4 ELEMENTARY PHYSIOLOGY 



The skull rests on the upper extremity of the backbone. It 

 consists in part of a large, hollow, bony case. Continuous with 

 the cavity thus formed is a tube which runs through the backbone. 

 Hence it is common to speak of the human body as enclosing two 

 distinct tubes or cavities : one surrounded by the skull and the 

 backbone, and the other the great cavity already mentioned on 

 page i. And this is true not only of the human body, but also of 

 the bodies of all the back-boned or vertebrate animals. The cavity 

 formed by the backbone contains the great nerve called the 

 spinal cord, a prolongation of the brain. 



The arms and the legs resemble each other very closely in 



the general arrangement of their 

 bones. The upper arm consists of 

 one bone, which corresponds with 

 the bone of the thigh. The fore- 

 arm consists of two bones arranged 

 side by side, as does also the leg. 

 The bones of the wrist and hand, 

 too, much resemble those of the 

 ankle and foot respectively. 



Muscular System. The bones 

 of the skeleton are all surrounded by 

 more or lessyfcsvfc or muscle. Each 

 muscle is composed of bundles of 

 fibres. These fibres have the power 

 of contracting in the direction of 

 their length and so shortening the 

 muscles ; and thus the bones with 

 which the ends of the muscles are 

 connected are brought nearer to- 

 gether. It is in this way that we 

 are enabled to exercise the body 

 in various ways. The muscles, too, assist very largely in giving 

 a rounded form to the body. 



Muscles are of two kinds voluntary and involuntary. The' 

 former are under the control of the will, while the latter are not. 

 We have examples of voluntary muscles in those of the face and 

 the limbs. The muscles of the heart and the stomach are of the 

 involuntary kind. 



Those muscles which move the bones are connected with them 

 by strong white fibrous masses called tendons (fig. 4 ). 



The digestive or alimentary system (Lat. alimentum, nourish- 

 ment) consists of \)I\Q. food-passage and the various organs which pre- 

 pare the digestive fluids. The food-passage consists of the mouth, 

 pharynx, oesophagus or gullet, stomach, and intestines. (See fig. 80.) 



Fig. 2. The Ligaments of the 

 Wrist. 



