560 HANDBOOK OF PHYSIOLOGY. 



divide up the sub-arachnoid space into a number of irregular sinuses. There 

 are some similar trabeculas, 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 connec- 

 tive-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. 



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 bulb, and ter- 

 minating below, about the lower border of the first lumbar vertebra, in a 

 slender filament of gray substance, ihefilum terminals, 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 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 portion or isthmus (fig. 350A). 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 tlK 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 

 commissures 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 gray aspect to this commissure. The 

 fibres of the commissures are mainly composed of collaterals. 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 three portions, columns, or tracts, an anterior, lateral, and posterior. 

 From the groove between the anterior and lateral columns spring the 

 anterior roots of the spinal nerves (4); and just in front of the groove 

 between the lateral' and posterior columns arise the posterior roots of the 

 same; a pair of roots on each side corresponding to each vertebra. 



"White Matter. The white matter of the cord is seen to be 

 njade U 1 J f medullated nerve-fibres, of different sizes, arranged longi- 



