SECTION II. Diseases of the Heart

DISEASES OF THE HEART

The heart is an organ whose duty, throughout the two classes considered in this study, remains entirely identic, purely a physical one in driving the blood through the corresponding vascular system. The physics involved naturally differs between mammals and birds, but energy is derived from the automatic power lodged in the cardiac musculature. Whether this be neuromyogenic, as seems to be the case in all mammals, or purely myogenic, as is probably the case for the birds in which MacKenzie and Robertson say there is no atrioventricular bundle, the result is the same, since in both classes there is some continuity of muscle fibres from auricle to ventricle. The gross anatomy varies little if any more than the physiology, albeit there is proportionately greater auricular capacity in the mammals than in the birds, and indeed there are differences within the classes which cannot now be readily explained; certain minor variations of valvular arrangement exist, such as the absence of the membranous light tricuspid in Aves.

When, however, one considers the cardiac power available for various animals, the subject becomes one of greater breadth and complexity, for no consistency obtains even within families, since the demand for cardiac strength will vary more with habits than with zoological relationships. Thus for example the domestic rabbit has a small heart volume while the wild hare has a great one. Although, of course, the size of an organ may not be an absolute measure of its efficiency (a flea’s leg muscle has relatively greater power than a man’s), yet size is the only physical gauge one has for estimating nature’s preparation for expected demand. Perhaps this will be shown later when after discussing the pathological anatomy of the heart in the wild mammals and birds, we can study these changes in light of statistics upon the relative size of the heart.

EXPRESSION OF CARDIAC DISEASE.

The diseases of this organ are known only by their physical effects, chiefly by causing physical or functional defects in other organs and to a minor degree by purely physiological irregularities in the heart itself (tachycardia, arrhythmia). All the latter and most of the former are subjects discovered by observation during life and unfortunately cannot be included in the study at hand. Both states are well known to the veterinarian who diagnoses them with reasonable ease in animals that can be handled; I saw one case of arrhythmia in a monkey for which no adequate gross morbid explanation was found postmortem. Some of these functional abnormalities are certainly caused by myocardial disease and cardiac failure has occurred among many orders. An interesting observation was made by Plimmer on several large birds (ostriches, storks, cassowaries) which apparently died from this condition; at autopsy he found myocardial degeneration, or epicardial edema or only a flabby heart. Lack of exercise was held responsible by this observer. Such cardiac deaths have probably been encountered at this Garden but we have accounted them to shock, or gastrointestinal disease; this matter will be discussed on a later page.

CORONARY ARTERY DISEASE.

If a degenerative sclerosis of coronary vessels be the cause of angina pectoris then perhaps paroxysms of this kind occur, for we have seen such anatomical changes in the heart of three widely separated varieties of animals, a Nylghaie, a Hamadryas Baboon, a Macaque, and a Brown Pelican. The history of these animals does not register anything resembling the clinical picture of angina pectoris in man, and they did not come to their death from the arterial changes in the heart alone since sufficient other pathology also existed.

KINDS OF PATHOLOGICAL CHANGE.

As an introduction to the strict pathology of the heart it might be well to outline the headings of the scheme upon which it seems desirable to study the subject. It is hardly profitable to take up seriatim the ordinary general pathological processes as discussed in systems of pathology for it is our purpose to show the distribution of basic aberrations from the normal in terms of zoological position. To this end one must consider the response of the heart (a) to damaging influences and (b) to a demand for increased work. In the first group come degenerations and inflammations, upon which may succeed an incompetency in the form of dilatation. The response of the normal heart to any physical demand greater than customary has usually been thought to lie in the direction of hypertrophy, but on occasion it has seemed to be in the form of dilatation, especially if the strain has been sudden and severe. Starling thinks that the primary and normal reaction of the heart to physical strain is always dilatation. The idea of hypertrophy must not be confused with an understanding of the relatively large hearts in animals whose habits demand great cardiac power, for then it is their norm and might be called “physiological cardiac hyperplasia.” I shall use the terms “increased muscle bulk” and “increased chamber space” as preferable to hypertrophy and dilatation; this also focuses attention upon the two features of an enlarged heart.

DEGENERATIONS AND INFLAMMATIONS.

Degenerative changes in disease are recorded in our system as amyloid, hyaline, fatty metamorphosis, granular and cloudy degeneration. While there is perhaps between some of these conditions and true myocarditis a matter only of degree, the records have been analyzed as filed and perhaps some lessons can be learned from the responses of the various zoological orders. In the accompanying Table 1 will be found the distribution of degenerative and inflammatory lesions through the zoological orders. The percentages speak for themselves but deserve as well some consideration from the standpoint of normal heart value; this will be taken up later after the other lesions have been discussed.

