THE NERVOUS-SYSTEM. g(JI 



tudiiially, and of a supporting material of two kinds, viz. : (a) ordinary 

 fibrous connective tissue with elastic fibres, which is connecteofSvith 

 septa from the pia mater which pass into the cord to carry the blood 

 vessels. (#) Neuroglia; this material is made up of the branching cells 

 (fig. 35lA), the bodies of which, in consequence of the high development; 

 of the branchings, are small. The processes of the neuroglia-cells are 

 arranged so as to support the nerve-fibres which are without the usual 

 external nerve sheaths. Neuroglia was formerly considered to be a 

 kind of connective tissue, but is now considered to be a distinct material. 



15 15 



16 



Fig. 850A. Horizontal section of the cord and its envelopes, at the middle of a vertebral body^. 

 (Schematic). 1, Spinal cord with a, its anterior median fissure; 3. its posterior median fissuref 

 4, anterior roots; 5, posterior roots; 6, pia mater (in red); 7, ligamentum dentatum; 8, connect; 

 ing fibres passing from the pia to dura mater; 9, visceral layer and 9', parietal layer of the 

 arachnoid (in blue); 10, subarachnoid space; 11, arachnoid cavity; 12, dura mater (in yellow); "13, 

 periosteum; 13', external periosteum; 14, cellular tissue situated between the dura mater and the 

 wall of the vertebral canal; 15, common posterior vertebral ligament; 16, intra-spinal veins; 17,; 

 vertebra i n section. (Testut. ) 



It is derived from the neural epiblast, and yields neuro-keratin. (See.p~ 

 107.) 



The general rule respecting the size of different parts of the cord ' 

 appears to be, that each part is in direct proportion in this respect to th 

 size and number of nerve-roots given off from it, and has but little rela- 

 tion to the size or number of those given off below it. Thus^the cord is 

 very large in the middle and lower part of its cervical portion, whence 

 arise the large nerve-roots for the formation of the brachial plexuses and 

 the supply of the upper extremities, and again enlarges at the lowest 

 36 



