42 HUMAN PHYSIOLOGY. 



The Vertebral Column. In all static and dynamic states of the body the 

 vertebral column plays a most essential role. Situated in the middle of the 

 back of the trunk, it forms the foundation of the entire skeleton. It is com- 

 posed of a series of superimposed bones, termed vertebrae, which increase in 

 size from above downward as far as the brim of the pelvic cavity. Supe- 

 riorly, it supports the skull; laterally, it affords attachment for the ribs, which 

 in turn support the weight of the upper extremities; below, it rests upon the 

 pelvic bones, which transmit the weight of the body to the inferior extremities. 

 The bodies of the vertebrae are united one to another by tough elastic discs 

 of nbro-cartilage, which, collectively, constitute about one-quarter of the 

 length of the vertebral column. The vertebrae are held together by liga- 

 ments situated on the anterior and posterior surfaces of their bodies, and by 

 short, elastic ligaments between the neural arches and processes. These 

 structures combine to render the vertebral column elastic and flexible, and 

 enable it to resist and diminish the force of shocks communicated to it. 



The amphiarthrodial character of the intervertebral joint endows the entire 

 column with certain forms of movement which are necessary to the perform- 

 ance of many body activities. While the range of movement between any 

 two vertebrae is slight, the sum total of movement of the entire series of 

 vertebrae is considerable. In different regions of the column the character, 

 as well as the range of movement, varies in accordance with the form of the 

 vertebrae and the inclination of their articular processes. In the cervical and 

 lumbar regions extension and flexion are freely permitted, though the former 

 is greater in the cervical, the latter in the lumbar region, especially between 

 the fourth and fifth vertebrae. Lateral flexion takes place in all portions of 

 the column, but is particularly marked in the cervical region. A rotatory 

 movement of the column as a whole takes place through an angle of about 

 twenty-eight degrees. This is most evident in the lower cervical and dorsal 

 regions. 



The skeleton may, therefore be regarded as a highly developed frame- 

 work, which determines not only the form of the body, and affords support 

 and protection to the various softer organs and tissues, but also, through the 

 mobility of its joints, permits of a great variety of complicated movements. 



GENERAL PHYSIOLOGY OF MUSCLE TISSUE. 



The muscle tissue, which closely invests the bones of the body, and 

 which is familiar to all as the flesh of animals, is the immediate cause of the 

 active movements of the body. This tissue is grouped in masses of varying 

 size and shape, which are technically known as muscles. The majority 

 of the muscles of the body are connected with the bones of the skeleton in 



