/; ,S/ J /.V.IA 



906 



the fourth ami fifth thoracic, and the coccygcal nerves) into a medial (internal) 

 branch and a lateral (external) branch. The medial branch turns inwards tnuards 

 I lie spinous processes of t lie vertebra-, and supplies I he hones and joints and t he mus- 

 cles about them, and may or may not supply t lie skin overlying them. The lateral 

 branch turns outwards and dorsal wards and also supplies the adjacent muscles and 

 bones, and. if the medial branch has not supplied t lie overlying skin, it terminates 

 in cutaneous twigs. In the upper half of the spinal nerves the medial branches 

 supply the skin; in the lower half, it is the lateral branches which do so. Both 

 branches of almost all the posterior divisions, especially those of the lower nerves, 

 show a tendency to run caudalwards and thus are distributed to muscles and skin 

 below the levels of their respective intervertebral foramina. They never supply the 

 muscles of the limbs, though their cutaneous distribution extends upon the buttock, 

 the shoulder, and the skin of the back of the head as far upwards as the vertex. 

 The posterior primary divisions, with the exception of those of the first three cer- 

 vical nerves, are much smaller than the anterior primary divisions. 



As their mixed function suggests, the posterior primary divisions contain both 

 nerve-fibres from the ventral roots and peripheral fibres (dendrites) arising from the 

 spinal ganglion-cells. If the nerve-trunk on the immediate peripheral side of 

 the spinal ganglion be teased, bundles of ventral root-fibres may be seen crossing 

 thi> trunk obliquely to enter the posterior division, and fibres from the spinal 



FIG. G72. DIAGRAMS ILLI'STHATIN<; THE ORIGIN AND DISTRIBUTION OF A TYPICAL SPINAL NERVE. 

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



XKIIIAL 

 SMAJfCM 



I i //'/. i;. 

 HllAXl'B 



IllllK 

 I'll I. VARY 





MEDIAL 

 H U.I XI II 

 LATERAL 

 IlllAXrll 



l:/OR 

 PRIMARY 



DtrniOK 



AXTKRIOR 

 PRIMARY 



Dirjsiot 



LATERAL 



UK 

 DORSAL 



Alimentary canal 





ganglion may also be traced into it. Also a few sympathetic fibres, derived 

 chiefly by way of the ranius eommunicans. are known to course in it for distribution 

 in the walls of the blood-vessels, etc., of the area it supplies. 



The anterior primary divisions run outwards and ventralwards. With the 

 exception of the first three cervical nerves, they are larger than the posterior primary 

 divisions, and appear as direct continuations of the nerve-trunks. Only in case of 

 most of the thoracic nerves do they remain independent in their course. In these 

 I hey run outwards and ventralwards in the body-wall. In general, these divisions 

 supply the lateral and ventral parts of the body, the limbs, and the perineum. In 

 the cervical, lumbar, and sacral regions they lose their anatomical identity by divid- 

 ing, subdividing, and anastomosing with each other so as to give rise to the three 

 great cerehro-spinal plexuses of the body the cervical, the brachial, and the 

 lumbo-sacral plexuses. The majority of the thoracic nerves retain the typical or 

 primitive character in both their anterior and posterior primary divisions. In them 

 the anterior division (intercostal nerve) divides into a lateral or dorsal and an an- 

 terior or ventral branch, both of which subdivide. The lateral branch is chiefly 

 cutaneous; it pierces the superficial muscles and. in the subcutaneous connective 

 tissue, divides into a smaller posterior and a larger anterior ramus. which respectively 

 supply the skin of the sides and the lateral part of the ventral surface of the body. 

 The anterior branch continues ventralwards in the body-wall, giving off twigs along 

 its course to the adjacent muscles and bones, and. as it approaches the ventral mid- 



