THE PHYSIOLOGY OF AN ANIMAL 211 



then passes to the exterior with the faeces, the one cloacal 

 opening serving, in the frog, for the exit of undigested food 

 as well as for the urine. 



Support. The skeleton serves to support the softer part of 

 the body. 



Motion. The motion of the frog is accomplished by the 

 muscles. The muscles are numerous, and each has its own 

 special attachment to the bones; Fig. 89. Every muscle pos- 

 sesses the power of shortening, but has no other function; 

 and the ordinary muscles are attached to two bones in such a 

 way that when the muscle shortens one bone is moved upon 

 another. All the motions of the body are produced by the short- 

 ening of the different muscles. Many of the muscles are in 

 pairs, one of each pair serving to bend a joint, the flexor, and 

 the other straightening it, the extensor. The details of their 

 actions we cannot consider here, but it will readily be seen that 

 with the many muscles present in the frog's body a great variety 

 of motions can be produced. The selection of the proper mus- 

 cles to produce any desired motion is a complicated process, 

 some motions indeed requiring the orderly selection of a large 

 number of muscles, which must act together in perfect harmony. 

 This power of selecting the muscles and causing them to act in 

 unison and in harmony with each other is called coordination. 



The Coordinating System. The nervous system of the frog 

 controls all other functions. As already seen, it consists of (1) a 

 central system, the brain and spinal cord; (2) the peripheral 

 system, the latter composed of the nerves distributed over the 

 body, and the various end organs of the nerves. The central 

 system is really the center of activity, and the nerve fibers are 

 merely paths for conducting impulses from one part of the body 

 to another. Some of the end organs at the outer ends of the 

 nerves receive impulses from the brain; others receive them from 

 the exterior and transmit them to the nerves to be carried to the 

 brain. The brain corresponds to the central station of a telephone 

 system, which is connected with all parts of the city by wires hav- 



