22 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



SPECIAL OSTEOLOGY. 



The human skeleton may be separated into three chief divisions: (A) The skeleton of the 

 trunk; (B) the skeleton of the head; (C) the skeleton of the extremities. 



From the standpoint of embryology and evolution the skeleton should be divided into: (i) The axial skeleton, 

 *. e., the vertebral column with its adnexa and the greater part of the base of the skull; (2) the appendicular skeleton, 

 *. e., the skeleton of the extremities; and (3) the membrane and visceral bones, *. e., the flat bones of the cranial vault 

 and the facial bones, and those portions of the skeleton which represent the visceral or branchial skeleton of the lower 

 vertebrates. 



The skeleton of the trunk is formed by the vertebral column and its appendages (the ribs 

 and the sternum), the skeleton of the head is represented by the skull, and the skeleton of the 

 extremities is further subdivided into the skeleton of the upper and that of the lower extremity. 



The Skeleton of the Trunk. 



The principal portion of the skeleton of the trunk is the vertebral column, which is com- 

 posed of a series of parts, the vertebral. A typical vertebra consists of a body and of arches, 

 these latter being subdivided into a posterior or dorsal and an anterior or ventral arch. The 

 posterior or dorsal arches surround the spinal cord, while the anterior or ventral arches, in the 

 form of the ribs, are well developed only in the thoracic portion of the vertebral column and 

 are rudimentary in the remaining vertebrae; they surround the vegetative cylinder of the body, 

 the intestine. While the dorsal arches are firmly united with the bodies of the vertebrae, the 

 ribs are paired bony arches articulating with the thoracic vertebrae behind and anteriorly with 

 a special bone, the breast-bone or sternum. 



The entire series of the vertebrae form the spine or vertebral column, and the thoracic ver- 

 tebrae with the ribs and the sternum form the thorax. The skeleton of the trunk consequently 

 consists of the vertebral column together with the thorax. 



THE VERTEBRAL COLUMN. 

 The True Vertebra. 



In the vertebral column two main subdivisions may be recognized. One subdivision is 

 formed by the true vertebra, the other by the false vertebra?, the former being separate bones 

 connected by ligaments and joints, while the latter are united by bony tissue to form larger 

 bones. The entire human spinal column consists of thirty-two to thirty-five vertebrae; Of these, 

 twenty-four are true vertebrae and eight to eleven are false vertebra:. The true vertebrae mav 

 be separated into three subdivisions: (1) The cervical vertebrae; (2) the thoracic or dorsal 

 vertebrae; and (3) the lumbar vertebrae. There arc seven cervical, twelve thoracic, and five 

 lumbar vertebrae. 



A typical vertebra is composed of: (1) The body; (2) the vertebral arch; and (3) a num- 

 ber of processes. 



