tt^Responses of Higher Animals: 

 The Effectors 



THE MOST important effector organs in 

 higher animals are the muscles and glands. 

 But some complex animals, including both 

 vertebrates and invertebrates, also possess 



(1) luminous organs, which give forth light; 



(2) electric organs, which generate high- 

 voltage electrical discharges; and (3) pigmen- 

 tary effectors, which execute rapid changes 

 in the body color of the animal. Moreover, 

 practically all multicellular animals possess 

 unicellular effectors, such as the amoeboid 

 cells of the blood and ciliated epithelial cells; 

 these were discussed in Chapter 11. 



THE SKELETAL MUSCLES 



Skeletal muscles (p. 286) execute all ex- 

 ternal movements of the body and its parts. 

 Typically each muscle has the form of a 

 strong elongate strand that stretches from 

 bone to bone of the skeleton (Fig. 24-1). 

 Accordingly, when a muscle contracts, or 

 shortens, it exerts a force upon some bone 

 and tends to produce a movement at a joint 



where the particular bone is hinged to the 

 rest of the skeleton. 



Usually the two ends of a muscle are not 

 exactly alike. At the proxima] end, which is 

 called the origin, there may be one or more 

 tendons (Fig. 24-1), although usually the 

 many individual fibers of the muscle fasten 

 quite directly to the bone (see the triceps, 

 Fig. 24-1). However, at the other end, which 

 is called the insertion, there is almost always 

 a tendon (Fig. 24-1). This tough cablelikc 

 strand of connective tissue intervenes be- 

 tween the muscle fibers and the bone and 

 transmits the force of the contraction to the 

 bone. Generally the origin maintains a rela- 

 tively fixed position when the muscle con- 

 tracts, and thus most of the force of contrac- 

 tion goes to produce a movement of the bone 

 to which the insertion is attached (Fig. 24-1). 



At least two muscles, acting antagonisti- 

 cally, control the movement at any particular 

 joint. In the knee, for example, one set of 

 muscles, called the flexors, acts to bend the 

 knee; whereas the extensors are used to un- 



431 



