10 PHYSIOLOGY OF FARM ANIMALS [CH. 



labour necessitates a means of communication between the 

 various organs, and this function is fvalfilled by the nervous 

 system. The nerves consist of strands of a peculiar kind of 

 tissue constituting the nerve fibres, and these connect together 

 the sense organs and muscles and all the various parts of the 

 body. The nervous system is presided over by the brain and 

 spinal cord. The former of these is contained within the skull, 

 while the latter consists of a hollow tube enclosed by the back- 

 bone or vertebral column. The brain and spinal cord together 

 constitute the central nervous system. The other nerves are 

 differentiated into afferent (sometimes called sensory) nerves, in 

 which the impulses pass from the sense organs to the central 

 nervous system, and efferent (sometimes called motor) nerves, 

 in which the impulses travel outwards from the central nervous 

 system to the muscles or other organs to which a message is to 

 be sent. All actions which are under the control of the will are 

 presided over by the brain, from which the voluntary nervous 

 impulses are transmitted. But there is another class of actions 

 in which either the brain or spinal cord is concerned, but which 

 are involuntary. These are called reflex actions. For example, 

 if a frog after being deprived of its brain be hung up by its jaw 

 and one of its toes pinched, its leg is drawn up. Such an act is 

 involuntary, and in this case is controlled by that part of the 

 central nervous system which is still intact, namely the spinal 

 cord. If this be destroyed the reflex action can no longer take 

 place. It is clear, therefore, that such an act involves a succes- 

 sion of processes, which are as follows. The pinching of the toe 

 supplies a stimulus which is transmitted as a nerve impulse by 

 an afferent nerve to the sjjinal cord. Another nerve impulse is 

 then transmitted by- an efferent nerve in the opposite direction, 

 passing from the spinal cord to the muscles of the limb, and this 

 causes them to contract. There are also cerebral reflexes, which 

 are likewise involuntary. For example, the secretion of the 

 saliva and the secretion of the gastric juice are reflexes which 

 are induced by the introduction of food into the mouth, messages 

 being transmitted m the first place in an afferent direction from 

 the mouth to the brain, and then in an efferent direction from the 

 brain to the secretory glands of the mouth and stomach. It is 

 interesting to note that different sets of nerves are concerned in 

 transmitting the impulses in opposite directions. Every reflex 



