SPINAL CORD. 471 



subarachnoidean space. The anterior space is situated between 

 the lobes of the cerebrum, immediately in front of the optic 

 commissure ; the middle, between the pituitary body and the 

 pons Varolii ; and the posterior, between the medulla oblongata 

 and the posterior part of the cerebellum. 



Yellow and white fibres, intimately blended, enter into the 

 composition of the arachnoid, the free surface being lined with 

 scaly epithelium. It surrounds the nerves at their apparent 

 origin, and accompanies them to their exit from the cranial 

 cavity, when the layers become continuous. 



The PlA MATER (tunica vasculosa) is closely adherent to the 

 entire surface of the brain, dipping into every fissure or depres- 

 sion, into the sulci between the convolutions, and passing into 

 the interior in several places. It is thinner, but much more 

 vascular, than the pia mater of the cord, its vascularity being 

 greatest on the surface of the hemispheres, where it is little more 

 than a ramification of minute vessels, part of which can be 

 traced some distance into the brain substance. 



SPINAL CORD. 



The spinal cord, medulla sphudis, or myelon, is a large, white, 

 iiTegularly cylindrical cord, flattened from above downwards, 

 which extends from the foramen magnum to the sacral portion of 

 the neural canal, where it terminates in a slender filament. In 

 order to allow considerable motion of the vertebral column, with- 

 out risk of injury to the cord, it is loosely suspended in the canal, 

 the other contents being the meninges and subarachnoidean 

 fluid, with numerous arterial and venous plexuses. In its course, 

 the cord varies in size, being dilated at its origin, where it joins 

 the medulla oblongata, and having a considerable enlargement 

 between the fifth cervical and second dorsal vertebrae, where the 

 large nerves which form the brachial plexus are given off ; and 

 again at and posterior to the third lumbar vertebra, where the 

 lumbar and sacral nerves which constitute the lumbo-sacral 

 plexus arise. Towards its termination the cord gradually tapers 

 to a point, to which portion the name conus medtdlaris has been 

 applied ; and here is given off an indefinite number of filaments to 

 supply the coccyx and neighbouring structures, forming a bundle 

 of nerves termed the cauda equina. The roots of the spinal 

 nerves have their origin opposite to the intervertebral foramina. 



