We have already, in our description of the varieties of form in the circulatory organs of animals, adverted to the intimate relation which very generally subsists between the structure and functions of the organs of circulation and respiration. We shall nol,v mention a few other circumstances connected with the func tions of circulation in the adult human body, which seem to depend upon this relation of the motion of the blood to the respiration.
The influence of the mechanical operations of respiration is not confined to the venous cir culation, for it has been shewn by direct expe riment that the force of the blood in the arteries varies also from the same cause, being greater during expiration than during inspiration. This greater force of the blood in the arteries during expiration, known to Haller, Lamure, and Lorry, was proved by the experiments of Hales, Poiseuille, and MagendieT formerly mentioned.
It is very probably occasioned in part by the assistance which the ventricular systole receives from the collapse of the parietes of the chest at the time that the air is expelled from that ca vity, and in part by pressure of the parietes of the chest upon its contents, and through them upon the trunks of the larger arteries. During inspiration the pressure must be, to a certain amount, removed from the larger arteries, and consequently the current of blood through them at that period will be less forcible and less rapid.
The well-known fact that rupture of aneu risms of the large arteries and effusion of blood within the cranium in apoplexy are more liable to occur during straining and other muscular efforts associated with forcible expiration, is a further illustration of the fact that the arterial pressure is greatest at the time of the collapse of the parietes of the chest.
The relation of the force and frequency of the pulse to the activity of the respiration is an interesting subject connected with the facts at present under consideration.* In many per sons, in ordinary and tranquil respiration, the force and frequency of the pulse vary percepti bly during inspiration and expiration, and in these persons, when the respiration is more forcible than natural, the pulse indicates very distinctly by its changes the varying states of the chest. During an unusually long and for cible inspiration the beats of the pulse are more rapid and weaker, and during a succeeding complete expiration, or even while the chest is kept expanded, the pulse is more full, strong, and slow. Some individuals have the power of occasioning an intermittent pulse, and some of causing the action of the heart to cease even by forcible exertion of the expiratory mus cles. VVe think it probable that it may have been in this or some similar indirect manner that the action of the heart was arrested in Colonel Townsend's case, described by Dr. Cheyne in his work on the English malady, and very often referred to as a proof of the pos session by Colonel Townsend of a voluntary power of influencing directly the heart's action.
There is in general a very constant propor tion in the ordinary state of the circulation be tween the number of the beats of the pulse and the frequency of respiration. The average number of respirations in a healthy person may be considered as from 15 to 20 in a minute, and taking the number of the pulse in the same time at from 72 to 75, this makes one complete respiratory motidn for nearly four beats of the heart. The force and frequency of the heart's action and consequent state of the pulse are well known to be considerably influenced by very slight muscular efforts, as well as by changes of position of the body even; but it is not observed that the respiration becomes inva riably more or less hurried in a corresponding degree with an increased or diminished fre quency of the pulse. In very violent exercise,
it is true, and more particularly in rapid nto tions which give rise to a great and immediate increase of' the frequency of the heart's action, the respiration becomes hurried and forcible, or there is panting ; but, on the other hand, it does not appear that the gradual changes of the pulse, which are liable to occur from one pe riod of the day to another, are accompanied by corresponding variations in the frequency of respiration ; and again, when by a voluntary effort we breathe very hurriedly, as for example, from 80 to 100 times in a minute, the fre quency of the pulse is not increased by more than 8 or 10 beats in a minute.* Some physiologists hold the opinion that the motion of the blood in the capillaries of the lungs and the system is considerably influenced by the chemical changes which the blood un dergoes in its passage through the minute pul monary and systemic vessels. VVe are not ac quainted with any facts or experiments which shew that the systemic capillary circulation is immediately dependent upon the change of the arterial into venous blood : on the contrary, such an opinion is much opposed by the facts that a free circulation of imperfectly arterialized blood takes place in the fcetus before birth, as well as in many children after birth affected with malformations of the heart or greater vessels, and that a completely venous blood circulates through the system in hybernating aniinals when in the state of deepest torpidity. There are, however, several circumstances which appear to justify the opinion that the motion of blood through the pulmonary capillaries has a more immediate dependence on the change of arte rialization.f In all those circumstances which cause imperfect respiration and prevent the ac customed necessary arterialization of the blood, or in approaching asphyxia, it seems to follow from the experiments of Dr. Kay, Alison, and Reid, that there occurs from the very first com mencement of the symptoms of impeded respi ration, a diminution of the quantity of blood which passes through the pulmonary capillaries. There is thus produced from the first com mencement of non-arterialization of the blood an accumulation of venous blood in the pulmo nary capillaries and arteries, but it is equally well proved that a certain quantity of venous blood does, as Bichat shewed, gain the left side of the heart and permeate the arterial sys tem. As the symptoms, however, of suffocation or asphyxia become more urgent, the accumu lation of blood in the pulmonary artery on the right side of the heart and in the systemic veins gradually increases, until by the time that the involuntary motions of respiration have ceased, there appears to be a complete stagnation in the lungs, although the heart continues to beat a little longer. During the occurrence of these changes the action of the heart also is no doubt gradually becoming weaker, a circumstance which may very probably contribute to the stag nation of the blood in the lungs, but there is good reason to think that the motion of the blood is first arrested in the pulmonary capillaries.