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Basic Physiology Of Horses
Genetic Considerations Of Horses
( Originally Published 1954 )
It is near the end of a long distance race. We are sure that we have picked the winner and he is still in the lead as they come thundering around the last turn into the stretch. Suddenly our horse seems to falter, his tail goes up, and he bears to the outside. The jockey goes to the bat but it only makes matters worse. The other racers go on as our favorite falters and fades. As he crosses the line a poor fifth we hear the murmur in the ranks of the two dollar bettors.
"The race was crooked!" "The jockey is an idiot!" "The horse is a no good dog!" "The trainer knows nothing!" "The horse was doped—poisoned!"
Only the last remark carries any inkling of the real truth. Actually he was poisoned; not by any outside action of humans, but by the waste products of his own great effort.
Before we can understand just what happened in the race we must have some understanding of the physiology of living animals in general, and of the Thoroughbred in particular. In all scientific articles it is customary to append a list of the authors whose researches have been relevant of the subject at hand, but in this article it seems better to include them in the opening paragraphs. I have drawn freely from the standard texts on physiology such as that of Howell, the researches of A. V. Hill, the London physiologist, THE STAYING POWER OF THE RACEHORSE, by Stewart McKay of Australia, and a previous article of mine on the subject of muscular fatigue. The book by McKay is a fascinating one and should be in the library of everyone who wishes to understand the amazing physiologic specialization which is embodied in the modern Thoroughbred.
Let us start with the blood as it comes, dark and waste-laden, through the great veins to the heart. Oxygen is reduced to a low level and carbon dioxide and other wastes are in high proportion. The heart is a two-sided and four-chambered organ of a specialized type of muscle. The rate and amplitude of the heart's contractions are controlled by a delicate nerve center located in the lower parts of the brain. This sensitive area reacts quickly to the accumulation of the by-products of muscle activity, and sends an impulse which starts the heart beating faster and with greater output at each beat.
The blood flows into the first chamber of the heart. This is called the right auricle. It acts as a booster pump to send the blood on to the second and more powerful chamber known as the right ventricle. When this heavier muscle con-tracts it forces the blood out through the arteries which lead to the lungs. When the flow reaches the lung proper the arteries divide and subdivide as do the branches of a tree, until at last the blood is in tiny capillaries which run along the walls of the millions of little air sacs of the lung. When this point is reached the carbon dioxide is exchanged into the fresh air of the lung and oxygen is picked up by the red blood cells. If no more oxygen could be carried by the blood than it will hold in simple chemical solution the amount would be small indeed. But nature has added a special method to increase this carrying power. Each red blood cell is filled with a substance known as hemoglobin. This has the ability to combine with large amounts of oxygen and form a new chemical compound called oxy-hemoglobin. As this is formed the blood loses its dark hue and becomes a brighter red. Thus filled with the chemical compounds needed in the body's work it runs into larger and larger veins until the heart is reached again.
Once more the pump action takes place, first by the left auricle and finally by the left ventricle. This latter is the largest and most powerful of the chambers of the heart and such it must be, for upon it depends all the pressure which sends blood to both the delicate brain above and the flying legs below.
All processes of the body which involve an output of energy may be regarded as chemical processes. The muscles and other tissues act as a furnace in which the organic substances are oxidized (burned) and reduced to simple compounds, such as carbon dioxide and water.
Foods may be classed in two general groups: 1. the carbohydrates (starches) and fats. 2. The proteins. The former substances are changed by the body digestive processes to glucose which is the burnable energy-producing sub-stance; one might call it the gasoline of the animal engine. The proteins also yield some glucose, but their chief function is to furnish the building stones or repair parts of the animal engine. These substances are highly complex organic acids, containing nitrogen compounds and other elements.
Glucose is stored in muscles and liver in a complex form called glycogen. During the exercise of muscles this glycogen is reconverted into glucose and is then burned by the body. Complete oxidation of glucose results in its destruction with the final products carbon dioxide (a gas) and water. Incomplete oxidation may leave the glucose in all intermediary steps, of which the most important is lactic acid.
