Part 43

The prevalence of heart disease in animals may be deduced from the fact that out of 150 horses, oxen and dogs dissected at Montfauçon by Leblanc in 1840, not less than one twentieth presented cardiac lesions. The supposition of an immunity of the lower animals has been largely due to the heavy muscular shoulder of quadrupeds which covers the upper and anterior regions of the heart shutting them out from physical exploration. In man the entire heart and connecting blood vessels are so open to examination that the physician can pronounce with the greatest accuracy not only concerning the existence of disease, but also its precise locality and nature. In the quadruped no such facility is open to us, and veterinarians have too generally refused to face the difficulty, preferring to ignore heart diseases, or still worse seeking to cover their ignorance by the assertion that such affections rarely exist. Now however we not only know that heart diseases are much more frequent in the lower animals than heretofore believed, but that as a general rule they are sufficiently manifested and recognizable by their distinctive symptoms.

=Position and exposure of the heart.= In the horse the heart has only its apex and a small portion of its left ventricle approached to the surface of the chest, at a point where it is felt to beat behind the left elbow. The apex approaches the surface in the interval between the fifth and sixth ribs and close above the breast bone. The posterior border of the ventricle follows a nearly vertical line upwards from this point, while the anterior border has a direction upward and forward crossing diagonally over the fifth rib. The part of the ventricle exposed extends about three inches upwards from the apex, and is about two inches in its transverse diameter. The great mass of the organ is covered by lung substance.

In the =ox= about the same extent of heart tissue is exposed. In =sheep= a portion about an inch in height and one and a half inches in breadth is left uncovered by lung. In the =pig= the heart is exposed only in a triangular space of about an inch across.

In the =carnivora= the heart lies more directly in the median line of the chest. It appears as if tilted forward so that its apex is directed backward and its base forward, while the body of the organ lies directly over the breast bone. The lungs invest it on both sides preventing any approximation to the walls of the chest laterally, and it can best be auscultated by applying the ear over the sternum.

In =birds= the heart is situated in the centre of the chest and enveloped by lung tissue so that its exploration is about equally difficult at all points.

The larger blood vessels at their origin from the heart are not open to examination in the lower animals except to a limited extent in the dog.

=Internal arrangement and structure of the heart.= In all warm blooded animals the heart is composed of two portions, the internal cavities of which are perfectly distinct from each other and contain blood in different conditions; the right portion holding the impure, purple or venous blood which has just circulated through the body, and the left portion being filled with the bright crimson or arterial blood, which has been ærated by circulating through the lungs. Each of these portions is divided into two distinct cavities, an upper (auricle) which receives the blood from the veins, and a lower (ventricle) which receives the blood from the auricle and transmits it into the arteries. The auricle is separated from the ventricle by a transverse musculo-membranous partition having a large central orifice furnished with valves (auriculo-ventricular), the free borders of which are turned downward so that they allow the blood to flow freely downward from the auricle but completely close the orifice and prevent any reflex when the ventricle contracts. The great artery which originates from the base of each ventricle is likewise furnished with a system of valves (semilunar) having their free borders turned into the artery, so that they allow blood to flow freely into that vessel during the contraction of the ventricle, but prevent any reflux into the heart when the ventricle again dilates. The apparatus may be likened to a force pump with two systems of valves, one to prevent the return of any water from the pump into the fountain; the other to hinder any reflux from the delivery pipe into the pump. Any interference with either of these valves entails a very serious and usually a fatal disorder of function.

These orifices differ considerably in size. Those between the auricles and ventricles are considerably larger than those at the commencements of the great arteries. Those on the right side of the heart too are greater than those on the left. They vary with the form of the heart. Thus in dilatation of an auricle and ventricle on one side of the heart, the auriculo-ventricular opening becomes equally widened and the valves remaining disproportionately small the blood is allowed to rush back into the auricle during ventricular contraction. The left auriculo-ventricular opening has been known to become contracted in some very flat and shallow chests; the blood failing to circulate freely through the lungs and to reach the left side of the heart in a full supply this orifice accommodates its size to the amount, and may become so narrowed that it forms a serious obstacle to the blood flow and a series of morbid changes result following the backward course of the circulation. The auricle first becomes overdistended and its muscular walls increase in thickness and consistency; the lungs tend next to suffer from a passive congestion, and lastly the right side of the heart becomes engorged and enlarged.

Any obstruction in the aorta which conveys the blood from the left side of the heart equally leads to dilatation of its internal cavity and abnormal thickness of its walls.

