CHAPTER XVI 



ON AN-ffiSTHETICS AND ANAESTHESIA-HUMANE DESTRUCTION 



738. The Nervous System.— The living body is 

 controlled by a wonderful mechanism known as 

 the nervous system. This system is divided into 

 two distinct branches : the voluntary branch, and 

 the involuntary branch or reflex branch. The 

 latter is responsible for the vital organs continu- 

 ing in their functional activity day after day. 



The whole of this wonderful system is con- 

 trolled by "minor offices" called local centres, 

 these being situated in the spinal cord. There is 

 a local centre for every one of the viscera and 

 organs in the pelvis, abdomen, thorax, neck, 

 face, etc. The whole of the blood supply is con- 

 trolled by local centres which contract and dilate 

 the minute blood capillaries, and thus regulate 

 the flow of blood. All these local centres are 

 controlled by one large centre, the " head office," 

 which is called the brain. The brain keeps per- 

 fect harmony between the local centres ; this 

 function is known as co-ordination. 



By means of co-ordination, blood is distri- 

 buted as required over the whole body, and 

 certain organs are not allowed to take too much 

 blood, thereby leaving other organs bloodless. 

 But for this main central control the minor 

 organs might demand all the blood and leave 

 none for the brain or other important organs, 

 and consequently cause death to the animal. 



After a meal more blood is required in the 

 stomach and intestines, and the blood supply of 

 the muscles and brain is therefore reduced ; 

 hence hard muscular or mental work should not 

 be allowed while heavy digestion is going on. 

 Horsemen must remember this, otherwise the 

 best value will not be obtained from the food, 

 but serious trouble may result. Indigestion is 

 the common sequence of neglect of the above 

 fact. 



The control of all internal viscera or other 

 organs, such as the blood-vessels, etc., is carried 

 on by the involuntary system, which is known 

 as reflex action. Every nerve passing from a 

 portion of an organ to the spinal cord has 

 another nerve passing back along the side of it. 

 That passing to the cord is the sensory nerve, as 

 it carries sensation, and that passing back is the 

 motor nerve, as it causes motion. 



A reflex action takes place in this manner : a 

 stimulus is given to the receiving surface, that 



is communicated to the sensory nerve ; this 

 passes it to the loced centre in the spinal cord, 

 the cord sends back a message along the motor 

 nerve, that conveys a stimulus to the active 

 part to which it is attached, which is called a 

 muscle fibre. Hence, a sensation felt by the part 

 causes a stimulus to be given to the muscle of 

 that part : this is a reflex action. The blood 

 supply is controlled entirely by reflex action. 



The voluntary system is also controlled to a 

 certain extent by reflex action. The voluntary 

 system controls all the red, or striated, muscles 

 (except the heart muscles), and is generally at 

 the entire command of the individual. 



729. The brain and spinal cord, which form 

 the nervous column, are composed of inner grey 

 matter and outer white ; the grey is the more 

 important part. From the brain and spinal cord, 

 all the way to the tail, numerous nerves, sen- 

 sory and motor, branch off ; the motor nerves, 

 strictly speaking, go out, and the sensory come 

 in. 



Nerves are always seen in bundles of minute 

 nerve fibres, each fibre being a nerve. The 

 bundles (like telephone cables) vary very much 

 in size. Sensory and motor nerves are mixed up 

 in the same bundle. 



There are twelve pairs of cranial nerves lead- 

 ing from the under surface of the brain : three 

 sensory, six motor, and three mixed. 



730. The first (olfactory) controls the sense 

 of smell ; the second, third, fourth and sixth 

 control the sense of sight — the third, fourth and 

 sixth being motor nerves. The fifth (trifacial) is 

 a very large mixed nerve, and controls most of 

 the face, mouth and teeth. 



The seventh (facial) is a large motor nerve, 

 and controls the face, ears and part of the teeth. 

 The eighth (auditory) controls the sense of hear- 

 ing. The ninth (mixed) controls the tongue and 

 pharynx. The tenth (mixed) controls the larynx 

 and neck. The eleventh (motor) passes to muscles 

 in the neck ; and the twelfth (motor) controls 

 the muscles of the tongue. 



731. Leading from the spinal cord, there are 

 eight double pairs of nerves in the neck (cervical 

 region) ; the sixth, seventh and eighth, together 

 with some of the dorsal nerves, form the brachial 

 plexus, that sends off the nerves of the fore 



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