TABLE 1. Table Showing Incidence, in the Orders, of Degenerations and Inflammations, or in Other Words the Response to Infectious and Toxic Agencies. ════════════╤════════╤═══════╤════════╤════════╤════════╤════════╤══════════ Heart of │Degener-│Myocar-│Pericar-│Endocar-│Total│ Cases │Percentage │ ations │ ditis │ ditis │ ditis │ │followed│in deaths │ │ │ │ │ │ by │per order │ │ │ │ │ │dilation│ ────────────┼────────┼───────┼────────┼────────┼────────┼────────┼────────── Primates │ 8│ 12│ 18│ │ 38│ │ 7.6 Lemures │ 1│ │ │ │ 1│ │ 1.2 Carnivora │ 19│ 14│ 11│ 10│ 54│ 5│ 11. Pinnipedia │ 2│ 1│ │ │ 3│ │ 15. Rodentia │ 5│ 7│ 6│ │ 16│ │ 8. Insectivora │ │ │ │ │ │ │ Chiroptera │ │ │ │ │ │ │ Proboscidea │ │ 1│ │ │ 1│ │ 33. Hyracoidea │ │ │ │ │ │ │ Ungulata │ 3│ 9│ 8│ 2│ 20│ │ 5.4 Edentata │ 2│ 3│ 1│ │ 7│ │ 44. Marsupialia │ 12│ 5│ 6│ 12│ 33│ │ 19. Monotremata │ │ │ │ │ │ │ │ │ │ │ │ │ │ Passeres │ 4│ 4│ 9│ 2│ 19│ │ 1.4 Picariæ │ 2│ 1│ 3│ │ │ 6│ 6.7 Striges │ │ │ 1│ │ 1│ │ .8 Psittaci │ 10│ 7│ 7│ │ 24│ │ 3.4 Accipitres │ 2│ 19│ 8│ 7│ 36│ 1│ 18.3 Columbæ │ 2│ 1│ │ │ 3│ │ 2. Pterocletes │ │ │ │ │ │ │ Galli │ 4│ 10│ 13│ 1│ 28│ 1│ 9.3 Hemipodii │ │ │ │ │ │ │ Fulicariæ │ 2│ │ 1│ │ 3│ │ 8.6 Alectorides │ │ 1│ 1│ │ 2│ │ 5.4 Limicolæ │ │ │ │ │ │ │ Gaviæ │ 1│ 1│ │ │ 2│ │ 10. Pygopodes │ │ │ │ │ │ │ Impennes │ │ 1│ │ │ 1│ │ 16. Steganopodes│ │ 1│ 2│ 1│ 4│ │ 20. Tubinares │ │ │ │ │ │ │ Herodiones │ 1│ │ 1│ 2│ 4│ │ 4.1 Odontoglossæ│ │ │ │ │ │ │ Palamedes │ │ │ │ │ │ │ Anseres │ 16│ 17│ 11│ 6│ 50│ 1│ 15.7 Struthiones │ 2│ 3│ 5│ 1│ 11│ │ 34. Crypturi │ │ │ │ │ │ │ ────────────┼────────┼───────┼────────┼────────┼────────┼────────┼────────── Total │ 98│ 118│ 112│ 44│ 367│ 8│ ════════════╧════════╧═══════╧════════╧════════╧════════╧════════╧══════════

In this and subsequent tables, figures in italics are for small groups of animals coming to autopsy, usually less than one hundred, and from which percentages may be misleading. The number of autopsies upon such groups may be found by consulting the list given on page 47.

ENDOCARDITIS, MYOCARDITIS.

Romberg said in his classical work on the heart that there is always some form of myocardial disease with endocarditis. The 44 cases of valvular disease detected at this Garden are 15 of chronic nature, 29 of acute or subacute character. In the former, the chronic, 9 showed some grade of muscular involvement, while 21 of the 29 acute cases of valvular inflammation were accompanied by myocardial damage; the percentages are 60 for chronic and 72 for acute, a relation that would be expected if one credit the theory that many valvular inflammations start at the root of the valves, but, at all events, they indicate that after the acute stages have passed the myocardial damage may be repaired.

FIG. 1.—VEGETATIVE AND ULCERATIVE ENDOCARDITIS OF AORTIC VALVE. OSTRICH (STRUTHIO AUSTRALIS). ORGANISMS ISOLATED CORRESPONDED CLOSELY TO BAC. AFANASIEFFICHESTER. ]

