ARTICULATION OF HEAD WITH SPINAL COLUMN.-RIBS. 475

dentatus (or tooth-like process), occupies the place of the body of the atlas; and by the rotation of the atlas around it, the movements of the head from side to side are accomplished. Wherever great freedom of motion is permitted, displacement or dislocation is necessarily more easy; and accordingly we find that the atlas and axis can be more easily separated from each other, than can any other two vertebræ. This dislocation may be produced by violence of different kinds; thus if the head be suddenly forced forwards while the neck is held back, the tooth of the axis may be caused to press against the spinal cord, and thus to interrupt or completely check its functions. Or, again, if the weight of the body be suspended from the head, and especially if it be thrown upon it with a jerk, the two vertebræ are liable to be dragged asunder, and the spinal cord to be stretched or broken. This is sometimes the immediate cause of death in hanging; and it has not unfrequently occurred when children have been held in the air by the hands applied to the head, a thing often done in play, but of which the extreme danger should prevent its ever being practised. Any serious injury of the spinal cord in this region must be immediately fatal, for the reason formerly stated (§ 470),-that it causes the suspension of the motions of respiration.

633. The number of the ribs which are attached to the bodies and transverse processes of the dorsal vertebræ, is, in the Human species, twelve on each side.1 The number in different animals may be judged-of by that of the dorsal vertebræ in the table already given (§ 627); since it is the attachment of the ribs that makes the essential difference between the dorsal vertebræ and the cervical or lumbar. The other extremity of each rib is connected with a cartilage, which is a sort of continuation of it; in Birds, the cartilages of the ribs are ossified or converted into bone. The cartilages of the first seven ribs (in Man), which are termed the true ribs, are united to the sternum or breast-bone, which forms the front wall of the thorax (fig. 163). The cartilages of the five lower ribs are not directly connected with this, and they are hence called false ribs; those of three of them, however, are

1 It is scarcely necessary here to state, that the common notion respecting the deficiency of a rib on one side of the body of Man is a popular error.

476

RIBS AND STERNUM.-BONES OF SHOULDER.

connected with the cartilage of the seventh rib; while the other two ribs, being completely unattached at the anterior ends, are termed floating ribs. The sternum or breast-bone is flat and of simple form in Man; but it is much larger in many other animals. In those which have need of great strength in the upper limbs, such as Birds, Bats, and Moles, it is not only increased in breadth, but is furnished with a projecting keel or ridge for the attachment of powerful muscles (fig. 250). In the Turtle tribe, on the contrary, it is very much extended on the sides, so as to afford, with the ribs, a complete protection to the contained parts (§ 83).

634. We have next to consider the structure of the members or appendages which are attached to this central framework. These are spoken-of as superior and inferior, when we are treating of Man, whose erect posture places one pair above the other but when the ordinary Quadrupeds are alluded to, they are termed anterior and posterior, one pair being in front of the other. Each member consists of a set of movable bones, which serve as levers; but the socket in which the first of these works, is formed by a bony framework, which is connected more or less closely with the spinal column. This framework, in the upper extremity, consists of the Scapula or blade-bone, and the Clavicle or collar-bone. In the lower extremity, it is formed by a set of bones, the union of which with the sacrum completes the Pelvis or bason at the bottom of the spinal column (fig. 223).

635. The Scapula is a large flat bone, which occupies the upper and external1 part of the back. Its form is somewhat

triangular; and at its upper and outer angle is a broad but shallow cavity, destined to receive the head of the humerus or arm-bone. Above this cavity is a large projection, termed the acromion-process, which is united by ligaments, &c, with the external end of the clavicle, and thus forms the bony eminence that we feel at the top of the shoulder. A little internally to this we find another process, the coracoid, which only serves in Man for the attachment of certain muscles, but which in Birds is developed into a distinct bone (§ 668). The hinder surface of the scapula is divided into two by a projecting ridge or keel, which gives a more extensive and

1 The term external is continually used in Anatomy, to describe the parts furthest removed from the central or median line of the body.

Fig. 226-MUSCLES OF THE BACK OF THE TRUNK, THE SUPERFICIAL LAYER BEING HOWN ON THE LEFT SIDE, THE MIDDLE LAYER ON THE RIGHT.

