157. The muscles may be arranged, in conformity with the general division of the body, into four parts: 1st. Those of the _Head_ and _Neck_. 2d. Those of the _Trunk_. 3d. Those of the _Upper Extremities_. 4th. Those of the _Lower Extremities_. -=-=-=-=-=-=-=-=-=-=-=-= 156. Give a reason why the limbs of youth are rounder than those of the aged. Describe fig. 37. -=-=-=-=-=-=-=-=-=-=-=-= [Illustration: Fig. 38. The superficial layer of muscles on the face and neck. 1, 1, The occipito-frontalis muscle. 2, The orbicularis palpebrarum. 6, The levator labii superioris 7, The levator anguli oris. 8, The zygomaticus minor. 9, The zygomaticus major 10, The masseter. 11, The depressor labii superioris. 13, The orbicularis oris. 15, The depressor anguli oris. 16, The depressor labii inferioris. 18, The sterno-hyoideus. 19, The platysma-myodes. 20, The superior belly of the omo-hyoideus. 21, The sterno-cleido mastoideus. 20, The scalenus medius. 23, The inferior belly of the omo-hyoideus. 24, The trapezius.[5] _Practical Explanation._ The muscle 1, 1, elevates the eyebrows. The muscle 2 closes the eye. The muscle 6 elevates the upper lip. The muscles 7, 8, 9, elevate the angle of the mouth. The muscle 10 brings the teeth together when eating. The muscle 11 depresses the upper lip. The muscle 13 closes the mouth. The muscle 15 depresses the angle of the mouth. The muscle 16 draws down the lower lip. The muscles 18, 19, 20, 23, depress the lower jaw and larynx and elevate the sternum. The muscle 21, when both sides contract, draws the head forward, or elevates the sternum; when only one contracts, the face is turned one side toward the opposite shoulder. The muscles 18, 19, 20, 21, 22, 23, 24, aid in respiration.] [5] In the plates illustrating the muscular system, the names of such muscles are given as are referred to in the paragraph "Practical Explanation." These names need not be committed to memory. If a pupil wishes to acquire a knowledge of the general attachment of the muscles represented in the plates, he can do so by _comparing_ the muscular plate with that of the skeleton, (fig. 28.) _Observation._ When we are sick, and cannot take food, the body is sustained by absorption of the fat. The removal of it into the blood causes the sunken cheek, hollow eye, and prominent appearance of the bones after a severe illness. 158. The number of muscles in the human body is more than five hundred; in general, they form about the skeleton two layers, and are distinguished into superficial and deep-seated muscles. Some of the muscles are voluntary in their motions, or act under the government of the will, as those which move the fingers, limbs, and trunk; while others are involuntary, or act under the impression of their proper stimulants, without the control of the individual, as the heart.