THE ABDOMEN, INTERIOR. 551 



part of the canal. It is held in place by the two layers of 

 the triangular ligament, between which it extends. It is 

 surrounded by the compressor urethrae muscle, and has the 

 glands of Cowper in close relation to it. 



The spongy portion of the urethra. This presents two 

 dilatations : one at its beginning, the pars bulbosa, and 

 one at its termination, the fossa navicularis. 



The external meatus is the vertical opening of the ure- 

 thra in the glans. Along the spongy portion of the tube 

 are several small blind depressions opening forward, called 

 lacunae. One of these, the lacuna magna, is situated about 

 an inch from the external meatus, and may catch the point 

 of a sound or catheter. The urethra shows on cross sec- 

 tion as a transverse slit in the membranous and spongy 

 portions of the urethra, and at the external meatus as a 

 vertical slit. 



The Sacral Plexus. Fig. 1 14. 



This is formed by the union of the Lumbosacral Cord, 

 (see page 529), and the anterior divisions of the first three 

 sacral nerves to form a broad ribbon at the entrance of the 

 great sacrosciatic foramen. It rests upon the origin of the 

 pyriformis muscle. The fourth sacral nerve enters into the 

 plexus indirectly by sending a branch to join the internal 

 pudic nerve. The fifth sacral nerve has no part in the 

 sacral, but enters into the formation of the coccygeal plexus. 

 The fourth sacral nerve has most of its filaments distrib- 

 uted independently of the sacral plexus, to the skin over the 

 lower internal portion of the gluteus maximus muscle, to the 

 integument of the anus, besides furnishing branches to the 

 external sphincter ani, coccygeus, and levator ani muscles. 



The Branches of the Sacral Plexus. 



(a) The superior gluteal. This is formed by two 



