DISEASES OF DO^nESTIC ANIMALS. & 



that the animal gradually grows. In the adult they are just about 

 equal, provided there is a certain amount of exercise. In old age the 

 waste is in excess, and the animal becomes smaller. In disease, some- 

 thing takes place, equilibrium is upset, and there is a change of function 

 or structure. We often see this taking place. An animal is attacked by 

 disease and reduced quite quickly — the result of this poison in the 

 system. Life is maintained in the body by the circulation of pure or 

 arterial blood through the system. This blood is the fluid of the body ; 

 whenever arterial blood ceases to circulate, death is the result ; or, if it 

 circulates, but becomes impure, there follows a similar result. If the 

 blood becomes changed but slightly, it produces disease very quickly ; 

 and if changed to any great extent, it produces death very quickly. Life 

 is an aggregate of the functions which resist death, and is maintained 

 by the blood. Death is the cessation of all the functions, the aggregate 

 of which constitutes life. Now death may occur in different ways, and 

 sometimes it is immaterial how an animal died, but at other times we 

 are called to tell how he died. If no blood circulates, death takes place 

 from syncope, from a want of a due supply of blood to the heart, and 

 the heart loses its power. Bleeding causes death from syncope, or from 

 necra^mia. The heart loses its power from want of its natural stimulus 

 — the blood Death from syncope may occur in other ways : the heart 

 may lose its contractile power from a blow over the heart or stomach, or 

 from poisons, or from fright, or from derangement of the nervous system. 

 Asphyxia, or Apncea ; access of air to the lungs is prevented ; as in 

 drowning, hanging, choking, and sometimes from tetanus ; again, from 

 coma. Death from coma begins in the brain — frequentlj' from medicines.* 

 The symptoms are drowsiness or comatose condition. 



BLOOD. 



Blood is the great and important fluid of the body, and is carried 

 through the system by means of a set of vessels ; they are arteries, 

 capillaries and veins The heart is the great centre of the circula- 

 tion. It is situated in the thoracic cavity, and acts as a force pump 

 to send the blood through the system ; but there is a power in the 

 vessels of drawing blood to them to a certain extent, somewhat as the 

 sap is drawn up in the tree. The arteries are vessels that convey 

 blood from the heart to various parts of the body. They are so named 

 from the former supposition that they contained only air, as they were 

 always found empty after death. They differ, also, in structure from 

 the vein-, and do not collapse as the veins do. They possess con- 

 tractility and elasticity, and their power of contraction is due to 

 muscular tissue in their walls, which consists of contractile flbre, cells, 

 which have the poorer of diminishing the calibre of the artery in which 

 they are situated, and can either arrest partially or completely the flow 

 of blood. The large vessels are especially elastic ; they have both 

 muscular and ^-ellow elastic tissue. They convey the blood to all the 

 tissues of the body, and when it has fulfilled its function, it is brought 

 back to the heart by the veins. 



Veins are the vessels which bring the blood to the heart. They have 

 thinner walls than the arteries, and have valves, which valves are fold- 

 ings of the inner lining of the vein. These valves do not exist in the 

 pulmonary vein. They tend to help the blood towards the heart. 

 When the veins are empty they collapse. Between the arteries and 



