PRINCIPLES OF PHYSIOLOGY 



97 



of the animal body, they are, in certain respects, the functional equiv- 

 alent of an exoskeleton. 



The vertebrate skeleton, lying within the soft tissues of the body, 

 provides an excellent framework for their support and does not inter- 

 fere with their growth. The arrangement of the parts of the skeleton 

 is essentially the same in all the vertebrates. The details of this will be 

 discussed in Chapter 25. 



The skeleton of vertebrates is composed of many individual bones 

 or cartilages. The region where two hard parts meet and move one on 

 the other is known as a joint. The fundamental differences in the me- 

 chanics of the vertebrate and arthropod joints are illustrated in Figure 

 5.7. The muscles of the vertebrate surround the bones; each is attached 

 by one end to one bone and by its other end to another bone. Its con- 

 traction thus moves one bone with respect to the other. The muscles of 

 the artliropod lie luitliin the skeleton and are attached to its inner 

 surface. The arthropod exoskeleton has certain regions— joints— in which 

 the exoskeleton is thin and ilexible so that movements may occur. The 

 muscle may stretch across the joint, so that its contraction will move 

 one part on the next. Or, the muscle may be located entirely within 

 one section of the body or appendage and be attached at one end to a 

 tough apodeme, a long, thin, firm part of the exoskeleton extending 

 into that section from the adjoining one. 



The movement of the wings of insects is achieved in a curious way: 

 the flight muscles are located within the body and are attached to the 

 body wall. The wings are attached to the body wall over a fulcrum, 

 the wing process (Fig. 5.8). The contraction of muscles arranged dorso- 

 ventraliy pulls down the tergum, a plate on the upper surface of the 



Endo skeleton 



Apod 



Exoskeleton' 



B 



Figure 5.7. A comparison of the vertebrate endoskeleton (A) with the arthropod 

 exoskeleton (B), showing the arrangement of the muscles and skeleton at a joint. 



