THE NERVOUS SYSTEM 



431 



communicans to the sympathetic ganglia. Fibres from the sympa- 

 thetic ganglia form the gray ramus communicans to the mixed spinal 

 nerve (Figs. 297 and 310). For further details regarding cranial 

 nerves see pp. 480, 481. 



The peripheral nerve consists of nerve fibres supported by con- 

 nective tissue (Fig. 298). Enclosing the entire nerve is a sheath of 

 dense connective tissue, the epineiirmm. This sends septa into the 

 nerve which divide the fibres into a number of bundles or fascicles. 

 Surrounding each fascicle the tissue forms a fairly distinct sheath, the 



Fig. 298. — From Transverse Section of Human Nerve Trunk. (Osmic acid fixa- 

 tion.) (Quain.) e/), Nerve sheath or epineurium surrounding the entire nerve and con- 

 taining blood-vessels iv) and small groups of fat cells (/); per, perifascicular sheath or 

 perineurium surrounding each bundle or fascicle of nerve fibres; end, interior of fascicle 

 showing supporting connective tissue, the endoneurium. 



perifascicular sheath or perineurium, which is covered with a layer of 

 flat cells. From the perineurium, delicate strands of connective tissue 

 pass into the fascicle, separating the individual nerve fibres. This con- 

 stitutes the intrafascicular connective tissue or endoneurium. In the 

 connective-tissue layers of the perineurium are blood-vessels, and 

 lymph spaces lined with endothelium, which communicate with 

 lymph channels within the fascicle. When nerves branch, the con- 

 nective tissue sheaths follow the branchings. When the nerve 

 becomes reduced to a single fibre, the perineural arid endoneural 



