58 PHYSIOLOGY AT THE FARM. 



posterior extremity is furnished with a circular muscle or 

 sphincter, which is cushioned in fat so as to give the well- 

 known prominence to the anus in the living animal. The 

 peculiarity of this sphincter muscle, and of other sphincter 

 muscles, is that their fibres are constantly in a state of con- 

 traction till overcome by some power occasionally made to 

 antagonise them. 



The intestines are freely supplied with blood. From the 

 inferior surface of the posterior aorta, two mesenteric arteries 

 come off, the anterior or great mesenteric, and the posterior or 

 smaller mesenteric artery. From these vessels branches pro- 

 ceed between the laminse of the mesentery, meso-colon, and 

 meso-rectum, to supply the coats of the intestines. Two 

 corresponding veins, the posterior and anterior mesenteric 

 veins, unite with the other veins coming from the chylo- 

 poetic viscera to form the portal vein. 



The nerves of the intestines are derived from the anterior 

 mesenteric plexus, the posterior mesenteric plexus, and the 

 hypogastric plexus, all coming from the splanchnic nerves of 

 the sympathetic system. Like the blood-vessels, the nerves 

 reach the coats of the intestines between the folds of the 

 mesentery, the meso-colon, and meso-rectum. 



Between the laminae of the mesentery in particular are 

 lodged the lacteal and lymphatic vessels, and the glands con- 

 nected with these, which take so remarkable a share in the 

 function of nutrition. 



The principal trunk of the absorbent system, termed the 

 thoracic duct, runs forward along the spine to terminate 

 in the left subclavian vein. This trunk, the thoracic duct, 

 is formed by the union of five or six vessels, two or three 

 of which come from and through the pelvis, two or three 

 from the mesentery, and a single one from the environs of 

 the stomach and liver. The vessels which convey the chyle 



