(From custodiendo; because they surround and keep in the lungs). The ribs. The costs, in anatomy, are generally twelve on each side, sometimes eleven, at others thirteen: their extremities next the vertebrae are rounder and stronger than those which join the sternum; the upper edges are more round than the lower, which are depressed internally for lodging the intercostal vessels and nerves; this channel is not observable at either extremity, which directs us to perform the operation for the empyema rather at the sides of the thorax than near the sternum or spine.

The ribs are articulated at each extremity, of which the posterior is doubly joined to the vertebrae; for the head is received into the cavities of the two bodies of the vertebrae by ginglymi, and the larger tubercle is articulated to the transverse process of the inferior vertebrae by arthrodia; they are thus guarded against luxations.

They are divided into true, called verae, and false, called spuriae, illegitimae, mendosae,nothae. The true are the seven superior, whose cartilages are joined to the sternum; these ribs include the heart and lungs. The false are five inferior, whose cartilages are not joined to the, sternum; unto all these the diaphragm is connected, and within them the stomach, liver, etc. are contained.

The cartilages of the false ribs are only connected to one another by the membrane which covers them; the two last are joined to the vertebrae by a round head, and their cartilaginous extremities are lost in the interstices of the muscles, so that they are more moveable than the other ribs.

The upper rib, contrary to the rest, is flat upwards and downwards, that it may not incommode the lungs, and leave room for the subclavian vessels and muscle. The anterior extremity of each rib is lower than the posterior; therefore, when elevated, the cavity of the thorax will be increased in its diameter backwards and forwards, and the middle part of the superior ridge is lower than the posterior part; thus, when elevated, the diameter of the thorax will be laterally increased.

The ribs are but little used in sleep, respiration being then chiefly carried on by the diaphragm.

Costae, in botany. The fibres of the leaves, or the long strings which run either across or lengthways through them, are called their ribs.