683. The lacteal vessels (figs. 175 and 176), take their origin on the surface of the villi, by open mouths, too minute to be visible to the naked eye, but distinguishable under the microscope. These minute, pellucid tubes, wholly countless in number, are composed of membranous coats so thin and transparent that the milky colour of their contents, from which they derive their name, is visible through them, and yet they are firm and strong. They present a jointed appearance (figs. CLXXVI. 4, and CLXXVII. 7). Each joint denotes the situation of the valves with which they are provided, and which are placed at regular distances along their entire course (fig. CXCII. 1 and 2). These valves, which are generally placed in pairs (fig. CXCII. 2), consist of a delicate fold of membrane of a semilunar form, one edge of which is fixed to the side of the vessel, while the other lies loose across its cavity (fig. CXCII. 2). So firm is this membrane, and so accurately does it perform the office of a valve, that even after death it is capable of supporting a column of mercury of considerable weight without giving way, and of preventing a retrograde course of the fluid. The lacteals are nourished by blood-vessels, and animated by nerves, and it is conceived that they must be provided with muscular fibres, or some analogous tissue, for they are obviously contractile, and it is by this contractile power that their contents are moved. The delicacy and transparency of the vessels, however, render it impossible to distinguish the different tissues which compose their walls. [Illustration: Fig. CLXXV. View of the inner surface of the ilium as it appears some hours after a meal. 1. The smaller branches of the lacteals, turgid with chyle, covering the surface of the intestine. 2. Larger branches of the lacteals formed by the union of the smaller branches.] [Illustration: Fig. CLXXVI. _View of the course of the Lacteals._ 1. The aorta. 2. Thoracic duct. 3. External surface of a portion of small intestine. 4. Lacteals appearing on the external surface of the intestine after having perforated all its coats. 5. Mesenteric glands of the first order. 6. Mesenteric glands of the second order. 7. Receptacle for the chyle. 8. Lymphatic vessels terminating in the receptacle of the chyle, or commencement of the thoracic duct.]