The imperfection of the valves is one of the most serious results of such changes in heart structure. The sounds by which such imperfection may be recognized will be presently noticed, meanwhile the mode of testing this in the heart of the dead animal will be referred to. If due to structural changes in the valves themselves, the new deposits, the cicatrices, the lacerations, etc., will be visible to the eye. Though no such disease-changes are seen the valves may still manifest imperfection by failing to fulfill their normal function when put to the test. Water is poured into one or other of the great arteries which arise from the ventricles, the vessel being held vertically, and if it fails to descend into the heart the valvular action is perfect. The auriculo-ventricular valves may be equally tested by filling the ventricle and observing whether there is a reflux into the auricle.

The =thickness of the walls of the heart= varies in disease. The auricular walls are invariably thin and flaccid except as above noted with diminution of the auriculo-ventricular orifice. The walls of the right and left ventricles differ in thickness in accordance with the distance to which they have respectively to propel the blood and the propulsive effort demanded. Thus the walls of the right ventricle which is only called upon to propel the blood through the lungs are only about ½ an inch in thickness and are thinnest at their lower part. Those of the left ventricle which have to send the blood to the most distant parts of the body are from 1 to 1½ inches except at the lower part where they form the apex of the heart, and are reduced to a tenuity resembling the walls of the auricles. They are thickest at the median part, and diminish slightly in an upward or downward direction. The bulk of these walls is excessively muscular, the fibres arranged as an elaborate double spiral and connected with a layer of white fibrous tissue placed in the interval between the auricles and ventricles and surrounding the auriculo-ventricular openings and the orifices by which the great arteries take their origin. It is at this point, where the muscular fibres of the ventricles are connected with the white fibrous rings, where rupture of the heart usually takes place.

The following measurements may be held to refer to medium sized animals of the different kinds mentioned.

─────────┬──────────────┬───────────────────────────────┬────────────── │ Longitudinal │ │ │ Diameter. │Transverse Diameter at the Base│Circumference. │ Inches. │ of the Ventricles. │ Inches. ─────────┼──────────────┼────────────────┬──────────────┼────────────── │ │Antero-Posterior│ Transverse │ │ │ Inches. │ Inches. │ ─────────┼──────────────┼────────────────┼──────────────┼────────────── Horse │ 10 │ 7½│ 5⅓│ 19½ Ox │ 9⅘│ 6⅘│ 4½│ 17½ Sheep │ 4 │ 3 │ 2 │ 7⅓ Pig │ 4½│ 3⅗│ 2⅓│ 9⅘ Dog │ 3⅘│ 3⅗│ 2 │ 8½ ─────────┴──────────────┴────────────────┴──────────────┴──────────────

The =internal capacity of the ventricles= is so modified by the amount of post mortem contraction that it differs widely from the actual capacity during life. The left ventricle of the larger domestic quadrupeds usually admits from 3½ oz. to over 5 oz., while the right ventricle whose walls are so much thinner and more lax will contain double that amount. In the smaller animals about a tenth of these quantities will be admitted.

=The weight of the heart= too can only be stated as an average or for medium sized animals. In the horse it may be from 4½ lbs. to 9 lbs.; in the ox from 3 lbs. 5 oz. to 4½ lbs.; in the sheep from 5½ oz. to 7 oz.; in the pig from 9½ oz. to 14 oz.; and in the dog from 5 oz. to 7 oz. This statement must be understood to apply to dogs approximating in size to the shepherd’s.

Taking into account the size of the particular animal any considerable deviation from these measurements and weights may be accepted as abnormal. The ratio to the body weight is about:—horse and dog 1:100, ox, sheep and pig 1:220. This necessarily varies with condition—fat or lean.

The =pulse= offers valuable indications in disease of the heart.

The number of the pulse in healthy full-grown animals may be set down as follows per minute:—horse, 36 to 46; ox, 38 to 42 (with loaded paunch or in a hot stable up to 70); sheep, goat and pig 70 to 80; dog 80 to 100; cat 120 to 140; goose 110; pigeon 136; chicken 140. In old age the pulse is less frequent. This diminution may extend to 5 beats per minute in the larger quadrupeds and to 20 or even 30 in the smaller. Youth and small size again are associated with a greater rapidity of the pulse. The pulse of the foal, at birth, is about three times that of the horse; in the colt of six months it is double; at a year old about one and a half times; and at two years old one and a quarter.

The smaller the animal, caeteris paribus, the more rapid is the pulse. Hot buildings, exertion, fear or any other exciting cause likewise accelerates it. It is more frequent with the nervous temperament, as for example in the English race horse, or the greyhound, than in the dull lymphatic cart horse or mastiff. In advanced pregnancy it is increased in number. In the cow and mare it undergoes a monthly increase of four or five beats per minute after the sixth month. (Delafond)

Independently of these conditions a rapid pulse indicates febrile excitement attendant on active inflammatory or other disease, or a state of weakness and debility. In this last condition the heart beats more frequently to secure a more rapid circulation in the capillary blood vessels, and thus make up to the craving tissues by frequency of contact, what is wanting in the quantity and quality of the nutritive fluid. This point cannot be too much insisted upon, as the fatal doctrine that a rapid pulse indicates force of the circulation is very misleading as to treatment.

