356 



THE PERIPHERAL NERVOUS SYSTEM 



plate nervous cords extend into the intermuscular spaces and end in the 

 premuscle masses. The developing skeleton of the shoulder splits the 

 brachial plexus into dorsal and ventral laminae. From the dorsal lamina 

 arise the musculo- cutaneous, median, and ulna nerves; from the ventral 

 lamina, the axillary and radial nerves. 



In 10 mm. embryos the lumbar and sacral nerves that supply the leg 

 unite in a plate-like structure, the anlage of the lumbo-sacral plexus 

 (Fig. 359). The plate is divided by the skeletal elements of the pelvis and 

 femur into two lateral and two median trunks. Of the cranial pair, the 

 lateral becomes the femoral nerve; the median, the obturator nerve. The 



Dorsal root 



Somatic sensory neuron 

 Visceral sensory neu 



Spinal cord 

 Visceral motor neuro 



Somatic motor m 

 Dorsal ramus 



Marginal layer 



Epcndymal layer 

 Mantle layer 



Ventral terminal division of / \ \ d t 



spinal nerve / 

 Ramus communicans Sympathetic ganglion 



FIG. 360. Transverse section of a 10 mm. embryo, showing the spinal cord, spinal nerves and 

 their functional nervous components. Diagrammatic. 



caudal pair constitutes the sciatic nerve; the lateral trunk will be the 

 peroneal nerve, the median trunk the tibial. 



Save for the neurons from the special sense organs (nose, eye, and ear) 

 that form a special sensory group, the neurons of the peripheral nerves, 

 both spinal and cerebral, fall into four functional groups (Fig. 360). 



(1) Somatic afferent, or general sensory, with fibers ending in the 

 integument of the body wall. 



(2) Visceral afferent, or sensory, with fibers ending in the walls of the 

 viscera. 



(3) Somatic efferent, or motor, with fibers ending on voluntary muscle 

 fibers. 



(4) Visceral efferent, or motor: (a) with fibers ending about sympa- 

 thetic ganglion cells, which in turn control the smooth muscle fibers of 



