THE VITAL FUNCTIONS. 175 



trate in a net-work of the most astonishing fineness to the remot- 

 est portion of the body, every portion of which, however minute, 

 being endowed by the nerves with the faculty of sensation. In 

 the sheep there are forty pairs of nerves, ten of which proceed 

 from the brain, and thirty from the spinal cord. Each of these has 

 its plexus, or system of branches and net-work. The nerves con- 

 sist of bundles of white filaments or threads. The different senses, 

 sight, smell, hearing, taste, and touch, are supplied by the nerves, 

 which proceed directly from the brain. Another important brain 

 nerve is the par vagum, or pneumo-gastric nerve, which proceeds 

 to the heart, lungs, stomach, liver, and the bronchial or breathing 

 apparatus. It is this nerve which gives the breathing, circulating, 

 and digestive organs their power of motion independent of the 

 will. Any injury or disease of this nerve affects these vital func- 

 tions, and its division or destruction occasions instant death. The 

 spinal nerves convey both sensation and motion. The spinal cord 

 of the sheep weighs one ounce and three-quarters. 



The vital functions of an animal are respiration, circulation, and 

 digestion or nutrition. By the first the system is supplied with 

 oxygen, which is one of the sources of animal heat and is the agent 

 by which the blood is purified. By the second, the tissues or 

 living structure of the animal are supplied with nutriment and 

 relieved from dead and used up matter, and by the last the blood 

 is replenished with fresh supplies of nutritive elements. 



Respiration commences at the moment of birth and continues to 

 the moment of death. It consists of an inspiration or in-drawing 

 and an expiration or out-forcing of air. The motions necessary 

 to these alternate actions are made by the diaphragm, a membrane 

 which divides the chest from the abdomen. When this membrane 

 is relaxed it is convex or rounded towards the chest. When its 

 fibers contract, it is straightened or flattened, and of course en- 

 larges the cavity of the chest and causes a rush of air to fill the 

 enlarged space. This alternate contraction and relaxing of the 

 muscles of the diaphragm cause the motion of the chest, which is 

 observed in breathing. A corresponding movement is also made 

 in the abdomen, as may be noticed in the flanks of hard driven 

 animals. The lungs occupy the largest portion of the cavity of 

 the chest, and enfold the heart. They are two in number, the 

 left and the light ; the left lung is divided into two parts, called 

 lobes ; the right is divided into four lobes, of which one is curved 

 around the heart. Fig. 68 shows this peculiarity, 1, 1, being the 

 right lung ; 2, 2, the left : 3, the trachea or windpipe ; 4, the heart ; 

 5, the carotid arteries ; and 6, the vena cava, or great vein which 



