138 



HISTOLOGY 



longitudinal nerve fibers, and thus they form the ganglionated trunk of 

 the sympathetic nerve (Fig. 128). 



From the ganglia of the trunk, bundles of nerve fibers grow out ven- 

 trally to supply the blood vessels and viscera. It is characteristic of these 

 branches that they unite with one another freely, forming net-like sym- 

 pathetic plexuses, within which there are many scattered nerve cells. 

 When the nerve cells in these ganglionated plexuses are particularly abun- 

 dant, the structure is called a ganglion, though generally retaining a 

 plexiform character. 



The principal branches of the cervical sympathetic trunk are the su- 

 perior, middle, and inferior cardiac nerves, which grow out from the corre- 

 sponding cervical ganglia. They extend to the heart (Fig. 128) and form 

 the cardiac plexus, associated with which is the cardiac ganglion, situated 

 under the arch of the aorta. These nerves, which are joined by branches 

 from the vagus, innervate the heart. The cervical sympathetic trunks 

 also send out nerves which form plexuses around the aorta and the pul- 

 monary, subclavian and carotid arteries to- 

 gether with their branches. These innervate 

 the smooth muscles in the walls of the vessels. 

 Some of the fibers accompany the thyreoid 

 arteries into the thyreoid gland and others are 

 distributed to the pharynx and larynx. 



The upper thoracic ganglia supply nerves 

 to the aortic plexus and pulmonary plexus, and 

 the latter enters the lungs. Large bundles of 

 fibers proceeding from the "fifth or sixth to the 

 ninth or tenth " thoracic ganglia of the sympa- 

 thetic trunk, unite to form the greater splanchnic 

 nerves, one on either side of the body, and 

 branches from the remaining thoracic ganglia 

 .__._. _ , form the lesser splanchnic nerves. These 



coe. g., cceliac ganglion; my.pl. -i ,. ., ..i i ? i 



myentenc plexus; sbm. pi., sub- splanciimc nerves pass into the abdominal cavity 



mucous plexus. . . . . 



and join one another, forming a large ganglion- 

 ated plexus on the sides and front of the aorta (Fig. 128). The sympa- 

 thetic trunks in the abdomen also send branches to join this plexus. The 

 great plexiform ganglion found around the cceliac artery, as it leaves 

 the aorta, is called the cceliac ganglion (or plexus). A similar plexus 

 surrounds the superior mesenteric artery. From these plexuses, as shown 

 in ^the diagram (Fig. 129), sympathetic nerves extend through the mesen- 

 tery, and they form a microscopic ganglionated plexus surrounding the 

 intestinal tube, lodged between the longitudinal and circular layers of 

 smooth muscle. This is the myenteric plexus (plexus myentericus). It 

 innervates the muscle and sends branches into the tissue beneath the mu- 



PIG I29 