Dubois-Raymond, an early physiologist, showed that the fatigued muscle is much more acid in its reaction than the resting muscle. This condition is due chiefly to the presence of lactic acid and other by-products. These substances exert a toxic or poisonous action upon the muscle, and de-crease and finally stop its action completely. In order to prove this, a test was made in which the fatigue products of an exhausted muscle were injected into a fresh muscle. This muscle at once showed signs of fatigue, while a control reaction in which an injection of extract from a fresh muscle was used showed no such phenomena. It is only after rest that these poisonous products are finally oxidized or re-moved and the muscle returns to its normal alkaline condition.
We are indebted to Prof. A. V. Hill for important discoveries in this field. He has shown that the body may "run into debt for oxygen," that is, under the stress of violent muscular work more glucose is destroyed than can be completely oxidized by the available oxygen. The process takes on the aspect of a mathematical equation. A molecule of glucose requires a definite amount of oxygen for its complete utilization and elimination. If the lungs are unable to furnish enough oxygen for this increased rate, the glucose cannot be completely oxidized and the intermediary products, especially lactic acid, accumulate and exert their toxic effect, limiting the muscular activity, in spite of the power of the impulse being sent from the nerve centers, which are being driven from both the will of the animal and the urging of outside influences.
When the body has gone into debt for oxygen it is necessary to continue violent breathing, even after the exercise has stopped, so that this insufficiency may be remedied. The poisons are destroyed and removed and the muscle once more takes on its normal resting condition. This explains why a horse blows and sweats for some minutes after a race, and also the reason for the long cooling-out and walking period. The average trainer may not know lactic acid or its chemical reactions, but he has learned by long and hard experience that failure to care for his racer after fatigue will certainly give him a sore or sick horse in the morning.
Now, perhaps, we understand why our horse acted as he did in the race described in the opening of the chapter. He was not a quitter, and no amount of punishment could keep his speed up. But why did he get himself all filled up with poisons while several others in the race were able to go on and finish without great distress? In order to answer that we must take a number of other factors ino consideration.
Since it is the most obvious thing we will, for a moment, consider the way our horse was ridden. We can assume that our jockey was doing his very best as he understood the race, and that failure in this case was not due to bad luck, crowding, or the other factors which upset the calculations of a horse race. An animal can go just so far at his top speed before the acid accumulations caused by his oxygen debt will bring him to a slower gait. The distance at which top speed is possible will vary from horse to horse, but a few furlongs is usually the limit of supreme effort. Please note that we mean supreme speed and not just a very fast pace.
In all races over a longer route the phenomenon of second wind comes into the picture. All athletes know that there is such a thing but few understand the manner in which it is called into play.
Professor Starling considers that the explanation of "second wind" is that after the first stage of exercise there is a rise in carbon dioxide, and after a time, in order to get rid of the substance, there is increased breathing which would last while the carbon dioxide was being eliminated. After its elimination the organism enters into a "steady state" and then the excessive breathlessness disappears and the individual can go on running steadily for a long time, having got his "second wind."
Professor Macleod says: "Since we know that lactic acid is produced by vigorous exercise at such a rate that it accumulates in the blood, but that it does not do so when the oxygen supply of the muscles is commensurate with the rate of production of the acid, it is likely that 'second wind' coincides with readjustment of the chemical processes in the muscles leading to a more thorough elimination of this metabolic product."
The two ideas expressed above really are very similar when considered in the light of our earlier discussion of the production of both carbon dioxide and lactic acid in the working muscle. All of which boils down to the facts known by experienced horsemen, namely, that if the animal sets the pace in a distance race and burns himself out in the first half-mile, he- is rarely able to go the rest of the long route, and the lead is taken by other runners who have saved their supreme effort. If a horse fills himself with poisons far from the finish line there is no way for him to regain his chemical equilibrium in the last few furlongs. The only exceptions to this rule are more apparent than real, and occur when a horse is so superior to his competitors, or so lightly weighted, that he can take the lead from the start and hold it to the wire. Actually he has not put in a supreme effort, because of his superior make-up, until the end of the race.
The next thing to consider is the fitness of the horse at the start of the race. All of this comes under the general heading of training and only a few general principles will be mentioned. There are many ways to train the Thorough-bred, and opinions on this subject form the basis of never-ending argument. It seems obvious that there are many routes to success and that they must vary with the animal trained and the task he is expected to do. Feeding, age, track conditions and other factors all play a part; but the experienced -eye readily detects the fact that a horse is really fit or, to use a common expression, is "off his feed." A horse which has had insufficient work or racing will be soft and unable to keep up the pace. This is because the muscles, the heart, and the blood vessels have not been trained to carry the extra load of speed over a distance. On the other hand, a horse may be "stale." This latter condition has been known for many years, but it is only recently that our re-search physiologists have thrown some light upon the problem.
