ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 609 



The average length of the spinal cord is eighteen inches. In the 

 foetus the cord extends the whole length of the vertebral canal. The 

 difference in relative length of the cord in the foetus and in the 

 adult is due to the unequal and more rapid growth of the spinal 

 canal than of the cord. The cord thus seems to ascend in its canal. 

 Instead of the spinal nerves of the lower portion of the cord leaving 

 their points of emergence horizontally, they sweep down like the 

 hairs in the tail of a horse to form the aforementioned cauda equina. 



Coverings. Not only is the cord protected by the spinal canal 

 in which it is suspended, but in addition it is enveloped by a triple 

 membranous container. The cord does not more than half fill the 

 lumen of the spinal canal. It is suspended in this cavity surrounded 

 by an aqueous medium : the cerebro-spinal fluid. 



The investing membranes have been termed, from within out- 

 ward, pia mater, arachnoid, and dura mater. They form a sheath, or 

 tlieca, which is considerably larger than the cord. It is separated 

 from the bony wall of the spinal canal by venous plexuses and loose 

 areolar tissue. 



The pia mater is a very delicate covering which is closely 

 adherent to the cord. It sends numerous septa into the substance 

 of the cord as well as into its anterior and posterior median fissures; 

 It is composed of blood-vessels and connective tissue. 



The arachnoid (spider's web) is, as its name implies, a very deli- 

 cate, reticular membrane. It is nonvascular. Hanging like a cur- 

 tain between the innermost and outermost membranes, it forms two 

 spaces which are termed subdural and sub arachnoid. 



The outermost and toughest membrane is the dura mater. It 

 is a very dense sheath and lies indirectly in contact with the canal- 

 wall. However, unlike the dura of the brain, it does mt form the 

 periosteum for the portions of the vertebra constituting the walls of 



i spinal canal. 

 Diameter of the Cord. The volume of the cord is not the same 

 ughout its whole extent. Although of a mean diameter of half 

 nch, yet it presents two decided enlargements. 

 The one enlargement is at the level of the inferior portion of 

 cervical region; the other at the lower portion of the dorsal 

 region. The first one is the cervical enlargement from which emerge 



* nerves of the upper, extremity. The name Irachial enlargement 

 been given to it. 



From the lower enlargement arise the nerves which proceed to 

 lower extremities. It is known as the lumbar enlargement. At 