The large number of cases of endocarditis among the Carnivora, Accipitres, Anseres and Marsupialia is noteworthy and can hardly be explained by other argument than a special vulnerability of this organ in these groups. However, the unusual number of cases in our only native marsupial, the opossum, seems worthy of a special note since ten of the twelve instances in the order Marsupialia affected this particular animal. When seen these ten cases were acute in five instances, subacute in three and chronic in two. The Streptococcus pyogenes was isolated in three of the five acute cases; bacteriology of the others was negative or not done. All of the acute and one of the subacute cases were combined with some evidence of general septicemia. The type of lesion was in no way peculiar, unless the facts that all were vegetative when acute and markedly deforming when chronic, be noteworthy. In two chronic aortic cases the valvular orifice was almost closed, yet the left ventricle could not be considered as greatly hypertrophied and no dilatation existed. In one acute aortic and mitral case, general dilatation existed. The mitral was involved nine times, three times alone, four times with the aortic, once each with the tricuspid and pulmonary. Once the vegetations were limited to the mural endocardium. Nine of these animals came from one enclosure over a period of two and a half years, during which time other opossums died from similar bacterial infections (pneumonia) despite repeated cleansing of the place. No unusual number of cases of this or similar kinds occurred elsewhere in the Garden at this time, but it would seem that we had in this cage a continued bacterial infection.

UNUSUAL PERICARDIAL CHANGES.

The appearance of a stiff gelatinous exudate in the pericardial sac has attracted our attention on ten occasions (8 birds, 2 mammals). The substance seems quite homogeneous and almost entirely acellular. One specimen became solid on heating and another became turbid when put into Kaiserling’s fluid. In two cases bacterial cultures were made; nothing grew. It has not been associated with tuberculosis or tumors nor has any one pathological lesion more than another appeared to accompany it.

A peculiar lesion occasionally seen in birds is “uratic pericarditis” a process not infectious at all, according to Plimmer, but due to renal disease. It has been seen here in association with retention of urates in the kidney, with gout of birds, and apparently quite independent of any renal or constitutional disease. Both layers of the sac are pearl gray or irregularly salted with a whitish granular material so that they are entirely opaque; occasionally the distribution is spotty. The deposit does not seem to penetrate the myocardium. There is at times some involvement of other serosæ, but this is usually much less marked than around the heart. It does not seem that this of itself should be fatal, but it has been the most decided pathological factor in some of the autopsies.

Aside from pericarditic exudates, twenty-one instances of pericardial effusion have been encountered. They offer little that is peculiar in etiology, chemistry or cytology, but as there has been some question of the position of the accumulation of the fluid in human beings, it might be well to note the position in our material. It is recognized in veterinary medicine that the cardiac dullness is increased especially to the right, and that most of the fluid will be on that side and posteriorly. At our autopsies on mammals this is the position usually occupied by the fluid, the apex being covered by pericardium, unless the quantity be great enough to make the sac taut, and this position is retained whether the animal be laid upon the one side or the other; nor does the fluid all leave the base of the heart when the body is placed prone. The crown of the heart is nearly always well covered. In birds, on the other hand, the fluid occupies the apical part of the sac, probably due to the fact that this membrane is attached by its tip to the transverse air sac wall which takes the place of a diaphragm, so that the tip of the heart is always free and the pericardium of the base fairly closely applied to the epicardium. This holds good even for the birds whose cardiac apex is normally attached to the pericardium by a fibrous band.

HYPERTROPHY AND DILATATION.

The response of the heart to a continued demand upon its working capacity will, as already indicated, lead to increased muscle bulk or to larger chamber size. Whether hypertrophy be purely the building of a bigger engine or be accompanied by, or due to, muscular disease as had been suggested by certain authors, was a question to which an answer was hoped, but it would seem that the solution is no nearer than can be obtained in human pathology. Fifteen of the 34 cases showed some degree of myocardial damage. Some of the other cases may have been instances of so-called essential hypertrophy, enlargements due to hard work or to low grade hidden infection. Aubertin ascribes such cardiac muscle increase to overwork under the stimulus of intoxication from intestinal sources or from irregular constitutional functions. For information concerning this and pathological enlargements one may consult the Table (2) of Hypertrophies and Dilatations; in advance the method of charting must be known. Since it is usually impossible to decide what may be the single important factor in the cardiac disease, all of the accredited factors have been listed with the hope that the resulting figures would be significant. Thus an animal may have recorded pericarditis, nephritis and arteriosclerosis—who shall say which was primary or most potent in the cardiac change.

Essential hypertrophy is limited to those cases for which there was no concomitant pathology that might have been responsible for the overgrowth. There was one in a carnivore (fox) and one in a raptatory bird (buzzard). Idiopathic dilatations on the other hand are much more common, but they still bear a relation to the apparent vulnerability of the heart. Their distribution is as follows: Primates 1, Carnivora 1, Pinnipedia 1, Ungulata 1, Marsupialia 4, Anseres 1. These may be cases such as Plimmer described, of cardiac failure, indicated by dilatation, the result of inactivity. Besides these special instances and the ones accounted for in the list, there were three acute dilatations apparently due to shock, two ungulates and one marsupial, probably incidental to fright when being caught by the keepers.

