PRIMARY DIVISIONS OF SPINAL NERVES 



967 



Nerves 



Third 

 Fourth 



Fifth 



First sacral 



Fifth 



Coccygeal 



(A) Middle of spine of tenth thoracic vertebra. 



(B) Just below spine of eleventh thoracic vertebra. 



(A) Just below spine of tenth thoracic vertebra. 



(B) Junction of upper quarter and lower three-quarters of spine of twelfth 

 thoracic vertebra. 



(A) Junction of upper third and lower two-thirds of spine of eleventh thoracic 

 vertebra. 



(B) Middle of spine of twelfth thoracic vertebra. 



(A) Just above lower border of spine of eleventh thoracic vertebra. 



(B) Lower border of spine of first lumbar vertebra. 



(A) Lower border of spine of first lumbar vei'tebra. 



(B) Just below upper border of spine of second lumbar vertebra. 



Relative size of the nerves. — The size of the different spinal nerves varies 

 greatly. Just as the spinal cord shows marked enlargements in the cervical and 

 lumbar regions necessitated by the greater amount of innervation required of these 

 regions for the structures of the upper and lower limbs, so the nerves attached to 

 these regions are considerably larger than elsewhere. 



The smaller nerves are found at the two extremities of the cord and in the mid-thoracic 

 region. The smallest nerve is the coccygeal, and the next in order of size are the lower sacral 

 and the first two or three cervical nerves. The largest nerves are those which contribute 

 most to the great nerve trunks for the innervation of the skin and muscles of the limbs: — the 

 lower cervical and first thoracic for the upper limbs and the lower lumbar and first sacral for 

 the lower Hmbs. The nerves gradually increase in the series in passing from the smaller toward.^ 

 the larger. 



Fig. 747. — Diagrams Illustrating the Origin and Distribution of a Typical Spinal 



Nerve. 



Medial 

 branch 

 Lateral 

 branch 

 Posterior 

 primary 

 division 

 Anterior 

 primary 

 division 

 Lateral 

 branch 



Anterior 



or ventral 



branch 



A, in thoracic region; B, in region of a limb (highly schematic). 

 A B 



Alimentary canal 



Medial 



branch 

 Lateral 



branch 



Posterior 



primary 



division 



Anterior 

 primary 

 division 

 Lateral 

 or dorsal 

 branch 



Limb 



Anterior or 

 Ventral branch 



The primary divisions of the nerve -trunk. — A typical spinal nerve (middle 

 thoracic, for example), just as it emerges from the intervertebral foramen, divides 

 into four branches: — the two large primary divisions; viz., the posterior primary 

 division [ramus posterior] and the anterior primary division [ramus anterior]; 

 third, the small ramus communicans, by which it is connected with the sympa- 

 thetic; and fourth, the smaller, ramus meningeus {recurrent branch), which im- 

 mediately turns centralward for the innervation of the membranes and vessels of 

 the spinal cord. 



In general, the posterior primary division passes dorsalward between the arches 

 or transverse processes of the two adjacent vertebrae in relation with the anterior 

 costo-transverse ligament, and then divides (with the exception of the first cer- 

 vical, the fourth and fifth thoracic, and the coccygeal nerves) into a medial (inter- 

 nal) branch and a lateral (external) branch. The medial branch turns toward the 

 spinous processes of the vertebrse, and supplies the bones and joints and the mus- 

 cles about them, and may or may not supply the skin overling them. The 

 lateral branch turns dorsalward and also supplies the adjacent muscles and bones, 

 and, if the medial branch has not supplied the overlying skin, it also terminates 

 in cutajieous twigs. 



