158 THE SURGICAL ANATOMY OF THE HORSE 



posterior gluteal sets of nerves and the great sciatic nerve are detached, 

 the greater portion of the band being expended in the formation of the 

 latter. 



The plexus is related anteriorly to the superior surface of the psoas 

 parvus muscle, which thus conceals this portion from view. Between 

 the contributions from the fifth and sixth lumbar nerves, which, excluding- 

 the connecting trunk, may be regarded as the last contribution to the 

 anterior division and the first contribution to the posterior division 

 respectively, we find the internal iliac artery. This constitutes a very 

 important relationship of the plexus. The posterior part of the plexus 

 may be found supero-laterally in the pelvis, near the greater sacro-sciatic 

 foramen. Outwardly it is related to the gluteal vessels, and inwardly to 

 the lateral sacral artery. 



From the situation of the plexus which we have described, it will be 

 seen that it is extremely well protected from external injury, differing 

 greatly in this respect from the plexus which supplies the fore limb. Its 

 anterior portion is protected by the vertebral column, whilst its posterior 

 portion lies within the pelvic cavity. Apart from the osseous protection 

 afforded it, we find superiorly the great thickness of muscle presented by 

 the longissimus dorsi and gluteus maximus. 



The branches given off by the lumbo-sacral plexus from before 

 backwards are : 



1. The iliaco-muscular nerves. 



2. The anterior crural nerve. 



3. The obturator nerve, 



4. The anterior gluteal nerves. 



5. The great sciatic nerve. 



6. The posterior gluteal nerves. 



The Iliaco-Muscular Nerves. — There are several small branches which 

 are given off by the anterior division of the plexus, and which were 

 designated iliaco-muscular nerves by Girard. Frequently one such branch 