The association of secondary dilatation with hypertrophy is only evident in three instances. One case and perhaps the most interesting, is that in which the principal antecedent pathology was thyroid hyperplasia and nephritis; the dilatation was perhaps agonal or shortly before the last struggles. It would seem that all of the dilatations occurred shortly before death because long standing passive congestions and dropsies of cardiac origin are exceedingly rare; only one certain case is recorded (carnivore).

Let us now examine the Table (2) according to orders and then as to causation. The Primates’ heart is apparently well able to increase in size in response to increased work, a demand most often made by pulmonary, pleural and pericardial diseases. Two of these cases occurred in animals suffering with pulmonary tuberculosis sufficiently extensive to impede cardiac action while in another case the tuberculous lesion was mild but a pericarditis existed. When the right hand columns are inspected it would seem that on occasion dilatation may occur; one of the tuberculous pulmonary cases had a dilated heart. The slothful lemurs apparently have no call upon their cardiac mechanism.

Carnivora with their large organ, which, it would seem, should be prepared for excess work either simply as a reserve or as an inherent ability to grow, present in about equal numbers, hypertrophy and dilatation. It is admitted that there are within this order, genera of differing habits, but analysis of the canidæ, felidæ and ursidæ for examples, in the first place, offer too few specimens for conclusions and, secondly, have upon trial actually shown nothing definite, so that we are forced to use the larger group, the order. It is interesting to note that long continued infection is in this order the most potent factor in enlargements of the heart. Four of the ten cases show myocarditis. Nephritis does not seem very important in relation to cardiac muscular increase, but occurs with great frequency in association with dilatation. Three of the cases of hypertrophy were associated with thyroid disease and two of these showed dilatation as well. The general causes of chamber distention are more diverse, and we see associations that do not appear with hypertrophy, namely arteriosclerosis and diseases of the chest.

TABLE 2.

Table Showing Incidence per Order of Hypertrophy and Dilatation, and the Principal Associated Lesions Believed to Have Etiological Importance.

═════════════╤═════════════════════════════════════════════════ Heart of │ Hypertrophy ─────────────┼─────┬────────┬────────┬────────────────┬──────── „ │Total│Percent.│Valvular│Arteriosclerosis│Diseases │ │ for │Disease │ │ of │ │ Order │ │ │Thoracic │ │ │ │ │ Serosæ ─────────────┼─────┼────────┼────────┼────────────────┼──────── Primates │ 4│ .8│ │ 1│ 3 Lemures │ │ │ │ │ Carnivora │ 10│ 2.1│ 1│ │ Pinnipedia │ │ │ │ │ Rodentia │ │ │ │ │ Insectivora │ │ │ │ │ Chiroptera │ │ │ │ │ Proboscidea │ │ │ │ │ Hyracoidea │ │ │ │ │ Ungulata │ 4│ 1.2│ │ │ 1 Edentata │ │ │ │ │ Marsupialia │ 1│ .6│ 1│ │ Monotremata │ │ │ │ │ Total Mammals│ 19│ │ 2│ 1│ 4 │ │ │ │ │ Passeres │ │ │ │ │ Picariæ │ 1│ 1.1│ │ 1│ Striges │ │ │ │ │ Psittaci │ │ │ │ │ Accipitres │ 8│ 4.1│ │ 4│ 1 Columbæ │ │ │ │ │ Pterocletes │ │ │ │ │ Galli │ 2│ .7│ │ 1│ Hemipodii │ │ │ │ │ Fulicariæ │ │ │ │ │ Limicolæ │ │ │ │ │ Gaviæ │ │ │ │ │ Pygopodes │ │ │ │ │ Impennes │ │ │ │ │ Steganopodes │ │ │ │ │ Tubinares │ │ │ │ │ Herodiones │ │ │ │ │ Odonotoglossæ│ │ │ │ │ Palamedes │ │ │ │ │ Anseres │ 2│ .6│ │ │ Struthiones │ 2│ 6.2│ │ │ 2 Crypturi │ │ │ │ │ Total Birds │ 15│ │ 0│ 6│ 3 ─────────────┼─────┼────────┼────────┼────────────────┼──────── Total │ 34│ │ 2│ 7│ 7 ─────────────┴─────┴────────┴────────┴────────────────┴────────

