Pathways of lymph flow from the intestine of the horse.

The intestine of horses differs from that of most other domestic animals in having a huge ascending colon and cecum, which together act as a large fermentation vat. The lymph drainage from this intestine occurs through a complex network of lymph vessels and lymph nodes, of which there are several thousand. The pathways taken by lymph through these vessels and nodes were studied by tracing injections of dye, by examining Microfil casts, and with light and transmission electron microscopy. Lymph vessels transporting lymph from the intestinal wall generally terminate on a single node within a group of primary nodes. Efferent lymphatics form networks in the immediate vicinity of groups of nodes. In these networks, the vessels branch, anastomose, entwine amongst one another, and sometimes form recirculating loops. Usually 2-9 lymphatic vessels course away from these networks to secondary or higher-order nodes in both local and more central locations. Vessels reaching these nodes generally terminate on more than one node within each group. This pattern of lymph distribution is repeated as lymph passes through a succession of five or more nodes before flowing into the cisterna chyli. The complexity of the lymph pathways, it is suggested, would promote the mixing of lymph from different parts of the intestine and from different nodes, thus collating, integrating, and potentiating the immunological responses of these nodes. The walls of lymphatic vessels are well endowed with smooth muscle cells and elastic fibres, suggesting that intrinsic contractility of these vessels is a major mechanism in the propulsion of lymph from the intestine of the horse.