VERTEBRATES 5 



tapering ends the muscles terminate in strong white cords, the tendons, 

 which are attached to the bones. The action of the muscles may be 

 easily demonstrated in the human arm. A large muscle (the biceps) 

 lies on the inner side of /the upper arm, and above is connected by two 

 tendons with the shoulder, and below by a single tendon with the fore- 

 arm. The contraction of this muscle is clearly observed if we place 

 our other hand on it in such a manner that the thumb touches its 

 upper end and the tips of the fingers come to lie on its lower end. As 

 a result of this contraction, the forearm is drawn up after the manner 

 of a drawbridge by its chain. Whilst, however, the bridge is raised by 

 a force from without, in the present case the force lies within the muscle, 

 viz., in its capability of contracting. 



This capacity is the property of all muscles, and all movements are 

 brought about by the contractions of the muscles. The motions of the 

 heart also, of the intestines, etc., are effected by muscles. These 

 movements, however, do not depend on the will of the animal, but are 

 involuntary, whereas the motion of the forearm was of a voluntary nature. 



4. Nervous System. — The biceps, as was said above, is contracted 

 at our will. Now, the will is one of the mental activities which have 

 their seat in the brain. (Brain and spinal cord form the main portion of 

 the nervous system.) The brain transmits its orders to the muscles by 

 special conductors, the nerves, which consist of fine white threads or fibres 

 passing out from the brain and spinal cord, and dividing, like the 

 branches of a tree, in finer and finer ramifications over the whole body. 

 In the same manner as the electric telegraph carries our words to far- 

 distant places, the nerves convey the orders of the brain to the muscles ; 

 and just as the telegraph calls forth in the distant place the motion by 

 which our words are written down, so the nerves likewise call forth a 

 motion, i.e., the contraction of the muscles. 



Take, for instance, the case of a cat which has caught sight of a mouse. 

 The animal at once carries out a number of purposeful movements having 

 for their object the capture, killing, and tearing to pieces of the mouse. 

 Again, if we approach a limb to a hot object, we at once withdraw it, and 

 so on. All these movements are replies to external stimuli. How do these 

 stimuli affect the consciousness of man or animals? How are they 

 transmitted to the brain ? This, likewise, is effected by means of nerves, 

 which, in contradistinction to the nerves of motion just referred to, are 

 termed nerves of sense. The nerves of motion (motor nerves) accordingly 

 conduct from within outwards (centrifugal nerves), while the nerves of 

 sense (sensory nerves) conduct from without inwards (centripetal nerves.) 



Thus, light acts as a stimulus only on the eye, sound waves only 

 affect the ear, odoriferous substances the nose, flavours the tongue, warmth 



