124 



THE POPULAR EDUCATOR. 



Senor, senora, senorita, senores, senoras, seuoritas, also are 

 used for sir, madam, miss, gentlemen, ladies, young ladies 

 respectively : as 



Buenos dias, seuor, good morn- I Buenas noches, seuores, gooa 

 tng, sir. | night, gentlemen. 



Senor and Senora are used as an additional mark of reaped 

 before the name of a relative in such cases as the following : 



4 Como esta su seuor hermano ? 

 hov> it your brother ? 



i Como esta su seuora madre ! 

 how is your mother ? 



HUMAN PHYSIOLOGY. VIII. 



CIRCULATION (continued). 



THE arteries are distributed to all parts of the body ; the only 

 portions which are destitute of them are the hair, the nails, the 

 outer covering of the skin, and the cartilages. They divide and 

 subdivide, the branches freely communicating with each other, 

 till they become diminished to a very small size, and at length 

 they terminate in a very delicate network of vessels, which, 

 from their great minuteness, are termed capillaries (or hairs). 

 The great artery of the body, called the aorta, starts from the 

 left ventricle of the heart, and first ascends, and makes a kind 

 of arch ; it then descends, passing downwards through the 

 thorax and abdomen, at the lower part of which it divides into 

 two. From the arch of the aorta are given off large branches 

 the innominate artery, which divides into the right common 

 carotid, and the right subclavian. The common carotid ascends 

 the side of the neck, and divides into the external and internal 

 carotids. From the first of these numerous branches arise, 

 which are distributed to the external parts of the neck, the 

 head, and the face ; the internal carotid passes up into the 

 skull, and is the principal channel for the blood going to the 

 brain. The subclavian gives off a large branch, the vertebral, 

 which enters the skull through the occipital foramen, and com- 

 pletes the blood-supply for the cerebral organs ; it then gives 

 off branches to the shoulder and external parta of the chest, 

 and terminates in a large trunk called the axillary, which 

 afterwards, taking the name of brachial, passes down the arm 

 to the bend of the elbow, where it divides into the radial and 

 ulnar arteries, which supply the fore-arm and hand. The left 

 common carotid and subclavian arise directly from the aorta, 

 without the intervention of an innominate artery. From the 

 descending trunk of the aorta branches spring which supply all 

 the viscera and the muscular walls of the thorax and abdomen, 

 and eventually the aorta splits into two halves, called the right 

 and left common iliacs ; these each again divide into external 

 and internal iliacs, the latter of which is distributed to the 

 pelvic organs, whilst the former, taking the name of the femoral, 

 at its exit from the abdomen, passes down the front of the thigh, 

 giving off branches to the muscles in that neighbourhood ; two- 

 thirds down it pierces the muscles, and appears at the back just 

 above the bend of the knee, there called the popliteal. Soon 

 after it enters the leg, it divides into two, an anterior and pos- 

 terior tibial, which supply the leg and foot. This is the general 

 arterial circulation of the body ; but in addition to this must be 

 mentioned the pulmonary artery, which springs from the right 

 ventricle, and immediately divides into a right and left pulmo- 

 nary artery ; these convey the blood to the corresponding lungs, 

 in the substance of wiiich they break up into a dense network 

 of capillaries, which will be more particularly described when 

 we come to speak of the structure of the lungs. 



The capillaries, or intermediate vessels in whieh the finest 

 branches of the arteries terminate, are extremely minute, their 

 average diameter being about ^ of an inch : they vary some- 

 what in size ; those of the brain and the intestines are the 

 smallest. These vessels form a dense network all through the 

 body, their number and the closeness of the network being 

 proportionate to the activity of the tissue they have to supply 

 with blood ; the walls of the capillaries are composed of a fine 

 transparent membrane, containing cells interspersed at intervals, 

 and offering little obstruction to the process of absorption. 