1. Trapezius; 2, occipital bone; 3, spine of the scapula; 4, acromion process; 5, deltoid; 6, infraspinatus; 7, teres minor; S, teres major; 9, latissimus dorsi; 10, its aponeurosis; 11, glutæ us magnus; 12, space between the latissimus dorsi and external oblique; 13, rhomboideus major; 14, splenius; 15, angularis scapula; 16, rhomboideus minor; 17, external oblique; 18, supraspinatus; 19, serratus magnus; 22, inferior serratus minor; 23, internal ob'ique; 24, crest of the iliac bone; 25, glutæus medius; 26, pyramidalis; 27, gemellus superior; 28, obturator internus; 29, gemellus inferior; 30), quadratus femoris; 31, semi-membranosus; 32, tuberosity of the ischium; 33, insertion of the gluteus maximus; 38, long head of the triceps extensor of the arm.

478 BONES OF SHOULDER: SCAPULA AND CLAVICLE,

firmer attachment to the muscles that arise from it (fig. 226, 3). The scapula is never deficient in animals that possess a superior extremity, though sometimes it is very narrow. The muscles attached to it are chiefly those which draw the arm upwards, and which turn it on its axis. In Man, their actions are very numerous and varied; but in animals that only use their extremities for giving motion to the body, the muscular apparatus is much simpler, and the scapula is narrower (fig. 229, o). This is particularly the case in Birds (fig. 250, 0), the raising of whose wings in flight is an action that requires very little power, though for their depression or pulling-down great muscular force is needed.

636. The Clavicle is a rounded bone, attached at one ex tremity to the acromion-process of the scapula, and at the other to the top of the sternum. Its principal use is to keep the shoulders separate; and we accordingly find it strongest in those animals, the actions of whose superior extremities tend to draw them together; whilst it is comparatively weak or altogether deficient in animals, the actions of whose limbs naturally tend to keep them asunder. In Birds, the violent drawing-down of whose wings in flight would tend to bring the shoulders together if they were not prevented, there not only a strong clavicle, but usually a second bone having a similar function (§ 668). In the Horse and other animals, on the contrary, the bearing of whose weight on their fore legs tends rather to separate the shoulders than to bring them together, the clavicle is deficient.

637. The Scapula is connected with the central framework of the skeleton by various muscles (fig. 226), which pass towards it from the spinal column and ribs, and which serve alike to fix it, and to assist in sustaining the weight which it sometimes has to bear. In Man these are numerous, and their actions are various; since the scapula is left very movable in him, that the actions of the arm may be more free. In Quadrupeds it is generally more fixed; and the trunk is slung from it, as it were, by a muscle (the serratus magnus, 9) of moderate thickness in Man, but in these animals of great strength, which passes from the scapula to be attached to the ribs.

638. The superior or anterior member itself is divided into three principal portions,-the arm, fore-arm, and hand. The arm is supported by a single long and cylindrical bone, which

MUSCLES OF THE FRONT OF THE TRUNK.

479

is called the humerus; this has a large rounded head, which is received into the glenoid cavity of the scapula; whilst its lower end is rather flattened, so as to articulate with the two bones of the fore-arm in the hinge-joint of the elbow. The

Fig. 227.-MUSCLES OF THE FRONT OF THE TRUNK, THE SUPERFICIAL LAYER BEING SHOWN ON THE LEFT SIDE OF THE FIGURE, THE MIDDLE LAYER ON THE RIGHT.

1, sternum; 2, sternal portion of the pectoralis major; 3, clavicular portion of the same muscle; 4, space between the deltoid and pectoral muscles; 5, deltoid; 6, 6, clavicles; 7, external oblique; 8, digitations of the serratus magnus; 9, 9, latissimus dorsi; 10, aponeurosis of the external oblique; 11, linea alba; 12, umbilicus; 13, pectoralis minor; 14, 14, crests of the iliac bone; 15, symphysis of the pubis; 16, crural arches; 17, rectus abdominis; 18, inferior oblique; 20, internal inter costal; 21, external intercostal; 22, infra-clavicular; 23, coracoid process; 24, inferior border of the internal oblique and transversalis muscles; 25, short head of the biceps; 26, long head of the biceps; 27, biceps; 28, sternomastoid; 32, adductor of the thigh; 33, rectus femoris.

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