36 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 33. — The thorax together with the left shoulder girdle, seen from behind (+). 



Fig. 34. — The thorax together with the left shoulder girdle, seen from in front (i). 



Fig. 35. — The skeleton of the trunk, divided by a medium longitudinal section, together with the 



shoulder and pelvic girdles, seen from the left side (\). 

 Fig. 36. — The skeleton of the trunk, divided by a median longitudinal section, together with the 



shoulder and pelvic girdles, seen from the median line (\). 



develops a broad mass which unites with the sacrum and is to be regarded as a sacral rib (lumbosacral vertebra, see 

 page 30), and the posterior arch of the atlas and the portions representing the arches of the sacral vertebrae sometimes 

 fail to ossify, so that the spinal canal remains open posteriorly (rhachischisis). 



The ribs frequently fork near the costochondral articulations, the two portions so formed usually uniting again, 

 so that a fenestration of the rib is produced. 



Foramina are not rare in the sternum and one is frequently found in the xiphoid process. At the upper end of the 

 manubrium, at the sides of the interclavicular notch, two small bones which are termed cpistcrnal bones occasionally 

 occur. 



THE SKELETON OF THE HEAD, 



The sum total of the bones of the head is designated as the skull or cranium, and this por- 

 tion of the skeleton differs from the others in that all of its constituents, with the exception of 

 the lower jaw, are firmly united even in the macerated condition (the exact nature of the union 

 is described under "Syndesmology," page 107), so that special means are required to separate 

 the individual bones from each other, and such a separation is not usually successful if the indi- 

 vidual is too old. A skull the bones of which have been isolated, is known as a disarticulated 

 skull. 



The completely formed adult skull is an extremely complicated structure, some of the 

 individual parts being united in such a manner that it is quite difficult to recognize them. Some 

 bones, indeed, are scarcely visible in the perfect skull, owing to the fact that they are to a great 

 extent covered or overlapped by the other cranial bones. Before describing the individual 

 cranial bones it will be advantageous to consider briefly the skull as a whole, in order to obtain 

 an idea of the topography of the individual cranial bones and of their chief component parts. 

 The skull will therefore be studied first from in front, then from the side, from below (without 

 the inferior maxilla), and from above, looking downward upon the great cranial cavity which 

 encloses the brain, and finally the outer and inner aspects of the cranial vault will be considered. 



THE ANTERIOR ASPECT OF THE SKULL. 

 If the anterior aspect of the skull (Figs. 37 and 38) be examined, it will be seen that the 

 bony forehead (jrons) is formed by the vertical portion of the frontal bone, and that toward the 

 vertex a slightly serrated suture, the corona/ .suture, separates the vertical plate of the frontal 

 from the two parietal bones. The frontal bone also forms the upper margin of the orbit, and 

 at the outer margin of the orbit it is separated from the contiguous zygomatic or malar bone by 

 a suture, the zygomatico- frontal suture. The process of the frontal bone articulating with the 

 zygomatic bone in this situation is known as the external angular or zygomatic process. To 

 either side of the frontal bone will be observed the anterior inferior or sphenoidal angle of the 



