68 MORPHOLOGY OF THE VERTEBRATE BODY 



their brain development, particularly that of the cerebrum. 

 In man the cerebrum is so large that it overlaps and com- 

 pletely hides all other parts except a portion of the cerebellum and 

 the spinal cord. It is known that the cerebrum is the part of the 

 brain associated with such functions as memory and the more 

 complex mental processes. Without a well-differentiated cere- 

 brum, no animal seems capable of these higher nervous activities. 

 Defects of the human cerebrum are correlated with defective 

 mental functions. If the matter is considered from an evolutionary 

 standpoint, one can hardly examine such a series of brains, each 

 correlated with the degree of intelligence exhibited by its pos- 

 sessor, without believing that the nervous system of the higher 

 vertebrates has gradually acquired its present complexity of 

 structure, and that in the course of this evolution the intelHgence 

 of these animals has increased by gradual stages. 



The Muscular and Skeletal Systems. — It is impossible to 

 understand either the muscular or the skeletal system when con- 

 sidered separately, because the two are so intimately related. 

 The skeleton of the frog or any vertebrate constitutes a frame- 

 work, which supports the softer parts and offers firm places of 

 attachment for the muscles. The relationship between muscles 

 and skeleton may be illusti'ated by typical examples. In bending 

 and straightening the arm at the elbow, one observes that the 

 movement is in one plane, like that of a hinge, and that a thicken- 

 ing, and hence a contraction, of the muscles can be felt in the 

 upper arm as the joint is " flexed " by the biceps and "extended " 

 by the triceps muscle. Dissection reveals that the mechanical 

 arrangement involved consists of two " antagonistic " muscles 

 which, acting against each other, move the bones in the manner 

 indicated. Thus, muscles '' pull," and never " push " to any 

 appreciable extent. Antagonistic muscles are found throughout 

 the body, as is well shown by the mechanism used in maintain- 

 ing the erect posture in man (Fig. 41). 



Similar relationships exist in the bodies of all animals possessing 

 a well-organized musculature. In some instances the extension 

 of a part is due to its elasticity, following compression by the con- 

 traction of a muscle; but commonly extension, as well as flexion, 

 is caused by the " pull " or contraction of a muscle or muscles. 

 Complex movements, like the swinging of the ball-and-socket joints 

 of the human hip and shoulder, or rotation of the forearm, necessi- 



