The two main veins are the basilic and the cephalic veins. Other forms include an M-pattern, where the vein branches to the cephalic and basilic veins. There are large tentacular nerves projecting into the cephalic tentacles. The "Scr" gene is responsible for cephalic and thoracic development in "Drosophila" embryo and adult. The cephalic portion curves in a frontal clockwise direction. Facial or cephalic sutures are the natural fracture lines in the cephalon of trilobites. Permanent pacemaker leads are often placed in the cephalic vein in the deltopectoral groove. The blade characteristically has two curves, the cephalic and the pelvic curves. Early in the fourth week the cephalic part bends sharply forward in a cephalic flexure. Eumalacostracans have 19 segments (5 cephalic, 8 thoracic and 6 abdominal). Stimulation of acid secretion occurs as it does in the cephalic phase. The cephalon or cephalic region refers to the head. The movement of the fetus to cephalic presentation is called "head engagement". Different phases of digestion take place including&colon; the cephalic phase, gastric phase, and intestinal phase. The cephalic fins are usually spiraled but flatten during foraging.