TEETH 113 



close to the tooth and turns down as a sheath surrounding the neck. At 

 the level of the upper part of the cement it ends abruptly. The connective 

 tissue of the gums blends below with the circular ligaments. It contains 

 few elastic fibers, but is very vascular and is often infiltrated with lympho- 

 cytes. Its lymphatic vessels drain outward, along the margin of the cheek 

 and gums, and inward, over the floor or roof of the mouth, as shown by 

 Schweitzer. 



MUSCULAR TISSUE. 



Contractility is a fundamental property of protoplasm. In muscle cells 

 it attains its highest development. Muscle cells are elongated structures, 

 known as muscle fibers, which contain numerous longitudinal fibrils within 

 their protoplasm. By the shortening of these fibrillated cells, muscular 

 action results. The muscle fibrils, or myoHbrils, may be free from trans- 

 verse markings, as in smooth muscle; or they may exhibit a succession of 

 dark and light transverse bands, as in striated muscle. Smooth muscle 

 fibers enter into the formation of the viscera, and their action, almost 

 without exception, is involuntary. Striated muscle, in so far as it consti- 

 tutes the entire system of skeletal muscles, is voluntary, or under the control 

 of the will, but the striated fibers of the diaphragm and upper part of the 

 oesophagus are apparently involuntary. The special form of striated mus- 

 cle, known as cardiac muscle, which makes the bulk of the heart and extends 

 some distance in the wall of the pulmonary veins, is involuntary. The 

 three principal forms of muscle, smooth, skeletal, and cardiac, are meso- 

 dermal in origin. Within the basement membrane of the sweat glands 

 there are elongated ectodermal cells which have been described as smooth 

 muscle fibers, but their contractile nature is still questioned. It is well 

 established, however, that the muscles of the iris, which control the size of 

 the pupil, are derived from ectodermal cells which bud off from those form- 

 ing the optic cup Ectodermal muscles in man are limited to these 

 examples. 



SMOOTH MUSCLE. 



Smooth muscle fibers are derived from mesenchymal or young connec- 

 tive tissue cells. Usually they are produced in layers which surround 

 some tubular organ, such as a blood vessel, duct, or a part of the digestive 

 tube. The fibers in these layers are generally parallel, and are usually 

 either circular or longitudinal in relation to the organ which they envelop. 

 Occasionally they are oblique, or irregularly interwoven. Fibers which 

 encircle an organ are called circular or transverse fibers; they may be cut 

 across or split lengthwise according to the plane in which the organ is 

 sectioned. The same is true of the longitudinal fibers, which run length- 

 wise of the organ. 



