354 



THE PERIPHERAL NERVOUS SYSTEM 



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 which the various nerves to the arm and shoulder arise. 



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

 in a plate-like structure, the anlage of the lumbo-sacral plexus (Fig. 341). 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 caudal pair constitute the sciatic 

 nerve; the lateral trunk is the peroneal nerve, and the median trunk is the tibial. 



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



Dorsal not 



Somatic sensory 

 neurone ' 



Visceral sensory 

 neurons 



Spinal cord 



Marginal layer 



Ipendymal layer 

 Mcwtle layer 



Visceral motor 

 fieunne 



rentral termintU. 

 division efjjyinaJ nerve 



us communicans 



-Sympathetic ganglion 



FIG. 342. Transverse section of a 10 mm. embryo showing the spinal cord, spinal nerves and their 

 function nervous components. Diagrammatic. 



form a special sensory group, the neurones of the peripheral nerves, both spinal 

 and cerebral, fall into four function groups (Fig. 342). 



(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 sympathetic 

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

 blood-vessels (spinal nerves) ; or (b) with fibers ending directly on visceral muscle 



