The Lymphoid Tissue in the lower portion of the ileum, just above the ileocecal valve, is the usual seat of the primary lesion. There is the formation of a tubercle with coagulation necrosis; the central portion is cast off and an ulcer with thickened edges and a yellowish base is formed. Several ulcers become confluent and a large irregular one results. Instead of remaining the shape of the Peyer's patch, the ulcer tends to increase in size laterally, unlike the typhoid lesion. This is due to the extension by means of the lymphatics which surround the intestines. The lesion usually involves the sub-mucosa as well as the mucosa, also the muscularis and sometimes the serous coat. As a rule, the peritoneal covering at the site of the ulcer is the seat of numerous small tubercles in clusters. There may also be found white lines connecting neighboring tubercles; these are probably lymph-vessels that are stopped up by caseous matter.

A peculiar hyperplastic form, involving chiefly the ileocecal region, is characterized by extensive round-cell infiltration and the formation of a large amount of connective tissue. Tubercle formation, caseous degeneration and ulceration are usually very slight. On account of the thickening of the wall the lumen is much narrowed and at times almost completely stenosed.

Perforation is the most dangerous complication, but it does not frequently occur, on account of adhesions that have been formed during the progress of the disease. Tuberculous peritonitis may result from tuberculosis of the intestine. The ulcers may be present in all stages, some completely healed while others are undergoing active changes. As a result of the position of the ulcer the cicatrization is more likely to be followed by stenosis than is the case in typhoid fever. Stenosis, however, may occur when the tendency to organization exceeds the tendency to destruction of tissue. The mesenteric nodes are usually involved and at times may show much more marked disease than is seen within the intestine.