1296 



THE ORGANS OF DIGESTION 



The veins correspond to the arteries, and the venous blood passes to the superior mesenteric 

 vein, which, it will be remembered, unites with the splenic vein to form the portal vein. The 

 mesenteric veins are devoid of valves. 



The lacteals are lymphatics (Figs. 1017 and 1024) which arise in the villi. Lymphatics also 

 beo-in in sinuses at the base of the solitary follicles and in Peyer's patches. There is an extensive 

 Ivmphatic plexus in the submucous coat, another in the muscular coat, another under the peri- 

 toneum. The submucous plexus is formed by lymphatics from the villi and mucous membrane. 

 This plexus is joined by lymphatics from the bases of the solitary follicles, and the lymph passes 

 by vessels to larger vessels at the mesenteric border of the gut. The muscular lymphatics are 

 placed between the two muscular layers. They form a plexus and communicate freely with 

 the lymphatics from the mucous membrane, and empty themselves in the same manner into the 

 commencement of the lacteal vessels at the attached border of the gut. The vessels from all 

 sources of Ivmphatic supply pass up between the two layers of the mesentery, being connected 

 with the mesenteric nodes (Fig. 572), and unite to form a trunk, the intestinal lymphatic 

 trunk, which opens into the receptaculum chyli, or the vessels unite to form several trunks, 

 which open separately into the receptaculum chyli. 



Fir,. 1031. Nerve endings 

 in the villi of the sm-ill intes- 

 tine of a rabbit. (Miiller.) 



FIG. 1032. Meissner's plexus. (Ramon y Cajal.) 



The nerves of the small intestine (Figs. 1031 and 1032) are derived from the coeliac plexus 

 about the superior mesenteric artery. They pass along within the mesentery with the superior 

 mesenteric artery and reach the intestine. They pass to the plexus of nerves and ganglia situated 

 between the circular and longitudinal muscular fibres (Auerbach's plexus), from which the nerve 

 branches are distributed to the muscular coats of the intestine. From this plexus a secondary 

 plexus is derived (Meissner's plexus). It is formed by branches which have perforated the cir- 

 cular muscular fibres. This plexus lies in the submucous coat. It is also gangliated, and from 

 it the ultimate fibres pass to the muscularis mucosae, to the villi, and to the mucous membrane. 

 The nerves of the intestine are amyelinic, and some of the fibres are derived from the vagus. 



Applied Anatomy. The applied anatomy of the small intestine, especially the surgical 

 anatomy of the hernias, is given on page 1316.' 



THE LARGE INTESTINE (INTESTINUM CRASSUM) (Figs. 959, 1033). 



The large intestine extends from the termination of the ileum to the anus. It 

 is about five feet or more in length, being one-fifth of the whole extent of the 

 intestinal canal. It is largest at its commencement at the cecum, and gradually 



