410 



ORGAN SYSTEMS OF MAN 



cated in each muscle and tendon. These 

 are sensitive to changes in tension. As 

 the various muscles contract, propriocep- 

 tors send out impulses which eventu- 

 ally bring about an intei-play of various or 

 all muscles and tendons to perform a co- 

 ordinated act. This goes on without con- 

 scious knowledge and functions without 

 vision, since, for example, a person is able 

 to play a piano in the dark. It is not only 

 necessary to know that a muscle is contract- 

 ing but also to know where the appendage 

 is at all times. In such a simple action as 

 grasping a fork and bringing some food to 

 the mouth it is necessary to contract the 

 proper muscles to extend the arm. Hex the 

 lingers on the fork, and further flex the arm 

 in order that the fork be brought to the 

 mouth. The contraction must be of just the 

 right amount or the fork will over- or under- 

 shoot its target with resulting catastrophe. 

 It is the proprioceptors that regulate the 

 amount of contraction to bring this act to 

 fruition. In a way, the muscles can be con- 

 sidered sense organs because sensory im- 

 pulses are coming from them almost as 

 rapidly as motor impulses are going to 

 them, though the cells involved in the two 

 cases are different. The tendons are also 

 abundantly supplied with proprioceptor 

 end organs but they receive no motor 

 nerves and cannot contract . 



It is because of the proprioceptors that 

 one can judge weight. If a 24-pound weight 

 is compared with a 25-pound weight, it is 

 difficult to distinguish between them. There 

 is no difficulty, however, in deciding that 

 a 5-pound object weighs more than a 4- 

 pound weight. In other words, one can de- 

 termine the relative differences in weight 

 and the heavier the objects the greater must 

 be the difference to be discernible. For ex- 

 ample, in the illustration above, there is 

 a one-pound difference between the two 

 lighter objects which is a 20 per cent dif- 

 ference; in the heavier weights, although 

 there is also a difference of one pound, the 

 relative difference is 4 per cent. People 



show considerable variation in their ability 

 to detect this difference and this skill prob- 

 ably plays an important role in manual dex- 

 terity. Some people have great difficulty in 

 using their hands effectively whereas others 

 are very proficient in this regard. In select- 

 ing a life work such ability should be taken 

 into consideration, and certain kinds of 

 vocational aptitude tests are designed to 

 determine this ability. 



Now that we have learned something 

 about how stimuli are received we must 

 consider next the portion of the coordinat- 

 ing mechanism that adjusts these incoming 

 impulses. This is the central nervous system 

 with all of its ramifying nerves. 



ADJUSTORS 



The nervous system 



When an animal is dissected, the brain 

 together with its many ramifying nerves 

 is most conspicuous even to the beginning 

 anatomist. Early morphologists, however, 

 thought the brain was an inert part of the 

 body and could not assign a function to it. 

 Today the anatomy of the nervous system 

 is well known, although the last word is 

 far from being written concerning its func- 

 tions. 



In its evolution it might be expected that 

 the nervous system originated from the ex- 

 ternal part of the body because it is this 

 part of the organism that contacts the out- 

 side world. Embryology and the history of 

 the animal groups both bear this out. As 

 animals became more complex, certain cells 

 in the outside layer became specialized to 

 receive and transmit stimuli. These cells 

 later confined their entire attention to the 

 job of receiving stimuli and thus became 

 the receptors or sense organs such as those 

 of the eye and the ear. Transmission of the 

 impulses was left to other cells which com- 

 bined to form the sensory nerves. These, in 

 turn, carried the impulses to a central sta- 

 tion, the brain and spinal cord, where 

 adjustment and interpretation took place. 



