OBJECTS AND METHODS OF STUDY.
Pathology—general—special. Morbid anatomy. Pathological chemistry; Disease. Health. Death—Somatic—partial—necrosis. Syncope. Apuœa. Asphyxia. Coma. Death from old age.
The principles and practice of Veterinary Medicine should embrace all that is known of the causes, nature, symptoms, prevention and cure of disease in domestic animals. Incidentally it includes diagnosis and prognosis.
Pathology is the science which tells of the causes, and nature of disease, and the functional and structural changes by which it is characterized. In modern usage the term pathology is understood to refer to the intimate nature of disease, but this necessarily involves an enquiry into its sources and the predispositions to its occurrence; its phenomena whether in changes of function or structure; and its results in the form of perverted function, structural changes, degenerations, dependent disorder, etc. The field of pathology is further divided into general pathology and special pathology.
General Pathology treats of disease processes in their generic form, and as they appear in many different diseases. Thus inflammation and fever are the prominent phenomena in a great many different diseases which differ in their seats, their causes, manifestations and results. Inflammation and fever are therefore subjects of general pathology. Similarly all forms of degeneration—fatty, fibrous, calcic, amyloid, etc., are disease processes found in many different organs and under very varied conditions and they are accordingly included in general pathology. Hypertrophy and atrophy are also possible in every organic tissue irrespective of kind or seat, they belong therefore to this particular field.
Special Pathology on the contrary is confined to a particular disease and not only elucidates the causes, phenomena and results of such disease, but seeks to do this in such a way as to differentiate this malady from all others however closely related to it. Thus inflammation of a bone is known under the general name of osteitis, this may be due to a great variety of different causes, and each would have its own special pathology. The osteitis of simple mechanical injury is essentially different from the osteitis of rheumatism, of purulent infection, of tuberculosis, of actinomycosis. So with the inflammations of every other tissue. Each may suffer from a variety of inflammations, springing from different causes, attended with characteristically unlike tissue changes and tending to different issues, and every one of these forms has therefore its own special pathology.
General Pathology may be said to deal with typical disease processes to a large extent irrespective of the individual disease in which they may appear, while Special Pathology deals with the morbid phenomena which distinguishes the individual malady from all other diseases however closely allied to it.
Pathological (Morbid) Anatomy deals with structural changes, the cause, the accompaniment or result of disease. These morbid changes are microscopic or macroscopic. Both constitute morbid anatomy, but the microscopic alterations come under the special name of morbid (pathological) histology.
Pathological Chemistry is that branch of pathology which treats of chemical changes produced by disease in the blood, lymph, tissues, secretions and excretions. It demands a previous knowledge of the condition of these tissues and fluids in health, in the particular genus of animal and under the same dietary and environment. Physiological Chemistry is therefore an essential prerequisite to pathological chemistry, just as anatomy, physiology and histology are indispensable to the appreciation of pathology and morbid anatomy.
Disease is an injurious deviation from the normal function or structure. The morbid process resulting in disease is usually in the nature of a modification of the normal or physiological condition, so that it is often difficult to set the exact limits of health and disease. What is a purely physiological process under given conditions, would be distinctly pathological under others. The free kidney secretion of cold weather and the profuse perspiration of a hot season are both purely physiological and in the main balance each other. Each under its special environment fulfills an essential work in eliminating from the system toxic materials which would prove hurtful if retained, and thus each is not only physiological but beneficial. If, however, they occurred, not in this mutually compensatory manner, but simultaneously in this profusion, they might well be dreaded as morbid conditions. Again if either were to occur apart from its normal causative environment, if for example the polyuria appeared in hot weather and the perspiration in cold, the phenomenon might fairly be called pathological. In any case if the excessive secretion induced a lowering of the general tone of health the process would be essentially a morbid one. In pronouncing therefore upon a morbid process one must take fully into account the corresponding physiological process, the attendant conditions, and whether the result is injurious or otherwise.
The same is true of structural changes. What under given conditions would be essentially a morbid structure, might under other conditions be a simple adaptation to an unwonted environment, and a means of protection from injuries that would otherwise accrue. Excessive growth of cuticular tissue in the epithelioma, wart or corn is injurious and essentially pathological, while the callus on the camel’s knee or the workman’s palm is purely protective and physiological. The local development of a mass of fatty tissue in the average man or beast is a disease, but the tendency to the uniform deposition of fat in the connective tissue of the improved breeds of meat producing animals, is the happy culmination of a long continued and skillful selection and regimen, without which the live stock industry of today would be a grievous failure. To constitute disease, therefore, modified function must be permanent, and not simply a compensating increase, decrease, or other change, and it must be in some way injurious to the animal economy. Similarly to constitute disease modified structure must be other than a simple protection or beneficial change, it must not be a simple evolution in the nature of accommodation to the environment but it must be a cause of injury to function or a distinct deformity.
