The =vena umbilicalis= originates in the umbilical region and empties into the vena hepatica magna sinistra at a point where it comes out of the liver (Fig. 70, No. 29). This is the remains of an embryonal vein which collected all the blood of the yolk sac, passed on the left side of the large intestine to the body, took up the vena mesenterica and ended as the vena umphalo-mesenterica. THE LYMPHATIC SYSTEM The peculiarity of the lymph vessels is that they are associated with organs in which lymph cells are formed. The lymphatic system consists of the lymph vessels and the cell-forming organs. In some instances the cell-producing organs are lymph follicles and in others lymph glands. For the most part the glands are replaced by plexuses which in many places surround the blood-vessels. The lymph of birds is similar to that of mammals. The larger lymph vessels are similar to the veins, although the walls are always thinner. Its tunica intima is rich in elastic fibers and has a layer of endothelial cells on the inner side. The tunica media is formed of rings of smooth muscle fibers. The adventitia is composed of loose connective tissue. The lymph vessels frequently form plexuses. The large lymph trunks follow the course of the larger blood-vessels, and frequently surround the arteries. All the lymph vessels of the body, exclusive of the lymph of the caudal region, form into a large trunk which originates on both sides of the celiaca communis and passes upward along the side of the abdominal aorta, reaching a point anterior to the celiaca. By receiving many vessels in this region it forms a plexus around the aorta, and finally divides into two vessels, the right and the left, ducti thoracici. The lymph vessels of the left side of the head, the neck, and the lung, and the left wing, and also lymph vessels of the proventriculus and the throat enter into the left ductus thoracicus. They accompany the jugular vein and are closely associated with the thyroid gland. The right thoracic duct receives the lymph veins from the right cervical lymph vein, and the right side of the head, the neck, the lung, and from the right wing. After the right cervical lymph vein has passed through the right thyroid gland, it divides into two branches, one branch emptying into the right thoracic duct and the other into the vena cava dextra. The lymph vessels of the liver, the stomach, the pancreas, and the duodenum enter near the root of the arteria celiaca into the large lymph trunk. The lymph vessels of the remainder of the intestines, of the kidney, and of the generative organs empty farther caudally.