═════════════╤════════════════════════════════════════════════════════ Heart of │ Hypertrophy ─────────────┼─────────┬───────┬─────────┬─────────┬───────┬────────── „ │Pulmonary│ Renal │ Chronic │ Acute │Thyroid│Myocardial │ Disease │Disease│Infection│Infection│Disease│ Disease │ │ │ │ │ │ │ │ │ │ │ │ ─────────────┼─────────┼───────┼─────────┼─────────┼───────┼────────── Primates │ 3│ 1│ 1│ 1│ │ Lemures │ │ │ │ │ │ Carnivora │ │ 2│ 6│ 1│ 3│ 4 Pinnipedia │ │ │ │ │ │ Rodentia │ │ │ │ │ │ Insectivora │ │ │ │ │ │ Chiroptera │ │ │ │ │ │ Proboscidea │ │ │ │ │ │ Hyracoidea │ │ │ │ │ │ Ungulata │ │ 4│ │ │ │ 2 Edentata │ │ │ │ │ │ Marsupialia │ │ 1│ │ │ │ 1 Monotremata │ │ │ │ │ │ Total Mammals│ 3│ 8│ 7│ 2│ 3│ 7 │ │ │ │ │ │ Passeres │ │ │ │ │ │ Picariæ │ │ 1│ │ │ │ Striges │ │ │ │ │ │ Psittaci │ │ │ │ │ │ Accipitres │ │ 3│ 1│ 1│ │ 5 Columbæ │ │ │ │ │ │ Pterocletes │ │ │ │ │ │ Galli │ │ 1│ │ 1│ │ 1 Hemipodii │ │ │ │ │ │ Fulicariæ │ │ │ │ │ │ Limicolæ │ │ │ │ │ │ Gaviæ │ │ │ │ │ │ Pygopodes │ │ │ │ │ │ Impennes │ │ │ │ │ │ Steganopodes │ │ │ │ │ │ Tubinares │ │ │ │ │ │ Herodiones │ │ │ │ │ │ Odonotoglossæ│ │ │ │ │ │ Palamedes │ │ │ │ │ │ Anseres │ │ │ 1│ 1│ │ 1 Struthiones │ │ │ 1│ 1│ │ 1 Crypturi │ │ │ │ │ │ Total Birds │ 0│ 5│ 3│ 4│ 0│ 8 ─────────────┼─────────┼───────┼─────────┼─────────┼───────┼────────── Total │ 3│ 13│ 10│ 6│ 3│ 15 ─────────────┴─────────┴───────┴─────────┴─────────┴───────┴──────────

═════════════╤═════╤═══════════════════════════════════════════ Heart of │ │ Dilatation ─────────────┼─────┼────────┬────────┬────────────────┬──────── „ │Total│Percent.│Valvular│Arteriosclerosis│Diseases │ │ for │Disease │ │ of │ │ Order │ │ │Thoracic │ │ │ │ │ Serosæ ─────────────┼─────┼────────┼────────┼────────────────┼──────── Primates │ 4│ .8│ │ 1│ Lemures │ │ │ │ │ Carnivora │ 11│ 2.2│ │ 1│ 2 Pinnipedia │ │ │ │ │ Rodentia │ 8│ 4.2│ │ │ Insectivora │ │ │ │ │ Chiroptera │ │ │ │ │ Proboscidea │ │ │ │ │ Hyracoidea │ │ │ │ │ Ungulata │ 11│ 3.│ │ │ 4 Edentata │ 2│ 12.5│ │ │ Marsupialia │ 8│ 4.5│ │ │ Monotremata │ │ │ │ │ Total Mammals│ 44│ │ 0│ 2│ 7 │ │ │ │ │ Passeres │ 1│ │ │ │ Picariæ │ │ │ │ │ Striges │ │ │ │ │ Psittaci │ 1│ .1│ │ │ Accipitres │ 1│ .5│ │ │ 1 Columbæ │ │ │ │ │ Pterocletes │ │ │ │ │ Galli │ 2│ .7│ │ 1│ 1 Hemipodii │ │ │ │ │ Fulicariæ │ │ │ │ │ Limicolæ │ │ │ │ │ Gaviæ │ │ │ │ │ Pygopodes │ │ │ │ │ Impennes │ │ │ │ │ Steganopodes │ │ │ │ │ Tubinares │ │ │ │ │ Herodiones │ │ │ │ │ Odonotoglossæ│ │ │ │ │ Palamedes │ │ │ │ │ Anseres │ 5│ 1.5│ │ │ 1 Struthiones │ │ │ │ │ Crypturi │ │ │ │ │ Total Birds │ 10│ │ 0│ 1│ 3 ─────────────┼─────┼────────┼────────┼────────────────┼──────── Total │ 54│ │ 0│ 3│ 10 ─────────────┴─────┴────────┴────────┴────────────────┴────────

