The Skeletal System 111 

 MISCELLANEOUS BONES 



Located in various parts of the body are bones which are un- 

 related to the other portions of the skeleton. For this reason, these 

 bones are simply grouped as "miscellaneous bones." 



One of the commonest types of bones of this class are those 

 known as the sesamoid hones. These may develop in any region where 

 friction is excessive, and are most often found in tendons and muscles. 

 The knee cap or patella is the best known of this type. Others are 

 normally present in the palm side of the hand. Occasionally these 

 bones develop when certain muscles are overused. Baseball pitchers 

 at times develop them in their arm muscles. In such a case, these 

 bones must be removed surgically. 



In the incompletely ossified skull of the infant, small islands of 

 bones develop in the fontanels. These are the Wormian hones. As os- 

 sification proceeds, these elements gradually coalesce and disappear as 

 individual elements. 



A few other unusual bones occur in different vertebrates. In 

 many mammals a strengthening bone, the penis hone, is found in the 

 penis. The cow, as well as some other ruminants, has an irregularly 

 shaped bone in the heart. Some lizards have special bones around 

 the eyes. A few mammals have special bones in the muscles, and 

 the mole has a sickle-shaped element in the digging hand. The antlers 

 of the deer, and the bony portion of the horns of cattle which were 

 discussed with the integument, probably should also be listed at 

 this point. 



HOMOLOGY 



Within each major animal group, a certain basic body plan is 

 apparent. This makes it possible to study a few representative mem- 

 bers in order to gain an understanding of the entire group. In the 

 vertebrates particularly this similarity of body plan is evident. Thus 

 to learn about man's structure, you may profitably dissect a cat. 



The basic plan of portions of the skeleton may be highly modified 

 and individual units may assume different functions. For example, 

 the ear ossicles of the mammal are derived from the gill arches. The 

 basic bones are the same, but the function and location have changed 

 completely. Often the ancestral pattern of a structure can be dis- 

 covered only by a study of the embryology of the animal. 



