604 STRUCTURE OF THE SPINAL CORD. [CH. XLII. 



Pacchionian bodies are growths from the sub-arachnoid network of 

 connective-tissue trabeculas which project through small holes in the inner 

 layers of the dura mater into the venous sinuses of that membrane. The 

 venous sinuses of the dura mater have been injected from the sub-arach- 

 noidal space through the intermediation of these villous outgrowths. 



In the chapters preceding this one we have seen how all per- 

 vading nervous action is ; in connection with circulation, respira- 

 tion, secretion, peristalsis, etc., the way in which such functions 

 are regulated by nervous activity has taken up a considerable 

 amount of space. Some of the facts there described will be 

 better understood, or be seen in a clearer light if the student 

 turns back to them and studies them once more after he has 

 grasped what we are going to consider in the chapters that 

 follow this on the physiology of the central nervous system. 



Tt would also be advisable before he begins this subject, that 

 he should once more read Chap. XVII. on nerve-centres, in order 

 to refresh his memory concerning the elementary and funda- 

 mental problems in relation to nervous activity in these regions. 



CHAPTER XLII. - 



STRUCTURE OF THE SPINAL CORD. 



THE spinal cord is a column of nerve-substance connected 

 above with the brain through the medium of the bulb, and 

 situated in the spinal canal. In transverse section it is approxi- 

 mately circular, but the cord is not of the same size throughout 

 its covirse. It exhibits two enlargements, one in the cervical, 

 the other in the lumbar region. These are the situations whence 

 the large nerves for the supply of the limbs issue. The cord 

 terminates below, about the lower border of the first lumbar 

 vertebra, in a slender filament of grey substance, the filum 

 terminale, which lies in the midst of the roots of many nerves 

 forming the cauda equina. 



It is composed of grey and white matter ; the white matter is 

 situated externally, and constitutes its chief portion ; the grey 

 matter is in the interior, and is so arranged that in a transverse 

 section of the cord it appears like two crescentic masses (the 



