392 A MANUAL OF ANATOMY. 



The Spinal Pia. Fig. 81. 



This, like in the brain, is the vascular membrane which 

 encloses the cord from the foramen magnum to the end of 

 the cord proper at the second lumbar vertebra, beyond 

 which, under the name of the filum terminale (mostly pia 

 but little cord), it extends to the base of the sacrum. 



From the lateral margins of the cord (pia) saw-teeth-like 

 processes pass outward to be attached by the apices of the 

 teeth to the inner surface of the dura (and thus pin the arach- 

 noid to the dura at these points). There are 21 of these 

 teeth, and they are termed the ligamentum denticulatum. 

 They serve to hold the cord steadily in its proper position 

 within the centre of the spinal canal. 



The space between the arachnoid and the pia is the spinal 

 subarachnoidean space and communicates with the similar 

 spaces of the brain. It contains the cerebrospinal fluid, 

 which can thus flow freely from the spinal canal into the 

 subarachnoid spaces at the base of the brain, and thence by 

 way of the foramen of Majendie into the ventricular cavi- 

 ties of the same. 



The pia dips into the anterior and posterior fissures of the 

 cord. 



The pia and arachnoid are connected by filaments of 

 connective tissue constituting the subarachnoid tissue. It 

 becomes thickened around the vessels which pass to (and 

 from) the cord at its posterior fissure and here forms an in- 

 complete partition called the septum. 



The Spinal Cord. Fig. 81. 



The spinal cord is 16 to 18 inches in length, ex- 

 tending from the margin of the foramen magnum to the 

 first or second lumbar vertebra. Its diameter is from two- 

 to three-fifths of an inch. It weighs about one ounce. 



