The term diagnosis, signifies the art of discriminating diseases, to determine their character and situation. It is sufficiently obvious, that a distinctive knowledge of diseases is of great practical importance in reference to their management. Treatment cannot be judiciously applied until a diagnosis has been reached. It may be based on the presence of characteristic signs peculiar to certain affections alone. Thus the crepitant sound denotes the existence ot pneumonia. But there are very few signs which are inseparable from a disease, being found in that and no other. It must be remembered, that all the symptoms typical of a disease will be but rarely present.

In some, possibly many instances, the disorder is readily apparent, and the exact location of it detected. Cough, and rapid breathing would point at once to the chest as the seat of the trouble. In other cases much difficulty will be experienced. Some symptoms may be absent, and others unduly prominent. Patient watching, with a careful analysis and study of each individual case will, however, usually dissipate all doubts.

The previous history is essential in reaching a diagnosis. The duration of the symptoms materially assist in determining whether we have an acute or chronic disease to contend with. If the animal were recently to all appearances in good health, and the attack more or less sudden in its invasion, the disease is probably the former, whereas, if the dog has for a long time shown certain signs of ailing, the disease is probably chronic in character. A highly effective method of reaching the diagnosis of a disease is called, " reasoning by way of exclusion".

In a case of doubt, the problem is generally to decide between a certain number of diseases. The existing disease is one of two, three, or more, which may be suspected. Now if it be difficult to decide which one of these is the disease present from positive proof, it may be practicable to decide that there is insufficient evidence of the existence of one or more, and therefore they are excluded. By this process of elimination, the number ot diseases is diminished, and may be reduced even to one disease. To illustrate the application of this method. -

Our dog does not respond to the usual call. We seek his kennel for the cause, find him unable to leave it, and observe the following symptoms. His breathing is rapid and labored; his manner exceedingly dull; he opens his eyes only to close them at once; hangs his head, it falls as though he slept, to be lifted as he seems to waken, or disturbed from time to time by a dry hacking cough, with an attempt to vomit, occasionally raising a little colored sputa. Nose and body are very hot. He lays down only to assume at once a sitting position, with forelegs braced and separated.

The disease may possibly be in the throat, but is evidently in the chest. The previous history tells us that the symptoms are acute, therefore we can at once eliminate all chronic affections, and there are left laryngitis, bronchitis, asthma, pleurisy, and pneumonia.

An examination of the throat dispels the doubt respecting laryngitis. No knowledge of a previous attack, the absence of wheezing respiration, and husky, barking cough, and asthma is reasonably excluded.

In bronchitis, so early in the disease, we should not expect such marked constitutional symptoms; while some fever would probably be observed, it would scarcely run so high; again, while the breathing is often accelerated, it lacks in the early stages at least, that labored character. Pain in bronchitis is evident when the patient coughs, and is less apparent in the interval. His discomfort would tend to make him restless, and on lying down he would assume no unusual position. These facts considered, render bronchitis improbable.

Uneasiness of the animal is one of the marked symptoms of pleurisy. That indication is absent in the patient before us. The breathing too is different, while in pleurisy it is labored, it is also unmistakably painful, and inspiration is shortened from that cause. A dog affected with that disease, would seem to avoid taking more air into his lungs than absolutely possible. There would be a restraint in the working of the muscles of the chest, that plainly told of pain. This too would be shown in the cough, dry and shortened, with little or nothing raised. While fever is present in pleurisy, it seldom in the early stage, runs as high as observed in this case.

If these differences in symptoms noted are insufficient, an examination of the chest by the ear will remove what doubts remain.

Pleurisy then excluded we come at last, by this method, to the disease of the animal before us - pneumonia.