NER VES. 299 



arteries. The capillaries of the cord everywhere form networks 

 with meshes somewhat lengthened in the direction of the long 

 axis of the cord. In the white matter the capillary network 

 is much less abundant than in the gray matter, where it is 

 richest in the region of the cell groups. 



The veins of the spinal cord follow the course taken by the 

 arteries. The central veins are relatively small in relation to 

 the central arteries, and are connected with the peripheral veins 

 by various well-developed anastomoses. On the dorsal surface 

 of the cord there are found much larger venous networks than 

 on the ventral side. From the venous plexuses of the pia 

 rnater the blood reaches the outside through the veins of the 

 ventral and dorsal roots. 



In the cerebrum and cerebellum we meet with a dense capil- 

 lary network around the cell groups of the cortex (Figs. 223 

 and 224). In the cortex the arteries break up into a fine capil- 

 lary network, which in the medullary substance becomes 

 coarser. The meshes of the network run in the direction of 

 the nerve fibres. 



The subclural and subarachnoidal spaces must be considered 

 as lymph spaces. It is believed also that they are in commu- 

 nication with the lymph-vessels of the nasal mucous membrane, 

 and those of the peripheral nerves. All the blood-vessels of 

 the central nervous system are surrounded by a perivascular 

 space which is to be regarded as a lymph space. 



2. PERIPHERAL NERVOUS SYSTEM. 

 A. Nerves. 



The cerebro-spinal nerves consist almost entirely of medul- 

 lated nerve fibres, which always are joined together into bundles 

 by a loose connective tissue. In this connective tissue there 

 can be distinguished a layer which surrounds the whole nerve. 

 This is known as the epineurium (Fig. 225). From this there 

 run into the interior of the nerves connective-tissue strands 

 between the so-called secondary bundles of nerve-fibres. These 

 strands are arranged around the secondary bundles in concen- 

 tric lamellae, which vary in thickness with the size of the 



