these two portions of the sanguineous system, that, whether in acute or chronic disease, it is seldom found that there is any serious disturbance in the one without a corresponding change in the condition of the other.

system.

Although the offices in the circulation which the veins per- The venous form are quite subservient to those of the arterial and capillary systems, being chiefly employed to return to the heart the blood, after it has undergone certain changes in the organs of secretion and excretion, mixed with the chyle, and arterialised in the lungs, there are, likewise, several distinct functions performed by different subdivisions of the venous system, and also by the veins of some organs.

neous veins.

For these purposes there are particular arrangements The subcutaneous veins are placed externally to the muscles, The subcuta and have numerous valves to prevent any regurgitation of the blood that might be occasioned by external pressure or by the contractions of the subjacent muscles, whilst the intermuscular veins are placed in sheaths along with the arteries, and are in such relation to the muscles that they are com- The vena pletely under the influence of their contractions.

In number and size the veins, compared with the arteries, also differ exceedingly in different parts of the body. The size of the pulmonary veins scarcely surpasses that of the pulmonary artery, and in the intestines the number of arte ries and veins is nearly equal; but in the cerebro-spinal and portal systems their number, compared to that of the arteries, is very considerably larger.

comites.

The compa

rative number of veins and arteries.

ties of veins.

The tortuosities of the veins of the uterus and testis also The tortuosi, present peculiarities of the venous system; and, however inexplicable the office of the tortuosities may be, they must have a considerable influence, and co-operate along with the heart, arteries, and capillaries in the important office of the circulation of the blood in the respective organs.

But the office either of diminishing or of increasing the quantity of blood in certain parts of the body at particular times is a function of the circulation not depending alone upon the heart, but on the nervous system. When, for instance, food is taken into the stomach, its vessels become

The circula

tion also mo

dified by vital

causes.

;

more replete with blood. When the intellectual powers are excited, an additional quantity of blood flows into the brain and so also with regard to the erectile tissues of the corpora cavernosa and the vessels of the uterus and testes, whenever these organs are called upon to perform their respective functions.

In like manner we shall find that the heart itself requires a different quantity of blood in its various conditions; and it becomes an interesting subject of inquiry to discover how that supply of the vital fluid to the heart is always furnished with the requisite regularity;-a function which I shall endeavour to demonstrate is performed by the muscles or organs of active motion,

CHAPTER II.

THE MUSCULO-CARDIAC FUNCTION.

The musculo-cardiac function explained; some organs perform several different functions; influence of muscles on the circulation; they accelerate the return of the venous, and impede the cxit of the arterial, blood; some muscles employed to compress arteries; some arteries are exposed to, and others protected from, muscular pressure; why veins accompany only some arteries; influence of muscular contractions on the vasa propria of muscles; anatomy of the vertebral and spinal arteries; anatomy of the carotid arteries; erroneous opinions of the use of tortuosities of the cerebral arteries; phenomena explained by the musculo-cardiac function; these phenomena enumerated; influence of the musculo-cardiac function on diseases of the heart; many symptoms explained by it.

The simpli

city of Na

ture's works.

HOWEVER difficult it may hitherto have been satisfactorily to explain any of the phenomena of living bodies, yet, whenever a step has been gained in physiological science, we are struck with the remarkable simplicity in the means which Nature employs for accomplishing what had to us appeared a most complicated function in the animal economy. In endeavouring, therefore, to explain any of those phenomena of living beings which have hitherto evaded research, it may be anticipated that, if such inquiries be successful, they will not lead to the discovery of new laws, but probably unfold the same simplicity of means for performing those operations of the economy which have already been disclosed by human intellect. It has, indeed, been justly observed, in allusion to the progress of chemical science, that it has shown "from what a small store of primitive materials have all that we behold and wonder at been created!" And in like manner Chenevix. it appears that, as physiology has advanced and we have been. able to explain any of the phenomena of living animals, the laws which govern them are found to be equally few and

E

Some organs perform several functions,

simple; so that it is by no means improbable that some such general law as that of gravitation, which governs inanimate matter, may yet be discovered to govern and regulate all the phenomena of organised beings!

In the arrangements of the animal economy it may be frequently observed that one organ is destined to perform several distinct functions-functions, indeed, in some instances so dissimilar and apparently so unconnected with one another, that it could not be well anticipated that they should be executed by the same physical apparatus. Thus, for example, the muscles of the jaw are not only employed to masticate the food, but they at the same time, and by the same muscular effort, evacuate the salivary glands, squeezing the saliva into the mouth by the pressure which they produce during their and those of contractions on the salivary apparatus. The urinary organs

as those of mastication

urine,

and the bones,

and the organs of respiration,

and the muscles.

of the male, moreover, are employed for the transmission of the seminal fluid, as well as to afford a passage for the urine. And the bones not only support the body like a framework, but they afford fixed points for the origin and insertion of the various muscles, and also form walls of defence for the internal organs, as the brain and the thoracic and pelvic viscera.

The functions of the respiratory apparatus are still more multiplied. Besides arterialising the blood, conveying odorous substances through the nasal passages to the organ of smell, assisting the return of the venous and expelling the systemic blood, and producing the voice, they also, as I shall subsequently endeavour to demonstrate, are employed in modifying the supply of blood to the heart. In birds we find that the functions of the respiratory apparatus are even more numerous, the air passing into membranous sacs within the chest and abdomen, as well as into their hollow bones, in order to assist their flight.

In like manner we shall find that the voluntary muscles, besides being the active organs of motion, destined to perform the various movements of the body, are essential auxiliaries in the circulation of the blood, in the arteries as well as in the veins, performing both these offices merely by the

pressure which they produce during their contractions upon the adjacent vessels. And further, when such muscular efforts are required, an additional vigour being at the same moment wanted for the heart, this office is performed by the same muscular contractions which are employed to accomplish the effort; and thus it is wisely constituted that the very same organs which perform the movements, and necessarily cause a certain degree of exhaustion, are also the means of invigorating the heart itself.

aid the

circulation.

Evidence of

comparative anatory.

A general view of the means by which the circulation of The muscles the blood is carried on throughout the animal creation, as systeinic well as the phenomena of diseases, will serve to point out the errors of those physiologists who have endeavoured to explain the systemic circulation by the action of the heart alone. The momentary flushings of the cheek, and the diseased changes in the cutaneous capillaries of a common pus tule, might surely have been deemed sufficient to show that these vessels are endowed with powers and perform functions in the circulation which are independent of the central organ; and the fact that the circulation of the blood is carried on in some of the lower animals, as in worms, which are without a heart, only by the almost incessant motions of their bodies, might surely have indicated that muscular contractions have at least some influence on the circulation of the blood in the arteries as well as in the veins.

So important, indeed, is the influence which the muscular movements have on the circulation of the systemic blood, that in various tribes of animals the heart is found developed more and more perfectly in proportion as the body is less exposed to motion. Hence the restless insect requires no heart to impel its blood, whilst the shellfish, fixed and motionless on its rock, is provided with a central organ or heart, to carry on the circulation. In man the duty required of the heart must therefore vary considerably with his bodily pursuits, the sedentary habits of some preventing it from deriving little or any assistance from the organs of locomotion, whilst the active occupation of others will materially aid the circulation of the blood.