Health may be said to be the harmonious exercise and mutual balance of all the bodily functions, and any interference with such mutual exercise or balance may be said to constitute disease. But as health passes into disease by insensible gradations, there is of necessity an extensive borderland which often cannot be allotted to one condition or the other, but which must often be left a disputed territory.
Again certain animal constitutions are innately strong and robust, while others are weak and feeble, yet the delicacy of the latter cannot be set down as actual disease, and by maintaining a due balance between the functions, a fair measure of health and even long life may be secured.
Death as the result of disease may be either partial or somatic.
Partial or local death may be molecular as in ulceration, or it may affect an organ or part of an organ, as in necrosis, sphacelus, or sloughing. Somatic death is a loss of vitality of the entire body and is manifested by a complete cessation of the bodily functions, including that of nutrition. Usually the arrest begins with one of the great vital processes, in advance of the others, and thus in different cases, we have death beginning at the heart, at the lungs, and at the brain.
Death from syncope or fainting, begins at the heart, which loses its irritability or contractility, or is seized with a tonic spasm. If there has been lack of contractility, the heart is found after death in a flabby, soft condition, and quite frequently filled with blood. If heart-spasm, it is contracted, firm, and empty or nearly so. Syncope may result from severe nervous shock (emotional), from the electric current, from insolation, or from heart sedatives like chloroform, or nicotine. It may, however be but the culmination of a gradually advancing debility, from exhausting diseases, from fatty degeneration of the cardiac muscles, or from starvation, or anæmia. Again the exhaustion coming from profuse hæmorrhage, or from violent over-exertion is a cause of fatal syncope.
In death beginning at the lungs (apnœa, asphyxia, or suffocation), the blood failing to receive oxygen and to give up its carbon dioxide is unable to maintain the various functions of the body and the arrest of the other vital processes speedily follows. The arrest of the respiratory process may occur from nervous shock, but more commonly it results from choking, strangulation, drowning, or the action of irrespirable gases. In diseases of the heart and lungs it is liable to occur from the obstruction of the pulmonary circulation and from the depression of the respiratory nervous centres. After death the lungs are found gorged with dark red—almost black—blood, which likewise distends the right heart and systemic veins, and all mucous and serous membranes have a dark red, congested aspect. When breathing has been arrested by mechanical violence there are, first, active contractions of the respiratory muscles, but no loss of consciousness; then as the brain becomes charged with venous blood, consciousness and volition are lost and convulsive movements ensue. Later still there is no respiratory effort nor convulsions, but the heart continues to beat for two or three minutes longer.
In death beginning at the brain (Coma) the sensory functions fail first, as evidenced by drowsiness, stupor, or complete insensibility, while the movement of heart and lungs are still temporarily continued. Pressure on the brain by a fractured bone or blood clot, or in cases of violent congestion or the rapid growth of tumors, usually operates in this way. It may also result from the direct action of certain poisons, like opium, belladonna, or chloroform, or the ptomaines or toxins of bacteria. Causes acting on the brain may, however, lead to death by syncope or asphyxia when the nerve centres presiding over circulation or respiration are the first to feel the full effects of the pressure or poison.
Death from old age, with a gradual failure of the natural processes of nutrition and tissue-growth, and the occurrence of atrophy and various degenerations of the organs is not a common occurrence in domesticated animals, so that it may be dismissed without further notice.
Actual somatic death is marked by the cessation of breathing and pulse, the dilated pupils and semi-closed eyelids, the coldness and pallor of the visible mucous membranes and skin, and the clenching of the jaws with slight protrusion of the tongue. Yet these symptoms may be present in syncope and it may even be impossible to detect the beats of the heart, though the subject still lives. Pressure of the finger on a white portion of the skin or on a mucous membrane may give a further indication. If the indentation made by the finger is slowly effaced and if the blood again slightly reddens the part the presumption is against death. Even this is not infallible, since by pressure of gas in the internal cavities or deeper blood vessels, the blood may be forced back into the surface capillaries giving an appearance of circulation, after actual death. On the other hand any exudation or œdema will retain the imprint of the finger even in life. The general relaxation of the muscles and their lack of response to electric stimulation, and the setting in of cadaveric rigidity, and later still of putrefaction give more conclusive evidence of dissolution.