334 THE ANATOMY OF THE HORSE. 



of this nerve will be found in front of the circumflex iliac artery, 

 emerging from between the psoas magnus and parvus muscles, after 

 having penetrated the substance of the latter. It accompanies the 

 posterior division of the circumflex iliac artery to the front of the thigh, 

 where it is expended in cutaneous branches. Before it emerges it gives 

 branches to the psoas and quadratus muscles. 



Inguinal Nerves. There is considerable variation in the mode of 

 formation of these, but that figured in Plates 44 and 45 is probably as 

 common as any other. A nerve is there seen passing obliquely back- 

 wards over the circumflex iliac artery. It is formed by the union of 

 two branches which emerge at the inner side of the psoas parvus, these 

 being from the 2nd and 3rd lumbar nerves respectively. It divides into 

 three sets of branches, viz : — 



1. Muscular, to the internal oblique. 



2. Cremasteric, to the cremaster muscle. 



3. Inguinal, which descend in the inguinal canal to supply the 

 scrotum, prepuce (mammary gland in the female), and surrounding skin. 



The Lumbo-sacral Plexus (Plate 48). This is the plexus of nerves 

 for the supply of the hind limb. Like the corresponding brachial plexus, 

 the inferior primary branches of five nerves compose it, viz., the 4th, 

 5th, and 6th lumbar, and the 1st and 2nd sacral nerves. There is a 

 loop of communication between the first of these and the 3rd lumbar, 

 which to that extent also enters into the formation of the plexus. The 

 majority of its branches fall to be dissected with the pelvis, and a 

 complete account will then be given of it (page 361). In the meantime 

 only the most anterior of its branches will be dissected. 



1. Branches to the psoas magnus and iliacus. These are derived from 

 the 4th lumbar root of the plexus, or from the loop between that and 

 the 3rd. 



2. The Anterior or great crural nerve (Plate 45). This is a large 

 nerve which derives its fibres from the 4th and 5th lumbar roots, and 

 from the loop between the 3rd and 4th. Emerging between the psoas 

 magnus and parvus, it descends at the outer side of the external iliac 

 artery, but separated from it by the tendon of the last-named muscle. 

 It rests on the iliacus and psoas muscles, and crosses their common 

 termination to end in a fasciculus of branches for the extensors of the 

 leg. In this course it is covered by the sartorius muscle. It gives off 

 as a branch the internal saphenous nerve, whose origin is about opposite 

 the ilio-pectineal eminence. 



The Aortic Plexus of the Sympathetic Nerve (Plate 45). This is 

 the backward continuation of the solar plexus. Its branches interlace 

 around the aorta behind the kidneys, and unite with the posterior 

 mesenteric plexus. It receives some of the efferent filaments of the 

 lumbar ganglia. 