The =force and character of the pulse= differ in the various species. In the =horse= it is full, moderately tense and elastic. In the =ass= and =mule= it is smaller and harder, with an inequality of force in successive beats, and sometimes even a beat is suppressed or imperceptible. In the =ox= the pulse is full, soft and regular, appearing to roll forward beneath the fingers. In the =sheep= and =goat= the pulse is small but with a peculiar quick or sharp beat. The =pig’s= pulse is said to be firm and hard. That of the =dog= and =cat= is firm and hard coming with a sharp impulse against the finger. In the dog, however, successive beats are not always of the same force and an intermission or complete absence of a beat is by no means an indication of disease of the heart or other serious malady. It often attends the slightest excitement in a perfectly healthy animal.

In disease the pulsations may become:—=frequent= or increased in number; =slow= or decreased in number; =quick= or striking with a sharp impulse against the finger; =tardy= or without sharpness of stroke and as if they rolled slowly past under the finger; =full= and =strong= when the impulse is forcible and not easily compressed by the finger; =weak=, =feeble= or =indistinct= in the opposite conditions; =small= when though perfectly distinct and forcible they are wanting in fulness; =hard=, when forcible and jarring (this is sometimes called =wiry= or, if smaller, =thready=); =soft= when though the artery may be full the beat is devoid of hardness and easily compressible so as to be unfelt; =oppressed= when with a full rounded artery, the impulse is jerking though not hard and as if the distended vessels opposed the transmission of the impulse; =jerking and receding=—=leaping=, when with empty and flaccid arteries the pulse seems to leap forward with each beat of the heart—(this pulsation may be visible to the eye in the carotids); =intermittent= when after a number of beats at regular intervals there is a complete pause extending over that period of time which would have been occupied by a full beat; =unequal= when some beats are strong and others weak; =irregular= when without any distinct intermission for a period equal to that of a single beat, the intervals between successive beats are of varying length. The pulse further has a peculiar =thrill= or =tremor= in states of great debility with deficiency of blood and imperfect filling of the vessels.

Of these the =leaping=, the =intermittent=, the =unequal= and the =irregular= pulses are of special importance in their bearing on heart diseases.

The =jerking and receding= pulse is felt in cases of imperfection of the semilunar valves at the commencement of the great aorta, and which allows blood propelled into the arteries by the contraction of the ventricle to flow back into the ventricle during its state of relaxation. This pulse is met with in other conditions as in aneurism of the aorta, but if from heart disease it is distinguished by the presence of a blowing murmur with the second sound of the heart.

The =intermittent= pulse indicates functional derangement of the heart but it does not as is generally believed betoken structural disease. It is frequently observed in healthy asses and mules, and in dogs however slightly excited whether by fear or joy, or by the mere fact of their being handled, it is so common as to be almost the rule rather than the exception. It may be seen in a healthy horse as the result of excitement. During the early stages of convalescence from inflammatory affections of the lungs in the horse the pulse is often intermittent. The pulsations are at the same time unequal. There is a regular cycle of beats gradually decreasing in force and extending over a complete respiratory act. The cycle commences with the strongest beat during or immediately after the act of expiration, and the succeeding four or five beats are less and less forcible until the chest is fully expanded when there is a quiescent interval corresponding to the period of one beat. In many such cases there is no other indication of heart disease and the phenomenon appears due to the interference with the circulation by the hepatized lung, to the impaired nervous energy of the heart and to its compression between the distended lungs. A pulse simply intermittent and not associated with any further sign of heart disease does not then possess the significance generally attributed to it, but a careful examination of the heart should invariably be made when this functional disorder is observed. It exists or may be brought about by slight excitement in the great majority of heart diseases.

In case of intermittent pulse it is useful to ascertain whether there is also an intermission of the heart’s beat, since in softening of the heart, that organ may beat without being able to transmit the impulse along the artery.

A pulse at once =unequal and irregular= is a much more serious indication than a merely intermittent pulse. It is observed especially in fatty degeneration of the muscular substance, and with imperfection of the valves on the left side of the organ, though it may be present in other cardiac diseases independent of the existence of those lesions.

In hypertrophy of the left ventricle, the pulse is full and strong and the impulse appears prolonged, because of the greater length of time taken up by the ventricle in the act of contraction. When dilatation coexists with hypertrophy the impulse is still full and strong, more blood being transmitted through the vessel; but when dilatation is combined with attenuation of the ventricular walls the impulse is soft and weak by reason of the feebleness of the contractions.