610 PHYSIOLOGY. 



the site of each enlargement the cord loses its cylindrical form to 

 become somewhat flattened from before backward. 



The formation of the enlargements is in intimate relation with 

 the development of the members. In fishes we have only rudiment- 

 ary members, the cord is of uniform diameter throughout. In steel- 

 workers the cervical swelling is considerable. 



The weight of the cord is about one and one-fourth ounces; it is 

 equal to about one fortieth of the weight of the brain. 



The suspension of the spinal cord within the canal is main- 

 tained laterally by irregular fibrous tracts which form the tigamentum 

 denticulatum. Laterally the roots of the spinal cord give support; 

 below, the filum terminale fastens it to the coccyx ; above, its con- 

 tinuation as the medulla furnishes the most important support. 



Exterior Form of the Cord. Externally the cord has two longi- 

 tudinal median grooves: one anterior, the other posterior. They 

 traverse the entire length of the cord to divide it into two halves 

 which are usually perfectly symmetrical. The origins of the spinal 

 nerves are situated upon each side of these two parallel, longitudinal 

 lines. 



The anterior median groove divides the anterior surface of the 

 cord into two perfectly equal parts. It extends from the decussa- 

 tion of the pyramids to the caudal extremity of the cord. In depth 

 it occupies nearly a third of the thickness of this organ. In this 

 groove is folded a layer of pia mater; at its base is seen a layer which 

 passes from one-half of the cord to the other the while, or anterior,, 

 commissure. 



The posterior median fissure, deeper and narrower than the 

 anterior, extends from the nib of the calamus scriptorius to the 

 termination of the spinal cord. Into this groove the pia mater sends 

 but a simple partition; but it is very adherent to the walls of the 

 groove. The depth of the fissure is bounded by a commissure analog- 

 ous to that which is furnished to the anterior median groove, but of 

 a gray color. This is the gray, or posterior, commissure. 



Upon each side of the cord are seen two lateral grooves which 

 represent the lines of implantation of the anterior and posterior 

 roots. They are known as the anlero- and poster o-laleral grooves. 

 ,The latter is the more apparent of the two, showing itself in the 

 form of a dotted, longitudinal line. 



The antcro-lateral groove corresponds to the line of insertion 

 of the anterior roots of the spinal nerves. The two lateral grooves^ 





