104 THE SKELETON 



attachiiieiit to the costo-clavicular ligament. Between it and the first thoracic rib there was a 

 well-marked intercostal space occupied by intercostal muscles. It received the attachment of tlie 

 S(utleniis miticiis and innlins muscles, and it was crossi'd by the subclavian artery and vein. The 

 nerves of the intercostal space were supplied in pint by the eiglith cervical and tir.^t dorsal, 'i'he 

 artery of the space was derived from the dee]) cervical, which, with the superior intercostal, 

 arose from the root of the vertebral. The head of the first thoracic lib in this specimen articu- 

 lated with the seventh cervical, as well as with the first thoracic vertebra. An interesting fact 

 is also recorded in the careful account of this specimen : — There was no movable twelfth thoracic 

 rib on the same side as this well-developed cervical rib, and the twelfth thoracic vertebra had 

 mammillary and accessory processes, and a strong elongated costal process, and was in linear 

 series with the lumbar transverse processes. 



Grnl)er and Turner from a careful and elaborate study of this question summarise the varia- 

 tions in the cervical rib thus : — It may be very short and possess only a head, neck, and tubercle. 

 A\'lu'n it extends l)eyond the transverse process, its shaft may end freely or join the first thoracic 

 rib : this union may be effected bj' bone, cartilage, or ligament. In very lare instances it may 

 have a costal cartilage and join the manubrium of the sternum. Not unfrequently a jirocess, 

 or eminence, exists on the first thoracic rib at the spot when it articulates with a cervical rib. 



Lumbar ribs are of less significance than cervical ribs and rarely attain a great length. 

 Their jiresence is easilj' accounted for, as they are the diff"erentiated costal elements of the trans- 

 verse processes. They are never so complete as in ceiTical ribs, and articulate only with the 

 tran.sverse processes ; the head never reaches as far as the body of the vertebra, and there is no 

 neck or tubercle. An extra levator casta' muscle is associated with a lumbar rib. 



Not the lea.st interesting variation of a rib is that known as the bicipital rib. This condi- 

 tion is seeti exclusively in connection with the first thoracic rib. The vertebral end consists 

 of two limbs which lie in different transverse planes. These bicipital ribs have been especially 

 studied in whales and man. This abnormality is due to the fusion of two ribs, either of a 

 cervical rib with the shaft of the first thoracic ; or the more common form, the fusion of the first 

 and second true ribs. 



Among unu.sual variations of ribs should be mentioned the replacement of the costal cartilage 

 ami a portion of the rib-shaft by fibrous tissue, a process which occurs as a normal event, 

 during development, in the eighth rib. 



Sometimes the shafts of two or more ribs may become united by small quadrilateral jilates 

 of bone extending across the intercostal spaces. 



The sternum is a thin flat bone situated in the anterior wall of the thorax. In 

 the young subject it consists of six pieces, or sternebrae. Of these, the four middle 

 fuse together to form the gladiolus (mesosternum); the superior remains distinct 

 throughout life as the manubrium (pre-sternum); and the lower segment, also 

 distinct until very advanced life, is the xiphoid (metasternum). The anterior 

 surface of the adult sternum is convex, and gives origin to the pedoralis major nmscle 

 of each side; near its superior angle the sterno-mastoirl muscle arises. This surface 

 is traversed by five lines, indicating the former segmentation of this bone. The 

 posterior surface is concave, and traversed by five lines corresponding to those on 

 the anterior surface. At the upper part it gives origin to the sterno-hyoid and sterno- 

 thyroid muscles. The posterior surface of the lower four segments gives origin to 

 the triangularis .sterni. The xiphoid is usually perforated; a branch of each internal 

 mammary artery traverses the foramen; and on each side of it a portion of the 

 diaphrayn) is attached. Occasionally the xiphoid is bifid. The superior border 

 presents the interclavicular notch, to wliich the fibres of the interclavicular liga- 

 ment are attached; this border terminates at each end in an articular notch for the 

 sternal end of the clavicle. The margins of the notch give attachment to the sterno- 

 cjavicular ligaments. The lateral borders of the sternum present a series of depres- 

 sions, which receive the sternal extremities of the costal cartilages of the first seven 

 ribs, and o(;casionally that of the eighth (see fig. 112). The borders intervening be- 

 tween these depressions or notches are in relation with the internal intercostal muscles. 



In order to appreciate the nature of these notches, it is advantageous to studv 

 the sternum in a young subject. Each typical sternebra presents four angles; each 

 angle presents a demi-facet. Between every two sternebric there is an iiitersterne- 

 bral disc; when two* sternel)ra3 are in position, each notch for a costal cartilage is 

 formed by a sternebra nhovo and below with an intersternebral disc; in the middle, 

 thus repeating the relation of the rib-head to the vertebral centra. Later in life 

 thes(> fuse more or less together, except in the case of the first and second sternebraN 

 which usually remain sei)arate to the end of life. The first (pre-sternum) is the 

 most modified of all the sternebra?. and dift'ers from them in the fact that the costal 

 cartilage of the first rib is continuous with it, and in the fact that it supi>orts the 