Professor Crile has shown that after prolonged stimulation from any causes, certain degenerative changes take place in the cells of the brain, the liver, and the adrenal glands. If then a horse is called upon to exert himself to the uttermost it is quite possible that damages will take place of such a degree that he will never recover. From that time on he will never again reach the heights of which he was formerly capable. This is particularly true in high-spirited and willing horses which are raced early and hard.
This is about the place to pull the hornet's nest down firmly over my ears, but as long as we are on the subject of physiology and the efficient use of an animal body we might as well out with it. If, and admittedly it is a big "if," we can eliminate the monetary angle there is no justification for the racing of two-year-olds. Hearts, bones, tendons, and nervous systems are utterly unable to withstand the demands placed upon them.
The tracks of America are strewn with the wreckage of promising young racers which might have risen to great heights if they had been given a chance to mature before their supreme efforts. When the horses line up for the four and a half miles of the Liverpool Grand National we see the ranks filled with veterans of twelve and fourteen years, and a young horse seldom is able to match them for stamina or speed. Jennie Camp was an aged mare when she set her record for the twenty-two miles at the Berlin Olympics. J. A. Estes, in THE BLOOD HORSE, points out that the quality of the English Thoroughbred is not due to any fancied "purity" as determined by the Jersey Act, but rather to the fact that their horses are better tested by the conditions of age, weight, and length of the races in Great Britain.
Having considered general physiology, riding, training, and other factors, we are ready for the last, but far from least point of importance. Was the horse bred to go the route he was asked to do? The fact that nearly any sound Thoroughbred can outrun any other breed of horse is a marvel of genetic development, but easily understood when we know the heights to which this breeding specialization advanced. Starting with the oriental ancestors of our modern horse, step by step there has been an increase in the physiologic efficiericy of the structures which make for greater and still greater speed. When we study the anatomy of the Thoroughbred of a thousand pounds and compare it with the body of a cold-blooded horse of the same size, we find some startling differences.
In the former the heart is much larger. So are the blood vessels. The chin skin, with its many large veins lying just beneath the surface, increase the cooling mechanism and enable him to rid himself of the excess heat which is the result of the combustion of organic substances incident to the production of: energy. If the cold blood tries to race with the Thoroughbred he may succeed for a furlong or two, but by that time his small heart, thick skin, and poorer circulation will cause him to be hopelessly acid-filled, while his blood companions will go flying by, nostrils distended, and ears pricked toward the distant finish line.
Just as there are differences between the breeds of horses, so are there anatomic and physiologic differences between the various strains of the Thoroughbred. Stewart McKay has spent years on this subject, and his conclusions are not to be taken lightly. In his book he discusses the evolution of the modern stayer and shows its relations to the racers of earlier days. In former times a horse had to run four miles, but it was at a much slower pace, and called for what he has termed the "endurance heart," as opposed to the "true staying heart."
McKay really means business when he speaks of a true stayer, which he defines as "a horse which can run two miles either in a handicap carrying weight-for-age, or in a weight for age race, and be able to run the distance in 3:25, or less." It is a pity that we have done so little to encourage the breeding of this type of horse in America. Even though we have few such races, we can profit by studying the facts he has developed by years of analysis of distance runners.
Here is a condensed outline of McKay's basic conclusions: The heart that makes staying power possible is an inheritable characteristic. If that is not present, nothing else matters. A large number of pedigree studies prove that this is usually transmitted by the male. Eighty-five percent of all cases came from a sire of marked staying ability. A sire will not transmit staying powers unless he, himself, possesses it in good measure. If you want to breed true stayers do not use a stallion, even if he has proved himself to be a stayer, if his staying power has been derived from his dam and not from his sire. There is only one reliable test, response to effort; the pedigree suggests: response to effort proves.
The strains running in Australia are not numerous here, so there is but little point in trying to make his conclusions regarding blood lines fit ours except in principle. The sprinter has his important place in our scheme of things, but when we ask a horse to go a long route we should first study his heritage to see if he may reasonably be expected to fulfill our expectations.