George L. Streeter 9 



region of the cord extends from the •24th to the 4:-2nd segment. About 

 one-half of this space, from the 26th to 3Tth, is occupied by the lumbo- 

 sacral enlargement. 



A feature which contributes largely to the peculiar appearance of this 

 part of the cord is the change occurring in the posterior longitudinal 

 sulcus. What was a barely-perceptible furrow in the cervical and tho- 

 racic cord becomes, at the beginning of the lumbo-sacral region, more 

 distinct, and, where the 31st pair of nerves are given off, it rather abruptly 

 widens out into a broad boat-shaped groove, the sinus rhomhoideus 

 sacralis. This reaches ventrally to the commissura anterior, and spreads 

 apart the posterior funiculi from the 31st tp 36th segment, at which 

 point the sides again come together and are continued as the posterior 

 longitudinal furrow. This sinus is filled with a delicate gelatinous 

 tissue, the structure of which will be discussed later. 



A drawing of the dorsal surface is reproduced in Fig. 2, b; lateral to 

 the sinus can be seen the sharply-defined dorsal funiculi increasing in 

 size from below upward. Each dorsal funiculns is bounded laterally by 

 a dorso-lateral groove, at a point corresponding to the tip of the dorsal 

 horn. Entering this groove are the enormous dorsal nerve roots, grouped 

 into segmental fibre bundles. 



Fig. 2, a shows the ventral surface of the enlargement. At two 

 places the pial sheath has been left intact. In this part of the cord the 

 pia is considerably modified from the form which is present in other 

 regions. Beginning at the 26th segment there is a marked increase in 

 the size of the thickened strips of the pial sheath, or ligamentous bands. 

 The pia in the intervening spaces becomes thinner and more web-like. 

 Between the 30th and 37th segments the ligamentum long, ventr. sends 

 out tooth-like intersegmental processes which join the ligamenta long, 

 lat., and the ligamentous structure thus formed affords a strong support 

 where, owing to its specialized character, the cord demands more than 

 ordinary protection. 



On removing the pia, there is seen an enlargement of the fissura longi- 

 tudinalis ventralis, which forms a sinus resembling, to some extent, the 

 sinus rhomhoideus of the dorsal surface, though it is shorter and nar- 

 rower. Moreover it is not filled with the gelatinous semi-transparent 

 tissue as seen in that sinus, and at the bottom one can see the cross-fibres 

 of the commissura anterior. The space where the fissura long, ventr. 

 may be called a sinus', extends from the 31st to the 35th segment, and is 

 1.3 mm. wide. 



The great increase in the anterior horn elements, which occurs in the 

 enlargement, is segmental in character, and forms segmentally projecting 



