166 TOPOGRAPHICAL ANATOMY OF THE 



The spinal arachnoid^ (arachnoidea spinalis), the second tubular 

 investment of the spinal cord, is very delicate and transparent. Con- 

 tinuous anteriorly with the arachnoid of the brain, posteriorly it forms 

 a loose covering for the cauda equina, and ends by joining the other 

 meninges. Between the arachnoid and the underlying pia mater is 

 the subarachnoid space (cavum subarachnoideale), which is imperfectly 

 subdivided by a dorsal longitudinal septum (septum subarachnoideale) 

 and the right and left ligamenta denticulata. 



Dissection. — Remove tlie arachnoid from a length of the cord and so 

 expose the pia mater. 



The spinal pia mater- (pia mater spinalis) is a tough vascular 

 membrane — thicker than the corresponding covering of the brain — 

 closely applied to the surface of the cord and continued into the ventral 

 median fissure, opposite which it is thickened into a longitudinal band 

 called the linea splendens. Thin sheets of pia are provided for the 

 roots of the spinal nerves. 



Connecting the pia mater with the dura mater, and suspending the 

 cord in the dural sheath, the denticulate ligaraent (ligamentum 

 denticulatum) is attached continuously along the lateral border of the 

 spinal cord. Its connection with the dura mater, however, is interrupted, 

 and is in the form of a series of pointed teeth, which reach the dura 

 mater between the apertures of exit of successive spinal nerves. 



The spinal cord (Medulla spinalis). — The spinal cord, weighing 

 about 250 grammes, is a bilaterally symmetrical cylindrical mass of 

 nerve matter extending from the foramen magnum, where it is con- 

 tinuous with the medulla oblonorata of the brain, to the third sacral 

 vertebra. Its posterior extremity rapidly tapers, thus forming the 

 conus medullaris, beyond which is the JiluTn ^ tenninale. The last 

 named is a thin, thread-like structure, mainly composed of pia mater, 

 but containing some small amount of nervous tissue. 



A certain degree of dorso-ventral flattening is observable in most 

 of the spinal cord, but this is least in the thoracic region and conus 

 medullaris. The thickness of the cord is not uniform. Those parts are 

 thickest from which spring the nerves that form the limb-plexuses. 

 Thus it comes to pass that there are two enlargements, a cervical sivell- 

 ing (intumescentia cervicalis) and a lumbar sivelling (intumescentia 

 lumbalis). The longest stretch with a uniform diameter is the thoracic 

 part of the cord. 



1 dpaxfoeiorii (arachnojides) [Gr.], like a cobweb {dpdxvn). 

 ^ Fius [L.], tender ; Mater [L.], mother. 

 ^ Filum [L.], a thread. 



