CHAPTER XVIII. 



THE CEREBRO-SPINAL NERVOUS SYSTEM* 



THE physiology of the cerebro-spinal nervous system includes that of 

 the Spinal Cord, Medulla Oblongata, and Brain, of the several Nerves 

 given off from each, and of the Ganglia on those nerves. 



Membranes of the Brain and Spinal Cord. The Brain and 

 Spinal Cord are enveloped in three membranes (1) the Dura Mater, (2) 

 the Arachnoid, (3) the Pia Mater. 



(1.) The Dura Mater, or external covering, is a tough membrane 

 composed of bundles of connective tissue which cross at various angles, 

 and in whose interstices branched connective-tissue corpuscles lie; it is 

 lined by a thin elastic membrane, and on the inner surface, and, where 

 it is not adherent to the bone, on the outer surface also, is a layer of en- 

 dothelial cells very similar to those found in serous membranes. (2.) 

 The Arachnoid is a much more delicate membrane, very similar in struc- 

 ture to the dura mater, and lined on its outer or free surface by an en- 

 dothelial membrane. (3.) The Pia Mater consists of two chief layers, 

 between which numerous blood-vessels ramify. Between the arachnoid 

 and pia mater is a network of fibrous-tissue trabeculae sheathed with en- 

 dothelial cells; these sub-arachnoid trabeculse divide up the sub-arach- 

 noid space into a number of irregular sinuses. There are some similar 

 trabeculae, but much fewer in number, traversing the sub-dural space, 

 i. e., the space between the dura mater and arachnoid. 



Pacchionian bodies are growths from the sub-arachnoid network of 

 connective-tissue trabeculae 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-arachnoidal space through the intermediation of these villous out- 

 growths. 



A. The Spinal Cord and its Nerves. 



The spinal cord is a cylindriform column of nerve-substance con- 

 nected above with the brain through the medium of the medulla oblon- 

 gata, and terminating below, about the lower border of the first lumbar 

 vertebra, in a slender filament of gray substance, the filum terminate, 

 which lies in the midst of the roots of many nerves forming the cauda 

 equina. 



Structure. The cord is composed of white and gray nervous sub- 

 stance, of which the former is situated externally, and constitutes its 