═════════════╤════════════════════════════════════════════════════════ Heart of │ Dilatation ─────────────┼─────────┬───────┬─────────┬─────────┬───────┬────────── „ │Pulmonary│ Renal │ Chronic │ Acute │Thyroid│Myocardial │ Disease │Disease│Infection│Infection│Disease│ Disease │ │ │ │ │ │ │ │ │ │ │ │ ─────────────┼─────────┼───────┼─────────┼─────────┼───────┼────────── Primates │ 1│ │ │ 1│ │ 1 Lemures │ │ │ │ │ │ Carnivora │ 2│ 4│ 1│ 2│ 3│ 2 Pinnipedia │ │ │ │ │ │ Rodentia │ 3│ 2│ 2│ 4│ │ Insectivora │ │ │ │ │ │ Chiroptera │ │ │ │ │ │ Proboscidea │ │ │ │ │ │ Hyracoidea │ │ │ │ │ │ Ungulata │ 1│ 2│ 2│ 2│ │ 1 Edentata │ 1│ │ 1│ │ │ 1 Marsupialia │ 1│ 3│ 2│ 2│ │ 1 Monotremata │ │ │ │ │ │ Total Mammals│ 8│ 12│ 7│ 11│ 3│ 10 │ │ │ │ │ │ Passeres │ │ │ │ 1│ │ Picariæ │ │ │ │ │ │ Striges │ │ │ │ │ │ Psittaci │ 1│ │ │ │ │ Accipitres │ │ │ │ 1│ │ 1 Columbæ │ │ │ │ │ │ Pterocletes │ │ │ │ │ │ Galli │ │ │ │ 1│ │ 1 Hemipodii │ │ │ │ │ │ Fulicariæ │ │ │ │ │ │ Limicolæ │ │ │ │ │ │ Gaviæ │ │ │ │ │ │ Pygopodes │ │ │ │ │ │ Impennes │ │ │ │ │ │ Steganopodes │ │ │ │ │ │ Tubinares │ │ │ │ │ │ Herodiones │ │ │ │ │ │ Odonotoglossæ│ │ │ │ │ │ Palamedes │ │ │ │ │ │ Anseres │ │ 2│ 1│ 2│ │ 1 Struthiones │ │ │ │ │ │ Crypturi │ │ │ │ │ │ Total Birds │ 1│ 2│ 1│ 5│ 0│ 3 ─────────────┼─────────┼───────┼─────────┼─────────┼───────┼────────── Total │ 9│ 14│ 8│ 16│ 3│ 13 ─────────────┴─────────┴───────┴─────────┴─────────┴───────┴────────── For meaning of italics see foot note Table 1.

The rodents seem to have no power to increase muscle bulk, but a sufficient number of cases of dilatation occur to make one conclude that this is their method of response to unusual strain. Pulmonary disease, mostly of infectious nature, and myocardial degenerations are the principal causes.

The next order to show cardiac enlargement is the Ungulata where nephritis is the most frequent association with hypertrophy and disease of the pleura and pericardium with dilatation, or the reverse of the factor value in the Carnivora. These animals, fairly well prepared for flight, with moderately large hearts, seem more often to show dilatation than hypertrophy.

Two Edentata (armadillo) showed dilatation but no hypertrophy.

Marsupials behave somewhat like rodents in that the heart does not seem to increase muscle bulk, but our records do not explain this clearly. As already mentioned four cases had no sufficient internal reason for dilatation, but as one was probably the result of shock three only remain to be accounted for. Nephritis seemed to exist in all three, but two of them had kangaroo mycosis of the jaw and a general chronic infection.

If now our attention be given to the Aves we find the highly specialized Passeres and Striges not represented and their closely related well- organized orders Picariæ and Psittaci with only an isolated single case. This is the more interesting since the last order suffers reasonably often with arteriosclerosis. Accipitres, the birds of pugnacious habit and carnivorous diet, seem well able to increase their muscle upon demand, but do not often suffer dilatation. Vascular and renal diseases stand out most prominently in the etiology, and one-half of them show myocardial change. The Galli, which includes both ground and flying birds, are represented but fail to exhibit any unusual accompanying disease. Anserine birds apparently have a low power to increase the size of the heart, but most often allow it to dilate. Struthiones, large stalking and rapidly travelling birds, apparently have a good margin of safety in their cardiac mechanism.

SUMMARY OF LESIONS ASSOCIATED WITH HYPERTROPHY AND DILATATION.

Analysis of the associated pathology will reveal that among the mammals, renal disease, chronic infections and diseases of the thoracic serosa are most often responsible for hypertrophy, and that something over one- third of the hearts showed myocardial damage. Among the Aves arteriosclerosis and renal disease are most important in enlarging the heart; half of the cases had myocarditis. In so far as dilatation in mammals is concerned, renal disease and acute infections are decidedly more important than other influences, even than the next in order— chronic infections and pulmonary diseases; only one-fifth of the cases had myocardial disease. Acute infectious disease is the most potent cause of dilatation in birds; only two of the eight cases had degeneration of the heart muscle.

COMPARISON OF MAMMALIA AND AVES.

