67. Besides the bones and the teguments, the face contains a number of muscles, which for the most part are small and delicate (fig. XLIV.), together with a considerable portion of adipose matter; while, as has been stated, the face and head together contain all the senses, with the exception of that of touch, which is diffused, more or less, over the entire surface of the body.
Muscles of the face.
68. The second great division of the body, termed the TRUNK, extends from the first bone of the neck to that called the pubis in front, and to the lower end of the coccyx behind (fig. XXXIV. 2). It is subdivided into the thorax, the abdomen, and the pelvis (fig. XLV.).
69. The thorax or chest extends above from the first bone of the neck, by which it is connected with the head, to the diaphragm below, by which it is divided from the abdomen (figs. XLV. and LXI.). It consists partly of muscles and partly of bones; the muscular and the osseous portions being in nearly equal proportions. Both together form the walls of a cavity in which are placed the central organs of circulation and respiration (fig. LX. 2, 5). The chief boundaries of the cavity of the thorax before, behind, and at the sides, are osseous (fig. XLV.); being formed before, by the sternum or breast-bone (fig. XLV. 6); behind, by the spinal column or back bone (fig. XLV. 2, 4); and at the sides, by the ribs (fig. XLV. 7). Below, the boundary is muscular, being formed by the diaphragm (fig. LXI. 2), while above the thorax is so much contracted (fig. XLV.), that there is merely a space left for the passage of certain parts which will be noticed immediately.
70. The figure of the thorax is that of a cone, the apex being above (fig. XLV.), through the aperture of which pass the tubes that lead to the lungs and stomach, and the great blood-vessels that go to and from the heart (fig. LX.). The base of the cone is slanting, and is considerably shorter before than behind, like an oblique section of the cone (fig. XLV.).
71. The osseous portion of the walls of the thorax is formed behind by the spinal column, a range of bones common indeed to all the divisions of the trunk; for it constitutes alike the posterior boundary of the thorax, abdomen, and pelvis (fig. XLV. 2, 4, 6). It is composed of thirty distinct bones, twenty-four of which are separate and moveable on one another, and on this account are called true vertebræ (fig. XLV. 2, 4); the other five, though separate at an early period of life, are subsequently united into a single solid piece, called the sacrum (fig. XLV. 5). The bones composing this solid piece, as they admit of no motion on each other, are called false vertebræ (fig. XLV. 5). To the extremity of the sacrum is attached the last bone of the series, termed the coccyx (fig. XXXV.).
72. From above downwards, that is, from the first bone of the neck to the first bone of the sacrum, the separate bones forming the column progressively increase in size; for this column is the chief support of the weight of the head and trunk, and this weight is progressively augmenting to this point (fig. XLV. 2, 4). From the sacrum to the coccyx, the bones successively diminish in size, until, at the extremity of the coccyx, they come to a point (fig. XXXV.). The spinal column may therefore be said to consist of two pyramids united at their base (fig. XLV. 4, 5). The superior pyramid is equal in length to about one third of the height of the body, and it is this portion of the column only that is moveable.
73. The two surfaces of the spinal column, the anterior and the posterior, present a striking contrast (figs. XXXIV. and XXXV.). The anterior surface, which in its whole extent is rounded and smooth, is broad in the region of the neck, narrow in the region of the back, and again broad in the region of the loins (fig. XLV. 2, 4.). It presents three curvatures (fig. XLV. 2, 4); the convexity of that of the neck being forwards, that of the back backwards, and that of the loins again forwards (fig. XLV. 2, 4).