found in the primary classes. With advancing education the percentage steadily increases, till the largest number with defective sight are found among the graduates; also that the smallest percentage is found with the idle children, and the largest percentage with the most industrious. It would really seem from these statements that acquired near-sightedness may be accepted as an evidence of close and long-continued application during school life.

It has been found, however, that all children are not equally liable to acquire near-sightedness, and that next to those who are born with long eyes, the hyperopic children, who have flat eyes, being those who require over-muscular work for seeing near, fine things, are the children whose eyes are more disposed to suffer under the effort of close study. It is from the age of eight to fifteen, while the eye walls are still soft and yielding, that the eye-shell of flat-eyed children gives way by the bulging of the back wall of the eye. Once induced, this bulging is never afterwards effaced, and the eye becomes permanently weakened. It is before this straining effort has caused the eye to yield that the preventive remedy ought to be applied. Badly lighted, imperfectly ventilated, overcrowded schoolrooms and long sessions, with the bad health which such defective hygiene engenders, are conducive to a general weakening of the tissues. Then overwork of the eye, especially if the body be not strong, does the mischief.

To save good eyes to the adult population is a subject worthy of the attention of any scientific body. In this connection especially it is well known that an ounce of prevention is worth many pounds of cure. If the eyes of all children be carefully examined by competent physicians, and glasses be adjusted to those who need them on account of the defective shape of the eyeball, before the eyes are taxed by over-study, there is no calculating how much misery, in school as well as in after life, might be prevented.

It is this very important subject of How to prevent children from becoming near-sighted, which affects the wellbeing of the whole human race, that is at the present moment absorbing the attention of specialists in eye surgery.

INVITED PAPERS.

THE INFLUENCE UPON THE PULSE RATE OF VARIATIONS OF ARTERIAL PRESSURE, OF VENOUS PRESSURE, AND OF TEMPERATURE.

BY H. NEWELL MARTIN, A. M., M. D., D. Sc.,
Professor in the Johns Hopkins University.

Last year I had the honor of laying before the members of the Medical and Chirurgical Faculty of Maryland a brief account of a new method of studying the physiology of the heart of the mammal, which rendered it possible to separate the organ entirely, both as to nervous connections and circulation, from all remaining parts of the body except the lungs. Since then I have been mainly occupied with a study of the influence of various conditions upon the pulse rate of the so isolated heart. Having been invited by your Executive Committee to address you to-day, I have thought that possibly some account of these experiments might interest you, as the pulse-rate is so valuable an indication in many forms of disease. A practitioner may, nowadays, when making a professional visit, omit to say "Put out your tongue," without being thought to have neglected his duty, but the family doctor who fails to feel his patient's pulse seriously risks losing the confidence of materfamilias.

Apart, however, from the whims of those with whom we have to do, there does remain the primary and universally acknowledged fact that in many cases the pulse-rate is a most valuable factor in forming a diagnosis, making a prognosis, or deciding upon a treatment. I therefore venture to hope that what little I may be able to add to your knowledge of the causes which influence the rate of beat of the heart may be not unwelcome.

As the method of work has been somewhat improved since I last addressed you, it is necessary to begin with some brief account of it. The guiding idea is to prevent all circulation through any part of

the body of a warm-blooded animal but the heart and lungs. From want of blood, brain, spinal cord and sympathetic ganglia very soon. die, and so the heart is liberated from the control of nerve centres outside of itself. In the second place, the heart, thus isolated, receives only blood of constant composition and known temperature, sent into it under readily controlled conditions. The animal having been tracheotomized, placed under curari, morphia or chloroform, the common carotid arteries are exposed, tied, and cannulas placed in their central ends. The pneumogastric nerves are divided in the middle cervical region. The next step is to expose the heart and great thoracic blood-vessels by opening the thorax. To this end, artificial respiration, if not already employed, is started, the top of the sternum with the cartilages of the first pair of ribs is resected, and the cephalic end of the anterior mediastinum exposed. On its sides the internal mammary arteries are readily found, coursing forward from the subclavian to the sides of the sternum, and ligated. This having been. done, the whole front and sides of the thorax are cut away to within an inch or two of the vertebral column. In most cases this leads to a little bleeding; sometimes a few of the intercostals spirt vigorously, and require torsion or a hard pinch by finger and thumb or a pair of forceps to stop the bleeding.

The next steps have for their object to close all paths of the greater circulation but those through the coronary system. Before describing them it will be necessary to indicate some points in which the anatomy of the great vessels of the dog differs from that of the corresponding trunks in man.