If a comparison of the incidence of increased muscle bulk in the two classes be made it will be found to occur two and one-half times more often in mammals, while dilatation occurs nearly ten times more often among the mammals than among the birds. Hypertrophy is accompanied by myocardial change in 44 per cent. of the cases, whereas muscular degeneration was only seen in 24 per cent. of the dilatations; this change is conspicuously lacking in the Primates, Ungulates and Marsupials. The usual teaching has been that dilatation, which means enlargement of chambers and thinning of walls or at least no thickening thereof, implied an inability on the part of the heart to keep up with increased demand—a decompensation. If Starling be correct that dilatation is not a degeneration of pump value but merely one of adaptations to increased demand, then this method is more characteristic of mammals than of birds. There is, however, the reserve power to increase the muscle bulk inherent in the mammalian, not possessed or needed by the avian heart. The large-hearted class Aves certainly dilate their blood pump less frequently than mammals and indeed have less cardiac disease.

An analysis of the incidence of hypertrophy versus dilatation shows that hypertrophying power resides in the Primates, Accipitres and Struthiones, their hearts relatively seldom dilatating. Lack of such power and consequent dilatation resides in Rodentia, Ungulata, Marsupialia and Anseres. Hypertrophying power lies therefore chiefly in the heart of average size for its class, dilatation occurring in the small heart. (See page 63.)

AVIAN HYPERTROPHY.

There is little to be learned from the nature and anatomy of the hypertrophies and dilatations except perhaps their character among the birds, in which the physics of the circulation is somewhat peculiar. In this class both the hypertrophy and distention are predominatingly left- sided, a state probably explained by the pressure against which the pump must work in flight because then the lungs and the viscera are somewhat compressed by the pressure of an excess of air in the pneumatic sacs. At all events while concentric hypertrophy was mentioned once, it is difficult to estimate the degree of increase in the right chambers because they are not uncommonly well filled when diastole occurs at death. Grober asserts that the normally large heart (or what I have called “physiological hyperplasia”) shows a “hypertrophy” of the right ventricle because of the extra work entailed in flying. This is certainly not the case in the material we have seen under pathological conditions. Right sided increase might be expected if pulmonary or serous membrane affections were prominent, but left-sided increase, following arteriosclerosis and nephritis is the actual finding. The best examples of concentric hypertrophy are in the dogs with thyroid disease and the best examples of concentric dilatation in ungulates suffering shock.

SUMMARY.

The foregoing pathological data can now be summarized by grouping the facts under the headings of absolute and relative vulnerability of the heart. By the former is meant the actual number and quality of lesions in the various orders, but here at once one comes upon the irregularity of examples of zoological and pathological character, and if one trust entirely to the percentages, fallacious conclusions might be reached. Basing judgment upon the incidence of pathological lesions in mammals and birds, it is evident that the former has greater vulnerability, as 13 is to 6.2. This is noteworthy as we shall learn that the bird has a larger and apparently better prepared heart than the mammal. Attempts to discover the order or kind of animal having the greatest or lowest vulnerability are difficult for the reason given above. Thus, for instance, Pinnipedia, Proboscidea, Edentata, Gaviæ, Impennes, Steganopodes, and Struthiones present the highest percentages of cardiac lesions, but the total specimens examined are so few that these figures may well be misleading. (See Tables 1 and 2.) If, however, figures mean anything in such small groups, these are the animals which have the greatest cardiac vulnerability. They have little in common in regard to zoological relationships and habits; four of the seven orders are rather slothful and three are active. It is much better to limit our observations to those orders from which sufficient examples have been subjected to autopsy and upon which we have some standards for comparison in the heart-body weight ratio. It so happens that in the above seven orders I was unable to obtain any reliable figures of heart weight. Table 3 is a combination of data from Tables 1 and 2 for the principal orders from which we have enough material (at least one hundred autopsies) and for which it is possible to obtain as comparative standards figures indicating the weight of the normal heart in kilograms of body weight; Table 4 gives these ratios for normal hearts. The information about the weights was obtained from some of our own figures and the references given in the footnote. There are no extensive data upon weights and measures in exact terms, such as body weight, so that we are limited to the numbers quoted in parentheses besides the orders in the table. The ratios might be modified slightly by a greater number of examples, but they show certain things by comparison of the classes; in a rough manner the heart ratios correspond to the pathology.

