The 'levator labii superioris,' fig. 29, 1, arises from the lower margin of the orbit, and descends to be inserted into the orbicularis and the skin of the upper lip. The levator anguli oris,' fig. 29, c, arises below the suborbital foramen and descends, inclining outward, to the angle of the mouth, blending its fibres with those of the zygomatici and orbicularis. The zygomaticus major,' fig. 29, 3, is cylindrical, rising from the malar and descending obliquely inward to a similar insertion at the angle of the mouth. The zygomaticus minor, fig. 29, 3, arises in front of the zyg. major, and passing downward and inward to the angle of the mouth, where it is continuous with the outer margin of the levator labii superioris. The levator menti is a conical fasciculus arising from the incisive fossa of the mandible, external to the symphysis, and expanding as it descends to be inserted into the integument of the skin. The depressor labii inferioris,' fig. 30, d, arises from the inner half of the external oblique line of the mandible, and is partly also continued from the platysma: its fibres ascend, inclining inward to be attached to the lip, where they blend with those of the orbicularis oris. The 'depressor anguli oris,' fig. 29, t, arises from the external oblique line of the mandible: its fibres ascend and converge to the angle or commissure of the lips, blending with the other insertions at that part. The buccinator, fig. 30, b, arises from both upper and lower jaws and the pterygo-maxillary ligament: its fibres line the cheek and converge toward the angle of the mouth, where some decussate, the lower ones going to the upper segment of 30 the orbicularis, the upper ones to the lower segment, while other fibres are continued forward into the corresponding lip. The buccinator acts, in antagonism with the orbicularis, in spirting fluids from the mouth and in playing on wind instruments. In mastication the buccinator presses the food from between the cheek and gums into the cavity of the mouth. It assists also in deglutition when the mouth is closed, by pressing the food backward. The levator labii superioris alæque nasi' arises from the nasal process of the maxillary, descends obliquely outward and divides, a short strip being attached to the cartilage of the ala nasi, the outer and longer strip to the skin of the upper lip near the nose, and becoming blended with the orbicularis and levator labii proprius. The triangularis nasi,' or compressor naris,' figs. 29, and 30, n, arises from the maxillary external to the incisive fossa: its fibres proceed upward and inward, expanding to an aponeurosis continuous, over the bridge of the nose, with that of the opposite muscle. The 'depressor alæ nasi' is a short flat muscle radiating upward from the myrtiform or incisive fossa of the maxillary; it sends upper fibres to the septum and back part of the alæ nasi and lower ones into the orbicularis oris. The orbicularis palpebrarum,' fig. 29, o, surrounds the orbit and eyelids: it arises from the internal angular process of the frontal, from the nasal process of the maxillary, and by a short tendon at the inner angle of the orbit. It rapidly expands to form a broad thin elliptical plane of fibres: the palpebral portion is thin and pale: the orbital portion is thicker and of a reddish colour. The action of the muscle is that of a sphincter, the curved fibres in contraction approaching the centre: but as they are fixed at the inner side the skin to which the muscle is attached is drawn toward the nose, and becomes corrugated into folds which converge toward the inner canthus. The corrugator supercilii, is a small triangular muscle placed at the inner end of the eyebrow, arising from the same end of the superciliary ridge: its fibres pass upward and outward to be inserted into the under surface of the orbicularis palpebrarum. It depresses the eyebrow, and, in conjunction with its fellow, throws the integuments into vertical folds as in the act of frowning. The occipitofrontalis' consists of an anterior and posterior carneous expansion united by a broad epicranial,' aponeurosis. The anterior muscle, fig. 28, f, consists of two lateral portions, each connected inferiorly with the integument of the corresponding eyebrow, and slightly overlapped by the orbicularis.' The posterior or occipital portion, ib. o, also consists of a pair, attached inferiorly to the upper curved line of the superoccipital, and to the mastoid. The fibres are parallel and nearly vertical. The action of this muscle is most apparent upon the skin of the forehead and the eyebrows it raises the latter and throws the former into transverse wrinkles. 6 § 202. Locomotion of Mammals. In the movements of the human frame the three kinds of lever are exemplified. Those of the head upon the atlas are on the principle of the first kind, fig. 31, in which the fulcrum F is between the power P and the resistance When the body is raised on tip-toe by the action of the muscles on the heel-bone, fig. 37, k, the action is that of the second kind of lever, in which the resistance (of the tibia on the astragalus), as in fig. 32, w, is between the fulcrum F (afforded by the ball of the hallux), and the power a (tendo achillis). In lifting a weight in the hand by motion of the fore-arm only, fig. 33, the elbow-joint is bent; the power (of the flexors of the fore-arm) being applied (as by the biceps, p) at a, between the fulcrum (elbow-joint) f, and the resistance w or b, according to the third kind of lever exemplified in fig. 34. The mechanism of the pulley is exemplified in the passage of the tendons of the peronei muscles through the groove of the external malleolus of the human ankle-joint, in the tendon of the obturator internus gliding through the groove in the os ischii, in the tendon of the circumflexus palati passing through the hamular process of the sphenoid bone, in the tendon of the obliquus superior gliding through the ring attached to the frontal bone, and in several other instances where a change of the directions of the limbs results from tendons passing over joints, through grooves in bones, or under ligaments, by which the muscles are capable of producing effects on distant organs without disturbing the symmetry of the body, an effect which, owing to the limited power of contraction in the muscles, could be accomplished in no other way. The joints in the mammalian skeleton are chiefly of two kinds, 'ginglymoid' or hinge-joints, and 'enarthrodial' or ball-and-socket joints. In Man the former are less definitely fitted for motion on one plane than in most brutes. The 34 P A B Lever of the third kind. arm and fore-arm move in concentric planes upon the elbow-joint; the knee-joint allows a certain rocking motion of the leg upon the thigh; the ankle-joint has a greater latitude of motion, and the foot may be directed out of the plane of the leg's motion. Atmospheric pressure exercises its influence upon joints. Dr. Arnott estimates the amount of that on the knee-joint at 60 lbs. ; Weber of that on the hip-joint at about 26 lbs.: in the hip-joint of the Megatherium the pressure could not have been less than 150 lbs. A. Swimming. Quadrupeds with inflated lungs are of less specific gravity than water, and swim by alternate extension and flexion of their legs; the effective stroke being the act of extension, when the limb presents a larger area to the water than in flexion: this is seen in the Horse, which strikes the water with the expanded and subconcave surface of the hoof, but draws the convex conical part through the water in the bending of the limb preparatory to the next effective stroke. In the best water dogs the digits are connected by webs, which are stretched in the back or down-stroke, folded in the return movement. The feet of the Otter are broader, especially the hind ones, and more fully palmated. The Seals and Whales have the limbs fashioned as fins. Man, with the chest well expanded, is lighter than water: the presence of mind which counteracts the tendency produced by immersion in a cold and dense medium to expel the air from the lungs is the first safeguard against drowning; and next, if the art of swimming has not been learnt, to keep the head immersed to the mouth and nose, and to refrain from the misdirected struggles of terror which tend only to hasten on the catastrophe. In swimming, the hands and feet are employed so as to present the greatest surface to the water in the effective stroke, the least in |