324 THE ANATOMY OF THE HORSE. 



The Eeceptaculum Chyli. Separate the aorta and vena cava at the 

 origin of the anterior mesenteric artery, and look above them for this. 

 It is the dilated commencement of the thoracic duct. It is formed by 

 the union of a variable number of large lymphatic vessels, and it is 

 continued forwards by the duct. This is a thin-walled vessel of small 

 calibre which passes into the thorax between the pillars of the diaphragm, 

 being generally to the right of the aorta. 



Last Dorsal and First Two Lumbar Nerves (Plates 44 and 45). 

 The inferior primary branches of these nerves appear at the outer edge 

 of the psoas maguus, the last dorsal being close behind the last rib, and 

 the other tAvo issuing in series behind it. These nerves have already been 

 followed in the dissection of the abdominal wall, where they are distri- 

 buted in the region of the flank to the abdominal muscles, panniculus, 

 and skin. These lumbar nerves also furnish cutaneous branches to the 

 inside and front of the tliigli. 



3fiD Lumbar Nerve (Plates 44 and 45). The inferior primary branch 

 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 backwards 

 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. Miiseular, 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 349). In the meantime 

 only the most anterior of its branches will be dissected. 



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

 4th lumbar root of the plexus, or from the loop between that and the 3rd. 