 There is no definite line to mark where vessels cease to be 

 arteries and become capillaries, or where the veins commence ; 

 but the intermediate vessels have this peculiarity, that when 

 once they have attained a certain degree of minuteness, they 

 retain it, and do not continue to diminish, and the meshes of 

 the capillary network a?3 more even and uniform than those 



formed by the smaller branches of the arteries or the commencing 

 radicles of the veins. The veins take their rise from the capillary 

 network, first as very small vessels, and gradually join together, 

 forming larger and larger trunks, till they are all eventually 

 merged in two, which have been already mentioned, the superior 

 and inferior cavae. The veins are larger and more numerous 

 than the arteries, and convey back to the heart the blood which 

 has exhausted its nutritive properties. In structure their walls 

 resemble those of the arteries, but have very little elastic tissue 

 in them ; in shape they are not so completely cylindrical as the 

 arteries, and when empty their walls collapse : they also have 

 another important point of difference from the artery, in that 

 there are valves placed in all the larger veins that are subject to 

 much pressure. These valves, which are semi-lunar in shape, and 

 generally occur in pairs, are so arranged as to allow the blood 

 to pass onwards towards the heart, but prevent any backward 

 movement of the current. Veins may be divided into super- 

 ficial, deep, and sinuses. The superficial lie immediately beneath 

 the skin, and communicate with the deep ones. The deep veins 

 accompany the arteries, and are usually inclosed in the same 

 fibrous sheath ; to the larger arteries, such as the femoral or the 

 subclavian, there is but one vein to each artery ; but in the 

 smaller ones, as the radial or ulnar, there are a pair, one lyirg 

 on each side of the artery. In the brain, and some other parts 

 of the body, the arteries and veins take different courses, and 

 do not accompany each other. The venous sinuses only exist in 

 the interior of the skull ; they are large channels, formed between 

 the layers of the dura mater, which collect the venous blood 

 from the substance of the brain and discharge it into the internal 

 jugular veins. 



Having now examined the blood, and the apparatus by which 

 it is circulated, we pass on to consider the act of circulation 

 itself, and we may take as a starting-point the left ventricle of 

 the heart. When this chamber is filled with blood, it contracts 

 and forces the blood into the aorta ; this conveys it, by means 

 of its many branches, to all parts of the body. When the blood 

 has reached the extreme divisions of the arterial system, it 

 leaves them and enters the capillary network ; from thence it 

 makes its way to the ultimate radicles of the veins, which carry 

 it forward and empty it into the superior or inferior cava ; these 

 at their termination empty it into the right auricle of the heart ; 

 the auricle, when it is filled, contracts and drives the blood into 

 the right ventricle, which in its turn pumps it into the pulmonary 

 artery ; this vessel, dividing into two, conveys it to the lungs ; 

 here, whilst passing through the capillary network, it is exposed 

 to the action of the air ; leaving the lung, it is conveyed by the 

 pulmonary veins and discharged into the left auricle, which con- 

 tracting, drives it once more into the left ventricle, to commence 

 again the same unceasing round. 



In addition to the general circulation of the body, there is a 

 minor one of the liver, called the portal circulation. This has 

 been before alluded to in the article on Digestion. The veins 

 which collect the blood from the viscera of digestion join together 

 to form a large trunk, called the portal vein ; this vessel enters 

 the substance of the liver at its under surface, and divides like 

 an artery into a capillarj network, thus bringing the blood it con- 

 veys into intimate relation with the secreting cells of the liver. 

 This network terminates in a number of moderate-sized veins 

 called the hepatic veins, which unite into three large branches, 

 and finally empty themselves into the inferior vena cava. 



We must now consider the part which each constituent of the 

 circulatory apparatus plays in the performance of this function ; 

 and first in importance is, of course, the heart. In order to 

 understand the way in which the heart fulfils its duties, we 

 must constantly bear in mind that the heart is a muscular 

 organ, split up into four distinct chambers, and richly supplied 

 with nerve power. The action of the heart is made up of two 

 sets of motions, the dilatation and contraction alternately of the 

 auricles and ventricles : the auricles contract together in alter- 

 nation with the contraction of the ventricles, which is also 

 simultaneous ; the dilatations follow the same rule, and the 

 contraction of the auricles takes place at the same moment that 

 the ventricles are dilating, and vice versti. The interval between 

 the two sets of movements is, of course, very short, but is easily 

 made out when the heart is acting quietly. During the contrac- 

 tion of the ventricles the apex of thi heart is drawn upwards 

 and tilted forwards, striking the parieties of the chest, thus 

 riving that sensation which is described as the beat of the 



