THORAX AND ABDOMEN OF THE HORSE 153 



After the foregoing nerves have been recognised, they must be 

 followed to their several origins. This can only be done by removing 

 the psoas minor and major muscles piecemeal. This dissection reveals 

 the quadratus lumborum muscle, and the lumbar arteries will also be 

 open to examination. 



M. QUADRATUS LUMBORUM. — The quadrate^ muscle of the loins 

 lies between the ventral surface of the lumbar transverse processes and 

 the psoas major muscle. In the horse the muscle is much less 

 developed than it is in the other domestic animals, and is further 

 characterised by a large admixture of tendinous fibres. 



Partly fleshy partly tendinous bundles arise from the internal 

 surface of the vertebral extremities of the last two or three ribs and 

 from the transverse processes of the lumbar vertebra3. After a short 

 oblique (caudal and lateral) course, these bundles are attached to the 

 transverse processes of the lumbar vertebrae succeeding those from 

 which they arise, and to the ventral sacro-iliac ligament and the 

 adjacent part of the sacrum. 



Nn. LUMBALES. — The lumbar nerves, six in number, emerge from 

 the vertebral canal by traversing intervertebral foramina. The first 

 nerve emerges by the foramen between the first and second lumbar 

 vertebrae ; the sixth nerve leaves the vertebral canal between the 

 sixth lumbar and the first sacral vertebra. The first two or three 

 nerves are about equal in size and not larger than the last thoracic ; 

 but each of the last three lumbar nerves is larger than its predecessor. 



Each lumbar nerve behaves as a typical spinal nerve in dividing 

 into two rami — dorsal and ventral. The dorsal ramus (ramus 

 dorsalis) is much the smaller of the two, and, passing between adjacent 

 transverse processes, is distributed to the muscles and skin of the loins 

 and gluteal region. The ventral ramus (ramus ventralis) sends a 

 small white ramus communicans to a ganglion on the sympathetic nerve 

 cord and receives a grey ramus communicans therefrom. The ventral 

 ramus then passes in a lateral direction (generally oblique to the long 

 axis of the body) on a plane ventral to the transverse processes of the 

 vertebrae. 



It is to the ventral rami of the lumbar nerves that attention is now 

 to be directed. 



N, ILIOHYPOGASTRIC US. — The ilio-hypogastric nerve, the ventral 

 ramus of the first lumbar nerve, runs in a lateral direction between the 

 psoas major and quadratus lumborum muscles. After its emergence 



' Quadratus [L.], squared. 



