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DESIGN IN NATURE 



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length of the arteries and at right angles to the circular set. These two sets of muscles act harmoniously and 

 synchronously both in dilating and contracting the calibre of the arteries — elasticity pla5dng, relatively, quite a 

 subordinate part. In the arterioles, or smallest arteries, only the circular set is found. The rule is, that as the 

 muscular elements increase, the elastic elements decrease. 



The walls of the smaller arteries are composed of three layers, namely, an inner, middle, and outer layer. The 

 inner layer consists of fusiform epitheloid cells imbedded in a thin basement membrane with their long axes 

 parallel to the axes of the vessels. The cells of the membrane form an elastic layer {tunica irdima). The middle 

 layer consists mainly of pale or unstriated muscular fibres arranged circularly and spirally. It contains a small 

 quantity of connective tissue, and is known as the tunica media. The outer layer (tunica extima) consists of con- 

 nective tissue, the bundles of which are, for the most part, disposed longitudinally. It displays connective tissue 

 corpuscles and a relatively large number of elastic fibres. It will be observed that the elastic elements run, as a 

 rule, in the direction of the length of the arteries, and at right angles to the circular or spiral muscular fibres — an 

 arrangement which readily admits of their being put upon the stretch by the contraction of the arteries. Here 

 the muscular and elastic elements mutually assist each other ; the latter giving resiliency and helping the muscles 

 over their dead points. It is, however (and let this be noted), the muscular element which primarily confers the 

 closing and opening movements. The power of muscle to put elastic structures on the stretch is well illustrated 



by the effect produced by the left ventricle of the heart on the aorta. During 

 the systole or closing of the left ventricle the aorta is suddenly dilated. It 

 is not permitted to recoil or contract until the left ventricle withholds its 

 power, which it does during its diastole or opening movement. The left 

 ventricle and the aorta play, as it were, at cross purposes. The ventricle 

 closes, and in closing opens the aorta. The aorta in turn closes by recoil 

 when the ventricle opens. These are co-ordinated complementary movements, 

 and the result is a more rapid, equal, and perfect circulation of the blood. 

 Here, again, the muscular ventricle is the prime mover, and the aorta, which 

 is a passive structure, can only move at second hand, and when distended 

 by the blood forced into it by the left ventricle. The elastic properties of 

 the aorta supplement the movements of the left ventricle. They also confer 

 resihency on that part of the circulatory apparatus nearest the heart. The 

 resihency is an important property where shock is to be diffused and recoil 

 encountered, as is the case in the action of the left ventricles, where the move- 

 ments are sudden and jerky. 



The veins differ from the arteries in being considerably larger and in 

 having much thinner walls ; the walls containing, relatively, a larger 

 proportion of white fibrous tissue, and a smaller proportion of yellow elastic tissue. 



The structure of the small arteries and veins is given at Fig. 66, where it will be seen the fundamental elements 

 of both are arranged at right angles in a manner analogous to that in which the sarcous elements of muscle are 

 arranged at Plate Ixxxiii., Figs. 1, 2, 3, and 4. 



This hne of argument is greatly strengthened by a reference to the hollow viscera with sphincters, especially 

 the stomach, bladder, rectum, and uterus. The human stomach is provided at its narrow end with a powerful 

 sphincter (the pyloric valve), composed of two sets of strong muscles spirally arranged and crossing obliquely. This 

 valve during the digestion of food is so firmly closed that the thin walls of the stomach cannot open it by even their 

 most vigorous contractions. When the time arrives for the expulsion of part of the digested food (chyme) into the 

 duodenum (or upper part of the small intestine) the sphincter valve opens spontaneously. When the sphincter valve 

 opens the stomach, as a whole, closes or contracts. A double movement is inaugurated. The sphincter opens 

 when the viscus closes, and vice versd — the movements being alternate and rhythmic in character. The contraction 

 or closure of the body of the stomach does not open the sphincter, which, when contracted or closed, forms a solid 

 muscular mass. 



The stomach, as my dissections show, is provided with two sphincters — the pyloric sphincter or valve already 

 referred to, and another which I have designated the oesophageal or cardiac sphincter, from its being situated at the 

 root of the oesophagus, where it enters the stomach, and from its proximity to the heart. That these sphincters 

 act spontaneously and independently, and are not stimulated or set in motion by the food in the stomach, is proved by 

 this, that they are sometimes both open, sometimes both closed, and sometimes one is open and the other is closed. 

 They are both open when no food is in the stomach ; the oesophageal or cardiac sphincter is open and the pyloric 

 one closed when food is being ingested ; the pyloric sphincter is open and the oesophageal or cardiac one closed when 



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Fig. 66.— a. Small artery. B. Small vein : 

 both treated with acetic acid, a a, a' a', 

 Fibrous coats ; hb', nuclei of the muscular coat ; 

 c c, c' c', nuclei of the epithelial coat — magnified 

 300 diameters (after Huxley). 



