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238 NERVOUS SYSTEM motor fibres of the cord. The decussation of the sensitive fibres, however, exists through the entire length of the spinal cord, and not in the medulla alone, as in the anterior columns. The effect of this is shown when one lateral half of the spinal cord of a dog is divided ; sensation remains on the corresponding side of the body, but is lost on the opposite side. Reflex Action of the Spinal Cord. Thus far the spinal cord has been considered as a bundle of sensitive and motor nerves connecting the brain with the muscles and periphery of the body ; but we find that the central portion of the cord contains a considerable) amount of gray matter, and that it possesses in consequence the capacity of a nervous centre. When a frog has been decap- itated, sensation and volition are of course de- stroyed, the communication between the body and the centres in which these powers reside being cut off ; but it will be found, nevertheless, that irritation of either lower or upper extrem- ity will be followed by muscular contraction on the corresponding side ; and when the irrita- tion is excessive, contractions will occur in all the limbs. These phenomena, which are due to the action of the cord as a nervous centre, are not unfrequently observed as a consequence of disease or injury of the spinal cord. When- ever communication is cut off between any portion of the cord and the brain by disease, such as softening, or by fracture of the spine, sensibility and the power of motion are lost in the parts below the point of disease or injury ; but involuntary and uncontrollable contractions occur in response to any irritation applied to the surface, such as tickling of the soles of the feet, a draft of air, or the application of the galvanic current. For the production of these reflex phenomena it is necessary : 1, that the cord be in a state of integrity ; and 2, that the continuity of the spinal nerves be unbroken. If the spinal cord be broken up in the frog by the passage of a needle through the spinal canal, irritation of the extremities will produce no effect; and if the sciatic nerve, for example, be divided, the cord remaining perfect, no contrac- tion can be produced on the side of the division. It is inferred then from direct experiments and from the evidence furnished by disease that the spinal cord is a nervous centre, which sup- plies innervation to the muscular and cutaneous systems through the medium of sensitive and motor filaments; the sensitive filaments con- veying impressions from the surface to the centre, and the motor filaments transmitting impulses from the centre to the surface. Sym- pathetic or Ganglionic System. This system, which is sometimes called the nervous system of organic life, consists in a double chain of nervous ganglia occupying the visceral cavities of the body, and extending along the front and sides of the vertebral column from its anterior to its posterior extremity. There are four symmetrical pairs of ganglia in the head, three pairs in the neck, ten pairs in the chest, an irregular aggregation of ganglia in the abdo- men called the semilunar ganglion, and five pairs in the pelvis. These ganglia are all connected with each other by transverse and longitudi- nal commissural fibres, and they send off some filaments to join nerves from the cerebro-spi- nal system, and others to be distributed to all the viscera which are concerned in the main- tenance of organic or purely vegetative life. The state of knowledge concerning the special properties and functions of the sympathetic system of nerves is less advanced than that which relates to the cerebro-spinal system. The reasons for this imperfect knowledge are, the difficulties of experimenting on this system from the peculiarities of its situation, and the very complex connections of the sympathetic and cerebro-spinal systems. For further in- formation, see Longet, Anatomie et physiolo- gie du systeme nerveux (1842), and Traite de physiologic (1850-'70) ; Mayo "On the Ner- vous System" (1842); Carpenter, "Principles of Human Physiology" (1846; 7th ed., 1869); Todd's " Cyclopaedia of Anatomy and Phys- iology;" Todd and Bowman, "Physiological Anatomy and Physiology of Man " (1850) ; Bernard, Lecons sur la physiologic et la patho- logic du systeme nerveux (1858) ; Vulpian, Le- fons sur la physiologic generale et comparee du systeme nerveux (1866); Dalton, "Treatise on Human Physiology" (1859; 5th ed., 1871); Flint, "Physiology of Man," vol. iv. (1872); and other systematic works on physiology. NERVOUS DISEASES are properly divided into the functional and structural diseases of the ner- vous system. The functional diseases are those in which there is no morbid change or lesion in the nerve structures to account for the symptoms ; they manifest themselves by irreg- ular, depressed, or exalted conditions of the pecular functions of the nervous system, viz., sensibility and motion. They may be enumer- ated as follows : 1. The numerous varieties of neuralgia which are independent of dis- ease of the nerves or their centres. 2. The various forms of insanity, as delirium, mono- mania, dementia, and general paralysis, where no morbid change occurs in the brain to ac- count for the symptoms. 3. The protean nervous phenomena, neuralgia, paralysis of sensation and motion, convulsions, and the various exhibitions of mental and moral per- versity, constituting the disease known as hys- teria. 4. Delirium tremens, that derange- ment of the nervous functions, manifested by optical illusions, hallucinations, mania, and muscular trembling, which arises from ex- haustion of the nervous power produced by prolonged stimulation by alcohol. 5. Chorea or St. Vitus's dance, an affection occurring generally in young girls between the period of dentition and puberty, and consisting in irreg- ular contractions of the voluntary muscles. This disease has been graphically termed " in- sanity of the muscles." 6. The convulsions and paralyses that occur in infancy and childhood from the irritation of teething or from gastric