THE SKELETON: 



INCLUDING 



THE BONES AND THE JOINTS. 



Fig. 



133- 



The skeleton forms the framework of the bodv. In the widest sense it includes, 

 besides the bones, certain cartilages and the joints by which the different parts are 

 held together. The skeleton of vertebrates 

 is divided into the axial and the appendic- 

 ular ; the former constitutes the support- 

 ing framework of the trunk and head ; the 

 latter, that of the extremities. 



The Axial Skeleton. — The general 

 plan of the axial skeleton of the trunk is 

 as follows : a rod composed of many bony 

 disks (the vertebral bodies) connected by 

 fibro-cartilage separates two canals, a dorsal 

 and a ventral. In most vertebrates the rod 

 is in the main horizontal, with the dorsal 

 canal above and the ventral below ; but in 

 man the rod is practically vertical, with the 

 dorsal canal behind and the ventral in front. 

 The former is called the neural, because it 

 encloses the central nervous system ; the 

 latter, the visceral. The vertebral cohimn 

 has developed about the primary axis, the 

 notochord. The neural canal is enclosed 

 by a series of separate arches springing one 

 from each vertebra. The skeletal parts of 

 the anterior, or ventral, canal are less nu- 

 merous ; they are the ribs, the costal carti- 

 lages, and the breast bone. Above is the 

 bony framework of the head, or the skull. 

 This also is divided into a dorsal and a ven- 

 tral portion by a bony element which is 

 apparently a continuation of the bodies of 

 the vertebrae, and, indeed, is actually de- 

 veloped, in part, around the front of the 

 notochord. The cephalic axis, however, is 

 bent at an angle with the vertebral bodies, 

 so that the neural arches, which here en- 

 close the brain, are chiefly no longer be- 

 hind but above. Below and in front of 

 the brain-case is the face, which contains 

 the beginning of the digestive tube, of 

 Avhich the jaws and teeth are special organs. 

 In the head we do not find the separation 

 of the parts enclosing the brain into a 

 series of vertebrae, but they are clearly a 

 continuation of the vertebral arches, the 

 posterior, or occipital, division strongly suggesting a vertebra. The face is far more 

 complicated, the vertebral plan being lost. In short, the axial skeleton consists of a 



103 



The tinted portions constitute the axial skeleton ; 

 untinted, the appendicular skeleton. 



the 



