176 THE HUMAN BODY. 



formed reaching from the base of the skull to the coccyx. 

 In the trunk region these chains lie in the ventral cavity, 

 their relative position in which is indicated by the dots sy in 

 the diagrammatic transverse section represented on p. 6 in 

 Fig. 3. The ganglia on these chains are forty-nine in num- 

 ber, viz., twenty-four pairs, and a single one in front of the 

 coccyx in which both chains terminate. They are named 

 from the regions of the vertebral column near which they lie; 

 there being three cervical, twelve thoracic, four lumbar, and 

 five sacral pairs. 



Each sympathetic ganglion is united by communicating 

 brandies with the neighboring spinal nerves, and near the 

 skull with various cranial nerves also; while from the gan- 

 glia and their uniting cords arise numerous trunLs, many of 

 which, in the thoracic and abdominal cavities, form plexuses, 

 from which in turn nerves are given olf to the viscera. 

 These plexuses frequently possess numerous ganglia of their 

 own; two of the most important are the cardiac plexus 

 which lies on the dorsal side of the heart, and the solar plexus 

 which lies in the abdominal cavity and supplies nerves to the 

 stomach, liver, kidneys, and intestines. Many of the sympa- 

 thetic nerves finally end in the walls of the blood-vessels of 

 various organs. To the naked eye they are commonly grayer 

 in color than the cerebro-spinal nerves. 



Tho Sporadic Ganglia. These are found scattered in 

 nearly all parts of the Body except the limbs. They are for 

 the most part small, even microscopic in size, though several 

 large ones exist in the abdominal cavity. The} 7 are especially 

 abundant in the neighborhood of secretory tissues and about 

 blood-vessels, while a very important set is found in the 

 heart. Nerves unite them with the cerebro-spinal and sym- 

 pathetic centres, and probably most of them should be classi- 

 fied as belonging to the sympathetic system. 



The Histology of Nerve-Fibres. The microscope shows 

 that in addition to connective tissue and other accessory 

 parts, such as blood-vessels, the nervous organs contain tis- 

 sues peculiar to themselves and known as nerve-fibres and 

 nerve-cells. The cells are found in the centres only; while 

 the fibres, of which there are two main varieties known as 

 the white and the gray, are found in both trunks and cen- 

 tres: the white variety predominating in most cerebro-spinal 

 nerves and in the white substance of the centres, and the 



