94 The Vertebrate Organ Systems 



is kept constant. In cases of extreme demand (starvation or preg- 

 nancy), calcium and phosphorus may be removed from the bones. 



From the above discussion, it is apparent that the endoskeleton of 

 the vertebrates plays a more varied role in the life of these animals than 

 does the exoskeleton of the insects and their allies. The exoskeleton 

 is very effective as a protective covering and as a place for muscular 

 attachment. Undoubtedly this type skeleton has been one of the major 

 reasons for the success of this tremendously varied group of animals. 

 There are, however, certain inherent disadvantages to this exoskeleton. 

 For one, the animal is literally imprisoned within its inflexible armor. 

 For growth to occur, it must burst out at periodic intervals. During 

 these molts, the animal has a soft surface which is susceptible to drying 

 and attack by other forms. Further, since during the molting period 

 the soft tissues are unsupported, large size cannot be attained. Typically 

 the land-dwelling insects and their allies are relatively small animals. 



On the other hand, the size attainable by the vertebrates may be 

 tremendous. Compare the size of the modern elephant with the largest 

 of the insects, and you realize that the support offered by the endo- 

 skeleton is indeed great. It is a living growing part of the animal's 

 body; it changes with the changing body form and size of the animal. 



TISSUES OF THE VERTEBRATE SKELETAL SYSTEM 



The skeleton is composed entirely of supporting tissues of two gen- 

 eral types : cartilage and bone. Typically cartilage is a semitransparent 

 material with the living cells located within special small spaces called 

 lacunae. It is without penetrating blood vessels. Bone, the most abun- 

 dant of the skeletal materials, is a hard, opaque, calcified material. 

 Within it are the bone cells, likewise located in lacunae. Blood ves- 

 sels penetrate this hard material through special canals. 



Cartilage. — Cartilage (Fig. 26) is the principal component of the 

 skeleton of certain fishes such as the shark and of the embryos of the 

 other vertebrates. In most adult skeletons, cartilage is found only in 

 relatively small areas where special flexibility is desired. Three types 

 of cartilage are recognizable, depending upon the type of material found 

 in the intercellular substance or matrix. These types are hyaline, fibrous, 

 and elastic. 



Hyaline cartilage is intimately associated with various parts of the 

 skeleton. It is found at the ends of bones where it forms thick cushion- 

 ing articulating points ; at the ends of ribs ; and it enters into the forma- 



