CHAP. X.] THE SPINAL CORD. 255 



which invests the pineal gland. This sabulous matter is composed 

 of phosphate of lime, with a small proportion of phosphate of mag- 

 nesia, a trace of carbonate of lime, and a little animal matter."* 



Of the Ligamentum dentatum. This remarkable structure, found 

 in the sub-arachnoid space, requires a brief notice. It seems to be 

 a process of the pia mater, which exhibits more of the glistening 

 appearance of white fibrous tissue than the rest of that mem- 

 brane. It extends from the occipital foramen to the filiform termin- 

 ation of the pia mater, adhering by its inner straight border to that 

 membrane, and attached on the other hand to the dura mater by a 

 series of dentated processes which penetrate the visceral and parietal 

 layers of arachnoid, pinning them, as it were, down to the fibrous 

 membrane. They form a vertical septum between the anterior and 

 posterior roots of the spinal nerves. The dentated processes vary 

 in number from eighteen to twenty-two. The first is attached to 

 the dura mater which covers the occipital foramen just behind the 

 vertebral artery ; the rest are inserted between the orifices for the 

 exit of the spinal nerves, and the last is on a level with the first or 

 second lumbar vertebra. A considerable quantity of yellow fibrous 

 tissue exists in this structure, especially in its dentated processes. 



The Pacchionian glands or bodies are whitish granules, com- 

 posed of an albuminous material, which are found among the vessels 

 of the pia mater, on the edges of the cerebral hemispheres, which 

 push the arachnoid before them, and even project into the longitu- 

 dinal sinus. They do not occur in the earliest periods of life, and 

 are frequently absent even at the more advanced ages; but they are 

 so often met with in the brains of adult and old persons that many 

 anatomists regard them as normal structures. 



Of the Spinal Cord. The spinal cord is somewhat cylindrical in 

 shape, slightly flattened on the anterior and posterior surfaces. Its 

 anatomical limits are, the occipital plane above, and a point rang- 

 ing in different subjects between the last dorsal and second lum- 

 bar vertebra below. It tapers to its exterior extremity, which lies 

 concealed among the leash of nerves which comes off from its lumbar 

 region, the cauda equina. Superiorly the spinal cord is separated 

 from the medulla oblongata by the decussating fibres of the anterior 

 pyramids. 



In the cervical and lumbar regions the cord exhibits distinct 



swellings, of which the cervical is the larger. The cervical swelling 



extends from the third cervical to the third dorsal vertebra, the 



lumbar one commencing about the ninth or tenth dorsal vertebra, 



* Van Ghert, de plexibus choroideis. 



