THE ARTERIES 191 



the nerves which these foramina transmit. The vessel at its origin lies 

 above the peritoneum. Near the posterior end of the sacrum it termi- 

 nates by dividing into the ischiatic and lateral coccygeal arteries. During 

 its course the lateral sacral artery gives off four collateral branches, 

 which enter the spinal canal through the first four inferior sacral fora- 

 mina. In the canal they give off branches to the posterior extremity of 

 the spinal cord and to the cauda equina nerves, and then pass out by the 

 superior sacral foramina to be distributed to the muscles which lie above 

 the sacrum near the sacral spines. In addition, the lateral sacral artery 

 gives off a number of small but unimportant branches which are dis- 

 tributed to the surrounding structures. 



The terminal divisions of the lateral sacral artery are : 



{a) The Ischiatic Artery. — This vessel runs for a short distance on 

 the inner surface of the great sacro-sciatic ligament, which it then pierces 

 from within outwards, where it may be found under the upper extremity 

 of the posterior arm of the superficial gluteus muscle. It now takes a 

 course downwards and backwards, and divides into a number of small 

 branches which are distributed to the semimembranosus and semi- 

 tendinosus muscles near the tuber ischii. Anastomoses are contracted 

 by these branches with ascending branches from the femoro-popliteal 

 artery and with branches of the obturator and deep femoral vessels. 



[b) The Lateral Coccygeal Artery. — This may be regarded as the 

 continuation of the lateral sacral artery, since it continues the course of 

 the latter backwards. It is, however, much smaller than the parent vessel 

 owing to the detachment of the ischiatic branch. It runs the whole 

 length of the coccygeal region and is placed between the compressor 

 coccygis muscle and the bones. It gives off a number of small collateral 

 branches to supply the structures of the tail. 



The Middle Coccygeal Artery. — This branch usually leaves the 

 lateral sacral in common with the lateral coccygeal artery. It may, 

 however, leave the former before its division into the ischiatic and 

 lateral coccygeal vessels, as far forward even as the middle of the lateral 



