692 Journal of Agriculture. [8 Nov., 1907, 



The lymphatic vessels or lacteals placed centrally in the villi com- 

 municate with a smaller number of larger lymphatic vessels and these 

 pour their contents into the largest lymphatic vessel of the body, the 

 THORACIC DUCT, which passes up close to the spine and empties into a 

 vein near the heart. 



There are therefore \.\\o paths b\ which a substance, absorbed from 

 the gut, may get into the general circulation ; namely by the blood vessels 

 of the gut, in which case it must run the gauntlet of the ''ver ; or by 

 the lymphatics, in which case the absorbed substance can enter the general 

 circulation without passing through the liver, but its progress is much- 

 slower owing to the sluggish stream in the lymphatics. 



NERVES OF THE ALIMENTARY CANAL.— In the intestines, 

 and to a lesser degree in the crop and stomach, we find a local nervous 

 system which reminds one of that present in the lowest invertebrate 

 animals. It consists of a plexus or nerve-network with ganglia, in 

 between the muscle layers and in the submucous coat. This system can 

 work independently of the central nervous system and can carry out 

 the peristaltic and rippling movements of the walls of the canal. Under 

 normal conditions, however, it does not work independently, but is under 

 the direct subordination of the central nervous system through autonomic 

 nerves. In the first place the thoracic autonomic or sympathetic system 

 sends nerves to the canal from oesophagus to rectum which can exert two 

 functions, they can tighten up or constrict the small arteries in this 

 region, and secondly, can slow down or stop the various movements of 

 the canal muscles. On the other hand we have the cranial autonomic 

 system, through the vagus nerve, innervating the pharynx, oesophagus 

 and all the abdominal portions except the lower colon and rectum. Impulses 

 passing down the vagus increase the muscular movements of the canal 

 and can bring aljout forceful contraction of the entire musculature of the 

 stomach ; they can also start and increase secretion in the glands of the 

 stomach. In the lower colon and rectum the vagus is replaced by sacral 

 autonomics passing out of the spinal chord by the second and third sacral 

 nerves. 



The alimentary canal is also supplied with afferent or sensorv nerves, 

 giving in the mouth sensations of touch, taste, heat and cold and pain, 

 but, in the main portion of the canal, pain is the onlv sensation obtainable 

 except the special sensation of thirst and probably of hunger. 



The salivary glands in the mouth region have each a double nerve 

 supply, sympathetic and cranial autonomic ; the former have fibres which 

 constrict the blood vessels thus lowering the blood flow whilst the latter 

 have fibres which dilate the blood vessels thus increasing the blood flow. 

 Each has fibres acting on the gland cells but their distribution has not 

 been worked out with sufiicient clearness. It is known however that 

 certain nerve impulses can influence the composition of the saliva fitting 

 it for the particular substance in the mouth which has reflexlv excited 

 its flow. 



The nerve centres which regulate the movements of the alimentary 

 canal are all in the medulla oblongata, with the exception of that regulating 

 uefgecation which is in the lower part of the spinal cord. They are 

 excitable by appropriate afi^erent impulses due generally to the food 

 itself ; thus food in the mouth starts afferent impulses which excite the 

 centres for the salivary glands and for the secretion of gastric juice; 

 food in the pharynx excites the centre for' swallowing; faeces in the 



