90 



PRINCIPLES OF ANIMAL BIOLOGY 



upon another. Sometimes these curved surfaces are such as to permit 

 movement only in one plane, as in a hinge, while other joints allow a 

 rotary motion. The skeleton of insects and that of vertebrates differ, 

 however, in one important respect. In the insects it is on the outside, 

 covering all the fleshy parts, and here is known as an exoskeleton. In 

 vertebrate animals the skeleton is on the inside, everywhere covered by 

 flesh, hence of a type called an endoskeleton. The muscles which operate 

 the movable parts must work from the inside in the former but from the 

 outside in the latter (Fig. 77). 



Skeleton of Vertebrates. — To illustrate the main features of a typical 

 skeleton, that of the vertebrate animal is chosen. This skeleton is 

 composed of bones and cartilages united partly by ligaments, is covered 



TRUNK. 



Fig. 78. — Regions of the vertebrate skeleton (cat). {From Jayne, "Mammalian Anatomy.") 



by the soft parts of the body, and is supplied with blood vessels and 

 nerves. It may conveniently be divided into regions as indicated in 

 Fig. 78. On more fundamental anatomical grounds it is also subdivided 

 into the axial and the appendicular skeleton. The former lies in the 

 longitudinal axis of the body, and to it the latter is appended; hence the 

 names. 



Axial Skeleton. — The axial skeleton (Fig. 79) is made up of the skull, 

 hyoid apparatus, vertebral column, ribs, and sternum. The skull furnishes 

 a case for the brain, capsules for the organs of hearing and smell, and 

 orbits for the eyes. It also includes the bones of the jaws. To it is 

 attached the hyoid apparatus which is a bony or cartilaginous support 

 for the base of the tongue. 



The vertebral column is a jointed structure composed of a number 

 (different in different species) of vertebrae placed end to end. Together 

 they form a tube enclosing the s])inal cord, and their outer surfaces 

 form attachments for ligaments and muscles. The vertebral column 



