The lips [labia oris] form the anterior wall of the mouth cavity. The lower lip [labium inferius] is marked off from the chin by the sulcus mentolabialis. The upper lip [labium superius] extends upward to the nose medially and the sulcus nasolabialis laterally. The philtrum is a median groove on the upper lip extending from the septum of the nose above to the labial tubercle [tuberculum labii superioris] below, at the middle of the rima oris. On each side of the rima oris the upper and the lower lips are continuous at the angle of the mouth [angulus oris], which is usually opposite the first premolar teeth. Laterally, the lips are continuous with the cheeks [buccse], which form the lateral walls of the mouth cavity.
Our knowledge of the lymphatic system has been very greatly increased during the past ten years by studies on its mode of development. Previous to 1902 nothing definite was known about the primary development or the mode of growth of the lymphatic system. It was concluded by some (Budge, Gullard and Saxer) that the lymphatics arise from undifferentiated mesenchyme cells; Ranvier believed that they arise from veins by budding of the endothelium; while Sala described them as arising partly from the mesenchyme and partly from venous endothelium.
Closely associated with the lymphatic capillaries and vessels is a group of glandular structures known as lymphoid organs. They consist, essentially, of groups of round lymphoid cells, lying in a meshwork of reticulum fibers, and having often a definite relationship to the blood or lymph vessels.
The lymph which enters the lymphatic capillaries passes over into collecting vessels (ducts), which carry it through the lymph-glands (nodes) to the large veins at the base of the neck. The lymph-vessels course in the loose subcutaneous tissues, in the connective tissues between muscles and organs, often accompanying the arteries and veins, sometimes forming networks around them. An idea of their arrangement can be best obtained by glancing at the illustrations of the lymphatics of special regions.
The lymphatic capillary, like the blood-capillary, is the portion of the lymphatic system which is chiefly concerned in the specific function of this system. In the blood-capillaries, where the blood is separated from the outside tissues by a single layer of flat endothelial cells, there occurs the interchange of fluid substances and of cells, while the heart, arteries and veins serve to transport the blood, modified in the capillaries, to other parts of the body. Similarly in the lymphatic system, it is in the capillaries, both those most peripheral and those in the lymph nodes, where the absorption and interchange of fluid substances and of cells takes place. Consequently it becomes of prime importance to obtain a clear understanding of the structure of the lymphatic capillaries, their relation to the other tissues, and their mode of functioning. At the outset, however, it must be admitted that our knowledge on this subject is far from complete.
There are two distinct sets of articulations in the vertebral column:
Those between the bodies and intervertebral discs which form synchondroses and which are amphiarthrodial as regards movement.
Those between the articular processes which form arthrodial joints.
The ligaments which unite the various parts may also be divided into two sets, viz. - immediate, or those that bind together parts which are in contact; and intermediate, or those that bind together parts which are not in contact.
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