ISACRAL yERVE— SACRAL PLEXUS 809 



SACRAL AXD COCCYGEAL NERVES 



The anterior primary divisions of tlie uj)i)er four .sacral nerves escape from the 

 neural canal by passing through the anterior sacral foramina, while the anterior 

 division of the fifth passes forwards between the sacrum and coccyx. The 

 coccygeal nerve escapes from the neural canal by passing through thu terminal 

 opening. Its anterior primary division pierces the sacro-sciatic ligaments and 

 ])asses forwards. The sacral nerves decrease progressively in size, from the first to 

 the fifth. The first sacral nerve is the largest nerve in the body, while the fifth is 

 very small. Each sacral nerve is connected to the gangliated cord of the sym])a- 

 thetic by a gray ramus communicans. The anterior divisions of the first, second, 

 third, and part of the fourth nerves join the sacral plexus; another part of the 

 fourth has an independent distribution. The lower part of the fourth joins the 

 fifth sacral and the coccygeal nerve to form the coccygeal })lexus. 



The branches of the fourth sacral nerve and the coccygeal plexus may be 

 conveniently described first, as their distribution is confined to a limited area in 

 the innnediate neighbourhood of the sacral plexus. 



FOURTH SACRAL NERVE 



The fourth sacral nerve, called the "nervus l)igeminus, " gives of¥ an ascend- 

 ing branch to join the sacral plexus, and a descending twig to the coccygeal plexus. 

 In the interval between these communicating branches several ofi'sets arise directly 

 from the fourth sacral without entering into a plexiform arrangement. These 

 direct offsets are the i)erin8eal branch, and the muscular nerves to the coccygeus 

 and to the levator ani. 



The perinaeal branch of the fourth sacral pierces the pelvic diaphragm, 

 between the contiguous margins of the coccygeus and levator ani, and appears 

 close to the tip of the coccyx. It is then directed forwards, and ends in twigs to 

 the external s]ihinctor ani and to the integument of the anus. 



The branches to the coccygeus and levator ani enter these muscles on their 

 deep or pelvic surfaces. 



SACRO-COCCYGEAL PLEXUS 



The anterior primary division of the fifth sacral nerve divides into ascending 

 and descending branches. The ascendhig branch unites with the descending l)ranch 

 of the fourth sacral. The descending branch joins the coccygeal nerve. In this 

 manner two small loops are formed: these loops constitute the sacro-coccygeal 

 plexus. Anterior and posterior branches are given oflE by the plexus. The anterior 

 branches join the hypogastric plexus (Testut). The posterior branches i)ierce 

 the coccygeus muscle, an<l are distributed to the skin covering the posterior surface 

 of the coccyx. The coccygeal nerve gives a twig to the coccygeus and (according 

 to Testut) gives of^' a branch which pierces the great sacro-sciatic ligament and 

 ends in the lower fibres of the gluteus maximus. The plexus is placed in front of 

 the lower part of the sacrum and behind the second part of the rectum. 



SACRAL PLEXUS 



The sacral plexus, with the exception of the roots of the lumbo-sacral cord, is 

 situated in the ])elvis, l)ehind the ])arietal pelvic fascia. It lies, for the most part, 

 on the pyriformis. the fi1)res of that muscle often interlacing with the roots of the 

 plexus. 



It is behind the branches of the internal iliac vessels, two of which, the gluteal 

 and sciatic, pass through its loops. On the left side it is also liehind the first 



