NERVOUS SYSTEM 



443 



motion will be lost, but the reflex arc, o, a, s, e, m, 

 remains intact. We can sometimes control or 



Fig. 10. The Sympathetic Nerve; the right lateral 

 walls of the chest and abdomen, and the stomach, 

 intestines, liver, spleen, and pancreas being removed 

 to bring it in view : 



1. S, S, tie superior, middle, and Inferior cervical ganglia; 



4, the two lines from this figure include the twelve dorsal 

 ganglia; 6, include the four lumbar ganglia; 6, include the 

 dve sacral ganglia ; 7, the ganglion impar ; 8, cardiac plexus ; 

 9, solar plexus; 10, aortic plexus; 11, hypogastric plexus; 

 o, the larynx ; b, the trachea ; c, arch of the aorta ; <?, exter- 

 nal carotid ; c", internal carotid ; d, the heart; e, e. the dia- 

 phragm ; / the cardiac end of the oesophagus ; g, thoracic, 

 and /, abdominal aorta ; h. the kidnpy ; i, the supra-renal 

 capsule; *, the sacrum: (, the section of base of the skull; 

 m, the bladder ; n, the lower portion of the rectum. 



repress a reflex action voluntarily. The cell c in 

 the brain can so act on the cell * as to hinder or 

 inhibit its ordinary response to a stimulus, and 

 when the cell in the brain is destroyed the cell * 

 is more easily stimulated rettexly, apparently be- 

 cause some restraining influence is removed. Such 

 a restraining action is exerted on the heart by the 

 pneumogastric nerve, which, when stimulated, 

 glows or stops its movements; and in the same way 

 the secretion of glands may be interfered with by 

 abnormal stimulation of their nerve centres. This 

 influence is termed inhibitory action, and is one of 

 great importance. 



Secretory Nerves. The nerves which induce the 

 various acts of secretion leave by the motor cranial 

 nerves, or by the anterior roots of the spinal nerves ; 

 but the description of their distribution, &c. must 

 be relegated to the article SECRETION. 



The sympathetic system is, as already stated, 

 composed of a series of ganglia situated on either 

 side of the spinal column along its whole length. 

 In the dorsal, lumbar, and sacral regions the 

 ganglia correspond in number to the vertebra; ; but 

 in the cervical region there are only three, of such 

 large size, however, that they are generally supposed 

 to represent the fusion of a number of ganglia. 

 Below, the two chains unite in front of the coccyx 

 in a single ganglion. These ganglia are formed of 

 multipolar nerve-cells (fig. 5, A), and are united 

 with each other by gray nerve-fibres. Each gan- 

 glion gives to its corres[>onding spinal (or cranial) 

 nerve a gray non-medullated nerve, and receives 

 from it a fine white medullated nerve. The 

 fibres of distribution may be studied in fig. 10. 

 They pass to the blood-vessels and to the mucous 

 membranes and muscular coats of the various 

 internal visceras, and become united with each 

 other in fine networks or plexuses, on many of 

 which nerve-cells or ganglia are situated. 



The sympathetic chain is continued upwards as 

 a fine plexus of nerves on the internal carotid 

 artery, on the various branches of which it is dis- 

 tributed. From the superior cervical ganglion also 

 fibres pass to the various arteries in the neck and 

 face, and to form, along with the pneumogastric 

 and glosso-pharyngeal nerves, the pharyngeal plexus 

 on the muscles and mucous membrane of the 

 pharynx. 



From some of the cervical and upper thoracic 

 ganglia _ fibres pass into the chest, to form also, 

 along with the pneumogastric nerve, two important 

 plexuses, named pulmonary and cardiac, from 

 which branches pass to the lungs and heart, and 

 undoubtedly influence their functions. From the 

 thoracic ganglia also arise the three splanchnic 

 nerves which pass into the abdomen to enter the 

 solar or epigastric and the renal plexus. The 

 solar plexus is situated' at the pit of the stomach, 

 and is connected with two large semilunar ganglia, 

 which send branches to all the blood-vessels and to 

 all the organs within the alidomen. It is owing to 

 the relations and functions of the solar plexus that 

 blows in this region are so dangerous. The hypo- 

 gastric plexus arises from the lumbar ganglia, and 

 sends branches to the blood-vessels and to the 

 organs in the lower part of the abdominal cavity, 

 more especially the organs of generation, the lower 

 bowel, and the bladder. 



The functions of the sympathetic system are still 



