caudal or tail vertebrae. The sesamoid bones are pieces of bone 

 formed in the tendons. The kneecap is 'a good example of these. 



The principal functions of the skeleton are to support the body, 

 give it form, furnish a sturdy attachment for voluntary muscles, 

 and to protect internal organs. The bones performing the latter 

 function are usually flattened while those which serve for attach- 

 ment primarily are long, somewhat cylindrical, and are joined by 

 movable joints. 



It is customary to divide the skeleton of higher vertebrates into 

 axial skeleton and appendicular skeleton for convenience in study. 

 The axial portion extends in the main axis of the body and consists 

 of the skull, vertebral column, ribs, and sternum. The appendicular 

 portion includes the bones of the fore and hind limbs as well as the 

 pectoral and pelvic girdles which support them. 



The skull is rather conical in shape and somewhat elongated. It 

 is composed of the cranium, which encases the brain, the jaw struc- 

 tures, and the hyoid. For the most part these bones are joined to 

 each other by immovable joints or sutures which are usually marked 

 by a line of fusion. There are forty-two bones included in the skull. 

 The dorsal portion or roof of the cranium includes the paired frontal 

 bones, paired parietals, and the interparietal. The lateral walls are 

 composed of the tympanic and squamosal portions of the temporal, 

 alisphenoid, orhito sphenoid, and the lateral portions of the frontals 

 and parietals. The floor is formed by the hasioccipital, hasisphenoid, 

 and presphenoid. in order from posterior to anterior. The posterior 



