ARTEEIES. 107 



more scarlet ; 2nd, it has a greater specific gravity ; 3rd, its temperature 

 is slightly higher ; 4th, it contains more fibrin and oxygen, and less car- 

 bonic acid gas. It has also a greater capacity for rendering heat latent. 



207. Colour of Blood. 

 The colour of blood is not due, as has been often supposed, to the iron 

 which it contains. The colour remains after the iron has been removed. 

 Again the blood of many invertebrate animals contains iron, and yet is 

 colourless. The degree of colour really depends on the gases in the 

 blood. The carbonic acid gas found in venous blood distends the red 

 corpuscles and therefore allows the haematin or red pigment contained in 

 them to appear through their coats; whilst oxygen on the contrary 

 shrivels up the corpuscles, and thus gives a light scarlet tint to the blood. 



207a. The heart. 



The heart is the gi-eat force pump of the body, and may be described 

 as a hollow muscular organ of about 7^ lbs. weight when the blood is 

 washed out, in a horse of medium size, length about 10 inches and 

 breadth between 5 and 6 inches. 



It is in the form of a blunt cone, and is situated almost in the centre 

 of the chest, inclined to the left side, and is suspended from the dorsal 

 vertebra? by the blood-vessels which spring from it. 



The heart is enclosed in a distinct sac of its own called the pericardium, 

 a layer of which is reflected over the external surface. 



The heart is divided into right and left sides by a longitudinal septum 

 and into auricles and ventricles by a transverse septum, so that it will be 

 seen that this organ has four distinct cavities, two on the right side for 

 the reception of venous blood, and two on the left side for the reception 

 of arterial blood. 



The beats of the heart are caused by the contraction of its muscular 

 walls, by which the blood is forced into the lungs for purification and 

 from the lungs through the system generally for the purpose of nutrition. 



208. The Arteries. 



The arteries are elastic tubes fonned for the purpose of conveying 

 blood from the heart to all parts of the body. The blood from the left 

 ventricle of the heart passes into the Aorta, and from thence by innumer- 

 able channels to all parts of the body. The Pulmonary artery arises 

 from the right ventricle and conveys venous blood to the lungs. The 

 l3lood after being purified in the lungs by the action of the air is returned 

 by the pulmonary veins to the left auricle of the heart as arterial blood 

 for distribution through the body. 



The larger arteries are generally deep seated, and run on the inner 

 rather than on the outer sides of the limbs. So placed, they are better 

 protected from the chance of injury. In passing over joints, however, they 

 are most commonly found on the side, which is flexed by the motion of 

 the jomt. In this situation, any abnormal extension is obviated, and 



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