90 HAND-BOOK OF PHYSIOLOGY. 



noid is a much more delicate membrane very similar in structure to the 

 dura mater, and lined on its outer or free surface by an endothelial mem- 

 brane. (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 endothelial cells: 

 these sub-arachnoid trabeculje divide up the sub -arachnoid 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 outgrowths 

 known as " Pacchionian bodies." 



THE SPIRAL COED AND ITS NEEVES. 



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 grey 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 grey nervous sub- 

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

 portion, while the latter occupies its central or axial portion, and is so 

 arranged, that on the surface of a transverse section of the cord it appears 

 like two somewhat crescentic masses connected together by a narrower por- 

 tion or isthmus (Fig. 318). Passing through the centre of this isthmus 

 in a longitudinal direction is a minute canal (central canal), which is 

 continued through the whole length of the cord, and opens above into 

 the space at the back of medulla oblongata and pons Varolii, called the 

 fourth ventricle. It is lined by a layer of columnar ciliated epithelium. 



The spinal cord consists of two exactly symmetrical halves separated 

 anteriorly and posteriorly by vertical fissures (the posterior fissure being 

 deeper, but less wide and distinct than the anterior), and united in the 

 middle by nervous matter which is usually described as forming two com- 

 missures an anterior commissure, in front of the central canal, consisting 

 of medullated nerve fibres, and a posterior commissure behind the central 

 canal, consisting also of medullated nerve-fibres, but with more neuroglia, 

 which gives the grey aspect to this commissure (Fig. 316, B). Each half 

 of the spinal cord is marked on the sides (obscurely at the lower part, 

 but distinctly above) by two longitudinal furrows, which divide it into 



