356 



THE PERIPHERAL NERVOUS SYSTEM 



ventral laminae. From the dorsal lamina arise the musculocutaneous, median, and 

 ulna nerves; from the ventral lamina, the axillary and radial nerves. 



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

 in a plate-Kke structure, the anlage of the lumbosacral 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 caudal pair constitute the sciatic 

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



Dorsal root 



Somahc sensory neurone 

 Visceral sensory neurone. 



Spinal cord 

 Visceral motor neurone- 

 Somatic motor neuroni 

 Dorsal ramus- 



Lat. terminal 

 division 



Ventral terminal division of 

 spinal nerve 

 Ramus communicans 



■Marginal layer 



Ependymal layer 

 Mantle layer 



Sympathetic ganglion 



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

 functional nervous components. Diagrammatic. 



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

 form a special sensory group, the neurones 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 s>Tnpathetic 

 gangHon 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 

 fibers (mixed cerebral nerves). 



