1284 



HUMAN ANATOMY. 



THE COCCYGEAL NERVE. 



The posterior primary division (r. posterior) of the coccygeal nerve (n. coccy- 

 geus) does not divide into internal and external branches. It unites with the fourth 

 and fifth sacral to form the posterior sacro-coccygeal nerve, whose course and distri- 

 bution are described above. 



THE ANTERIOR PRIMARY DIVISIONS OF THE SPINAL NERVES. 



The anterior primary divisions (rr. anteriores) of the spinal nerves, like the 

 posterior (rr. posteriores), contain fibres from both the anterior and posterior roots 

 and, with the exception of those of the first and second cervical nerves, are larger 

 than the posterior. After liberation from the main trunk at the intervertebral 

 foramina, they pass ventrally and supply the lateral and anterior portions of the neck 

 and trunk, as well as the limbs. 



Shortly after leaving its foramen, each anterior division is joined by a slender 

 fasciculus from the gangliated cord of the sympathetic, called the gray ramus 

 commuyiicans (page 1357). Branches to the sympathetic system are given off from 



some of the thoracic, lum- 

 l^iG. lobs. I^^j. ^j^^ sacral nerves, in the 



shape of small fasciculi of 

 meduUated fibres, called the 

 white rami communicantes. 

 These are destined for the 

 various structures of the 

 splanchnic area and consti- 

 tute the visceral or splanch- 

 nic distrih(tio7i of the spinal 

 nerves. The remainder of 

 the fibres are supplied to 

 the body wall and ex- 

 tremities and constitute the 

 somatic distribution of the 

 nerves. 



In the case of the 

 cervical, first and some- 

 times second thoracic, lum- 

 bar, sacral and coccygeal 

 nerves, plexuses of a greater 

 or less degree of intricacy 

 are interposed between the 

 origin and distribution of 

 the nerves. This renders the tracing of any set of fibres a matter of extreme difificulty, 

 but in the greater portion of the thoracic region the original segmental and less 

 complex arrangement persists. 



A typical spinal nerve (Fig. 1085), such as one of those in the mid-thoracic 

 region, is arranged as follows. The constitution of the main trunk (page i278) and 

 the distribution of its posterior branch (page 1279) have already been described. 

 The anterior primary division (r. anterior) leaves the intervertebral foramen and 

 almost immediately is connected with the gangliated cord by gray and white rami 

 communicantes. It then enters an intercostal space through which it courses 

 between the external and internal intercostal muscles, both of which it supplies. At 

 the side of the chest it gives of^" a lateral cutaneous branch (r. cutanetis lateralis), 

 which distributes a few tiny motor twigs and then pierces the external intercostal 

 muscle to supply the skin over the lateral portion of the trunk. On reaching 

 the superficial fascia it usually breaks up into two branches, a larger anterior (r. 

 anterior) and a smaller posterior (r. posterior). Having given off the lateral 

 cutaneous branch, the main anterior primary division continues its forward course 

 nearly to the mid-line, where it pierces the muscle and becomes superficial as the 

 anterior terminal cutaneous branch (r. cutaneus anterior). 



Diagram illustrating constitution and division of typical spinal nerve ; 

 SC, spinal cord; AR. PR, anterior and posterior roots; SG, spinal gang- 

 lion; CT, common trunk; AD, PD, anterior and posterior primary divis- 

 ions; PC, LC, AC. posterior, lateral and anterior cutaneous branches; 

 RC, ramus communicans ; ^v, sympathetic ganglion and cord. 



