1278 HUMAN ANATOMY. 



by the unopposed action of the muscles of the opposite side. The nerve may be 

 injured by operative or other wounds in the submaxillary region or in the mouth, as 

 in gun-shot wounds. It can be easily reached in the submaxillary region by the 

 same incision as that used for ligating the lingual artery (page 736). It passes for- 

 ward to the tongue, just above the hyoid bone, and forms the upper boundary of the 

 small ' ' lingual triangle, ' ' which is exposed when the submaxillary gland is elevated. 



THE SPINAL NERVES. 



The cranial or cerebral division of the nerves having been considered, the spinal 

 group next claims attention, the visceral or splanchnic (sympathetic) nerves being 

 reserved for a final and separate description. 



The spinal nerves (no. spinales) include a series of usually thirty-one pairs of 

 symmetrically disposed trunks which pass laterally from the spinal cord and emerge 

 from the vertebral canal through the intervertebral foramina (Fig. 880). Each 

 nerve arises from the cord by a dorsal sensory and a ventral motor root, which sepa- 

 rately traverse the subarachnoid and subdural spaces and evaginate or pierce the pia 

 mater, arachnoid and dura mater. Within the intervertebral foramina the roots unite to 

 form a common trunk, which carries with it a sheath composed of the three membranes, 

 the contribution of the arachnoid and pia, however, soon ending, whilst the dural 

 covering is prolonged to become continuous with the epineural sheath of the nerve. 



Nomenclature. — The spinal nerves are designated not relative to the position 

 at which they arise from the cord, but according to their point of emergence from the 

 \'ertebral canal. They are divided, therefore, into the cervical, thoracic, lumbar, 

 sacral and coccygeal groups. With the exception of those in the cervical region, 

 the individual nerves are named according to the vertebra below which they emerge 

 from the vertebral canal. On account of the disproportion between the eight cervi- 

 cal nerves and the seven cervical vertebrae, this arrangement necessarily can not 

 prevail in the neck. The first cervical nerve, often called the suboccipital nerve, 

 emerges between the occipital bone and the atlas ; the second emerges below the first 

 vertebra, the third below the second and so on down to the eighth, which traverses 

 the foramen between the seventh cervical and first thoracic vertebral segments. 



Constitution. — Every spinal nerve arises by two roots, a posterior sensory 

 and an anterior motor, the latter being composed of the axones proceeding from the 

 motor neurones situated within the gray matter of the anterior cornu of the spinal cord, 

 whilst the fibres composing the posterior or sensory root are the axones of the neurones 

 within the ganglia which are invariably present on these roots. The formation of the 

 common trunk, by the union of the two roots, affords opportunity for the two varie- 

 ties of fibres to intermingle, so that the anterior and posterior primary divisions into 

 which the common trunk divides contain both sensory and motor fibres. In addition 

 to these fibres, which are destined for the somatic muscles and the integument, others 

 are added from the sympathetic neurones for the supply of the outlying involuntary 

 muscle and glandular tissue occurring in the regions to which the spinal nerves are 

 distributed. It is evident, therefore, that the terms "motor" and "sensory," as 

 applied to the somatic branches of the spinal nerves, are relative and not absolute, 

 since in all cases the nerves passing to the muscles contain sensory and sympathetic 

 fibres in addition to those ending as motor filaments in relation with the striated 

 muscle fibres. Likewise, in the case of the sensory branches distributed to the integ- 

 ument, sympathetic filaments (motor to the involuntary muscle of the blood-vessels 

 and secretory to the glands) accompany those concerned in collecting sensory 

 impulses. On the other hand, where they retain their typical plan, as in the case of 

 the thoracic nerves, the spinal nerves contribute motor fibres which end around the 

 sympathetic neurones to supply motor impulses either to the involuntary muscle 

 of the organs, by way of the splanchnic efferents, or to the outlying involuntary 

 muscle along the somatic nerves in the manner above described. 



The sensory, posterior or dorsal roots (radices postcriores) of the spinal 

 nerves are usually larger than the motor, a condition due to the increased number 

 of their filaments and the greater size of those filaments (fila radicularia). The fas- 

 ciculi which form the sensory root are attached to the cord along the postero-lateral 



