300 Journal of Comparative Neurology. 



Principal Rami. — The first nerve from the sacral plexus, 

 N. Gemellus (Fig. Gem.), is given from the ectal surface of 

 the first sacral nerve where it is joined by the lumbo-sacral 

 cord (Fig. L.-S. C.) at the caudal border of the first sacral 

 vertebra, and is directed dorsad; it lies close to the vertebra 

 for its central 5 mm., and then passes laterad into the gemellus 

 muscle, and is distributed by three branches. This muscle 

 has its origin along the sacral vertebras, but its relation with 

 the M. gluteus maximus and its insertion with the M. obtu- 

 rator internus, as well as the innervation, lead to its identifi- 

 cation as the M. gemellus superior. 



From the cephalic border of the first sacral nerve a branch 

 is sent laterad and around the ramus of the ilium cephalad of 

 the acetabulum to the M. pyriformis (Fig. Pyr. ). The nerve 

 trunk lies ectad of the sacral artery and vein, and leaves the 

 pelvis through the great sciatic foramen vipon the ectal sur- 

 face of the M. quadratus femoris, the M. obturator and the 

 M. pyriformis, and entad of the MM. glutei. 



About 10 mm. caudad of the foramen of exit a branch 

 from the ental surface is given to the M. gluteus maximus 

 (Fig. M. gl. max.), and 5 mm. still peripherad a branch 

 caudad and entad of the M. obturator internus (M. obt. in.), 

 which branch divides into two rami, the shorter being dis- 

 tributed to the overlying obturator and the longer terminat- 

 ing in the quadratus muscles. The nerve to the M. gluteus 

 is sometimes a branch from the second sacral nerve (see 

 Fig. F). 



THE SECOND SACRAL NERVE. 



The ventral division of this nerve is much smaller than 

 the first sacral nerve; it takes a ventro-laterad course, and 

 joins the first sacral just mesad of the trochanter. The union 

 of these two trunks constitutes the srreat sciatic nerve "q. v.". 



Principal Rami. — TJic dor.^al divi.sio)i is already described 

 in common with the other sacral nerves, v. " General De- 

 scription." Its caudal branch joins the plexus which innerves 



