THE SYMPATHETIC SYSTEM 185 



the anal canal, while others pass lateral to the rectum and 

 support the angle of union between the rectum and the anal 

 canal. When the levator ani contracts, it forms an unyielding 

 floor against which the pelvic viscera may be compressed by 

 the action of the abdominal muscles during defsecation. 

 Further, the anterior fibres hold the anal canal steady, when 

 a faecal mass is passing through it. 



THE SYMPATHETIC NERVOUS SYSTEM 



The sympathetic nervous system is subsidiary to the central 

 nervous system, and the fibres which it distributes and the 

 fibres which it receives are governed ultimately by nerve-cells 

 in the cerebral cortex. 



The system consists of two trunks (cords\ one on each side 

 of the body, which extend through the neck, thorax, abdomen 

 and pelvis, and each trunk possesses numerous ganglia, which 

 are interconnected by nerve-fibres. A typical ganglion corre- 

 sponds to one segment of the spinal medulla and is connected 

 to the anterior ramus (primary division) of the spinal nerve 

 of that segment by means of a grey ramus communicans. 



The grey rami communicantes contain fibres which are 

 passing from the sympathetic to the spinal nerves. When they 

 reach the nerve, some of the fibres pass centrally to the spinal 

 medulla, while others pass peripherally to be distributed by 

 the nerve to the blood-vessels, sweat glands, etc. Efferent 

 fibres from the central nervous system to the sympathetic do 

 not run in the grey rami. They are found in the white rami 

 communicantes, which are limited to certain ganglia. 



The white rami communicantes connect T. i or 2 - L. i or 

 2 and S. 2, 3 and 4 with the corresponding sympathetic 

 ganglia, and, in addition to efferent fibres, they contain the 

 afferent fibres from the viscera. They are not present in the 

 cervical region, and, therefore, the fibres which are distributed 

 by the cervical ganglia must descend through the cervical 

 portion of the spinal medulla and join the sympathetic via the 



