Ill] THE DIGESTIVE ORGANS 56 



food in their immediate neighbourhood. In this way the rotatory 

 motion of the contents of the ninien is produced. 



These movements are under the control of a network of nerve 

 fibres and ganglia, which though they are capable of going 

 on in the absence of any connection with the spinal cord, are 

 normal! V under the subordination of the central nervous system 

 through autonomic nerves. The cranial autonomics through the 

 vagus nerve supply the whole of the alimentary canal excepting 

 the hind part of the colon and the rectum. Impulses transmitted 

 along them excite muscular movement and increase peristalsis. 

 In the hind ]iart of the gut they are replaced bysacral autonomics. 

 The whole of the gut is supplied b}' S3'mpathetic or thoracic 

 autonomic nerves which in a general way act antagonistically to 

 those above mentioned. Impulses transmitted along them 

 decrease muscular movement, and by constricting the blood 

 vessels reduce the blood supply. The gut is also supplied with 

 sensory nerves conveying the sensations of hunger, pain, etc. 



The whole of the gut is well supplied by blood vessels bringing 

 arterial blood from the aorta. The veins of the alimentary tract 

 as already described unite to form the portal vein, so that the 

 blood drained from the gut cannot reach the heart and join the 

 general circulation without first traversing the liver. 



The lymphatic vessels discharge their contents either into the 

 large thoracic duct which opens into the left anterior vena cava 

 between the subclavian and jugular veins or else into the corre- 

 sponding smaller right lymphatic trimk which communicates 

 with the right vena cava. Through these vessels the lymph is 

 passed into the blood vascular system. 