TABLE 3. Table Containing a Condensation of the Two Foregoing Tables and Showing Figures for Degenerations, Hypertrophy and Dilatations for Orders Having the Largest Number of Autopsies. ═════════════════╤═════════════════╤═════════════════╤═════════════════ │ Degenerations, │ Hypertrophy │ Dilatation │ &c. │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── Primates │ 7.6│ .8│ .8 Carnivora │ 11.│ 2.1│ 2.2 Rodentia │ 8.│ 0.│ 4.2 Ungulata │ 5.4│ 1.2│ 3. Marsupialia │ 19.│ .6│ 5. │ │ │ Passeres │ 1.4│ 0.│ 0. Picariæ │ 6.7│ 1.1│ 0. Striges │ .8│ 0.│ 0. Psittaci │ 3.4│ 0.│ .1 Accipitres │ 18.3│ 4.3│ .5 Columbæ │ 2.│ 0.│ 0. Galli │ 9.3│ .7│ .7 Herodiones │ 4.1│ 0.│ 0. Anseres │ 15.7│ .6│ 1.5 ─────────────────┴─────────────────┴─────────────────┴─────────────────

TABLE 4. Table Showing Weight of Normal Heart in Relation to Body Weight. Number of Specimens used to Determine Weight Quoted in Parenthesis. ════════════════╤═══════════════════════════════ Average Heart of│Grams per Kilogram of Body. ────────────────┼─────────────────────────────── Man ( 4)│ 5.67 Primates ( 4)│ 6.56 Carnivora ( 6)│ 6.78 Rodentia ( 5)│ 5. Ungulata (10)│ 5.8 Marsupialia ( 3)│ 5.1 │ Average 5.82 Passeres (43)│ 19.8 Picariæ ( 9)│ 21.3 Striges ( 4)│ 7.33 Psittaci ( 6)│ 8.89 Accipitres ( 7)│ 12.32 Columbæ ( 4)│ 14.47 Galli (16)│ 11.08 Fulicariæ ( 3)│ 23.82 Limicolæ ( 2)│ 8.78 Anseres (14)│ 11.8 Struthiones ( 1)│ 12.7 │ Average 13.84 ────────────────┴─────────────────────────────── For meaning of italics see foot note Table 1.

However, there are many reasons why great caution should be used in evaluating the relative size of the heart. Welcher showed in his work that the proportion is greater in small and young animals than in large and adult ones. All the authors quoted agree that in birds and to less degree but still clearly in mammals, there is a direct relationship between the bodily activity of an animal and its cardiac bulk. This is fairly well shown in the list of avian heart ratios, but not so clearly in the mammals. What shall be considered the most active mammals—the monkey, perhaps, with his tendency to be occupied constantly, yet we find the greatest heart bulk among the Carnivora, animals prepared for travel and struggle, and the smallest among the Rodentia, quiet and timid animals. The avian order showing the greatest cardiac ratio, the Fulicariæ, shore birds, is made up of some quiet hiding varieties, and of some capable of very prolonged flight; the most constantly active fliers (Passeres) also have a high cardiac weight proportion. The inactive owls have the smallest heart bulk.

The contrast between the average heart-to-body weights of mammals and birds is striking, the latter having two and one-half times as much as the former, 5.8 vs. 13.8. Since this is the most prominent and best supported statement in the table of weights, it may be used to compare with the incidence of the pathology as seen in the two classes.

Degenerations and inflammations occur in mammals and birds as 9.5 is to 5.5.

Hypertrophies occur in mammals and birds as 10.3 is to 4.3.

Dilatations occur in mammals and birds as 2.4 is to .28.

In other words, mammals are much more susceptible than birds to degenerative and inflammatory processes, show an ability to increase the muscle bulk two and a half times as great and are liable to chamber distention nearly ten times as often. It might also be put that birds cannot or do not need to increase their muscle, and that the chamber and muscle balance is more perfectly arranged.

While in the preceding pages hypertrophy has been discussed rather from the standpoint of its value as a compensating and reserve capacity, and dilatation as a degenerative or decompensatory process on the part of the cardiac mechanism, it may be that dilatation of the mammalian heart is the usual method employed by the class in response to increased demand. It seems certain, however, that the originally and normally larger heart, both mammalian and avian, more often uses an increase of its muscle to this purpose.

Hypertrophy was accompanied by myocardial disease in 44 per cent. of the cases, while dilatation showed this change in only 24 per cent. This supports the theory that dilatation is a normal response of the myocardium under strain and the belief held in many quarters that the muscle increases its bulk because some of it is damaged.

The differences between classes are not so conspicuous between orders. However, the large heart of the carnivores increases both its muscle and chambers, while the small heart of the rodents and marsupials more often dilates. Analysis of the avian orders is inconclusive and somewhat contradictory. Let it suffice to say that the birds which fly most, with exception of the ducks, have a relatively low vulnerability, and the soaring carnivorous Accipitres and the largest birds, Struthiones, apparently have a high susceptibility to damaging influences and enlarge their muscle bulk in response to increased work.

Aneurysms of the heart are quite rare; only two have been seen. They were both located at the apex of the left ventricle in birds; they did not rupture. Myocardial damage is evident in both cases but the cause is not clear; parasites could not be demonstrated. Plimmer reports a case of cardiac aneurysm at the apex from infestation of the heart muscle by sarcosporidia.