232 



THE SKELETON. 



\ 



Tuberosity. two are present, they are generally situated one 



above the other, the upper one being the larger ; 1 

 the second piece has seldom more than one ; the 

 third, fourth, and fifth pieces are often formed 

 from two centres placed laterally, the irregular 

 union of which will serve to explain the occasional 



,5, , .*c ., occurrence of the sternal foramen (Fig. 184), or 



% : of the vertical fissure which occasionally intersects 



this part of the bone, and which is further ex- 

 . plained by the manner in which the cartilaginous 

 matrix, in which ossification takes place, is formed 

 (see page 115). Union of the various centres of the 

 gladiolus commences about puberty, from below, 

 and proceeds upward, so that by the age of twenty- 

 five they are all united, and this portion of bone 

 consists of one piece (Fig. 182). The ensiform car- 

 tilage becomes joined to the gladiolus about forty. 

 The manubrium is occasionally, but not invariably, 

 joined to the gladiolus in advanced life by bone. 

 When this union takes place, however, it is gen- 

 rally only superficial, a portion of the centre of the 

 sutural cartilage remaining unossified. 



Articulations. AVith the clavicles and seven 

 costal cartilages on each side. 



Attachment of Muscles. To nine pairs and 

 one single muscle : the Pectoralis major, Sterno- 

 cleido-mastoid, Sterno-hyoid, Sterno-thyroid, Tri- 

 angularis sterni, aponeuroses of the Obliquus ex- 

 ternus, Obliquus internus, Transversalis, Rectus 

 muscles, and Diaphragm. 



The Ribs. 



The Ribs are elastic arches of bone, Avhich 

 form the chief part of the thoracic walls. They 

 are twelve in number on each side ; but this 

 number may be increased by the development 

 of a cervical or lumbar rib, or may be dimin- 

 ished to eleven. The first seven are connected 

 behind Avith the spine and in front with the 

 sternum, through the intervention of the costal 

 cartilages ; they are called true ribs. The 

 remaining five are false ribs ; of these, the first 

 three have their cartilages attached to the 

 cartilage of the rib above : the last two are 

 free at their anterior extremities ; they are 

 termed floating ribs. The ribs vary in their 

 direction, the upper ones being less oblique 

 than those low r er down and occupying the middle 

 of the series. The extent of obliquity reaches its 

 maximum at the ninth rib, and gradually de- 

 creases from that rib to the twelfth. The ribs are 

 situated one below the other in such a manner 

 that spaces are left between them, which are called 

 FIG. 185. A central rib of right side, intercostal spaces. The length of these spaces 



corresponds to the length of the ribs; their 

 breadth is more considerable in front than behind, and between the upper than 



Sir George Humphry states that this is " probably the more complete condition." 