From the arch of the aorta in the dog arise (1) the two coronary arteries; (2) the brachio-cephalic; (3) the left subclavian. The brachio-cephalic gives off, first, the left carotid, and then divides into the right carotid and the right subclavian. Beyond the arch the usual bronchial and intercostal branches are given off by the thoracic aorta.

The first step after opening the chest is to tie the right subclavian below the origin of its first branch. The two carotids are already blocked by cannulas, as above stated. Next a ligature is put around the left subclavian below its first branch. Now all blood is cut off from the head save such as reaches it by anastomoses from vessels in the spinal cord, and, consequently, little arterialized blood reaching the medulla, the animal, unless curarized, exhibits dyspnotic symptoms, due to the deficiency of oxygenated blood in its respiratory centre.

As has been noted by other observers, however, the dog's medulla gets a good deal of blood by other roads than along the carotids and subclavians, so that after ligating all four arteries the dyspnoea does not pass on into the convulsions of a more extreme asphyxia, as it does later when the thoracic aorta also is closed.

Next a metal cannula, curved at one end so as to present a long limb and a short limb at right angles to one another, is put into the aorta. For this purpose a ligature is loosely placed around the vessel just beyond its arch, and a stout clamp put on it further down in the thorax. The thoracic aorta is then opened just above the diaphragm and the free end of the long limb of the cannula introduced. The clamp is removed, and the cannula, which is of such size as to barely fit into the aorta, is pushed along till its end reaches the aortic arch, when the ligature above mentioned is tied around it. The result of this is obviously that all circulation through the systemic arteries, except the coronaries, is blocked. The heart is the only organ now receiving blood from the left ventricle.

The next stage is to tie up the systemic veins leading to the right auricle. A ligature is quickly put round the inferior vena cava above the diaphragm; another around the vena azygos near its entry into the superior cava, and then the superior cava is ligated on the cardiac side of its last tributary. On the cardiac side of this ligature a large tube is introduced, which tube is in communication with a flask filled with defibrinated dog's blood, or the same diluted with one-third its bulk of 0.7 per cent. sodium chloride solution, or filled with defibrinated calf's blood.

The flask having been placed in connection with the heart, the carotids are opened, and all the blood present previously in the heart and the lungs of the dog is washed out and replaced by defibrinated blood. While this is being done a thermometer is placed in the left subclavian artery.

The animal (none of whose body but heart and lungs now has any circulation) is next transferred to a warm, moist chamber. The superior cava is connected with a Mariotte's flask, from which defibrinated blood under a known and easily controlled pressure enters the vein and thence the right auricle.

The aortic cannula is connected with a long rubber tube having at its distal end a bent glass tube, from which the blood forced out by each contraction of the left ventricle is poured into a funnel; from this funnel a tube leads to a Mariotte's flask exactly like that in con

nection with the right auricle. The blood taken by the right heart from one Mariotte's flask is thus pumped by the left heart into another when the first is empty, by altering the position of a couple of stopcocks the course of the blood is changed. The heart then receives it from the second Mariotte's flask and pumps it back into the first; and by altering the stopcocks from time to time as necessary, the blood circulates time and again through the heart as often as desired.

Under these circumstances the heart is under conditions allowing of accurate experiment with respect to various points.

Keeping the Mariotte's flasks at a constant level and the blood in them at a constant temperature, we can alter arterial pressure by elevating or lowering the exit point of the tube connected with the aorta. Keeping the level of the outflow of the aortic tube constant, we can maintain an approximately constant resistance to the systole of the left ventricle, while we alter venous pressure (i. e. pressure in the right auricle) by raising or lowering the supplying Mariotte's flask; and, finally, keeping venous pressure and arterial resistance constant, we can change the temperature of the blood supplied to the heart, and study by itself the influence of changes of temperature on the pulse-rate.

Let us first consider the effect of changes of arterial pressure on the pulse; a matter hitherto in much dispute.

Morey showed that in animals whose hearts were not separated from the influence of extrinsic nerve centres, a rise of arterial pressure showed the beat. Upon this he founded the very neat, but unfortunately incorrect, theory that when the heart had to pump against greater aortic pressure, each ventricular systole took longer and the pulse-rate was slowed.

Subsequent work proved that Morey's experimental results did not justify the conclusions which he drew from them. Increased pressure in the brain-case was shown to slow the beat of the heart quite independently of any general rise of aortic pressure; for instance, injecting a small quantity of liquid through a hole trephined in the skull greatly diminishes the pulse-rate although general arterial pressure is not raised.

This phenomenon (that of a slower pulse with increased cerebral pressure) is well known to the members of this Faculty as one of the concomitants of apoplexy, due to an effusion of liquid within the brain-chamber.