THE CIRCULATORY SYSTIQI 165 



to the direct sunlight. After the specimen has turned brown examine with the 

 low power. The outlines of the ceils should appear brown or black. 



(6) The endothelium of the smaller vessels and capillaries may also be demon- 

 strated in the specimen described under technic 8, p. 8^. 



(7) The elastic tissue of the blood-vessels is best demonstrated by means of 

 Weigert's elastic-tissue stain. Prepare sections of medium-sized vessels and of 

 the aorta, as above described (3), and stain as in technic 3, p. 28. 



The Heart 



The heart is a part of the blood-vessel system especially differ- 

 entiated for the purpose of propelling the blood through the vessels. 



The main mass of the heart wall consists of a special form of 

 muscle tissue already described as heart muscle (page 123). This 

 constitutes the myocardium. On its inner and outer sides the myo- 

 cardium is covered by connective- tissue membranes Uned, respect- 

 ively, with endothelium and mesotheUum and known as the erido- 

 cardium and epicardium. 



The MYOCARDiUii varies in thickness in different parts of the 

 heart, being thickest in the left ventricle, thinnest in the auricles. 

 A ring of dense connective tissue, the aurictdo-ventricular ring, com- 

 pletely separates the muscle of the auricles from that of the ventri- 

 cles, except in the median septum where the auricular and ventricu- 

 lar muscle is continuous. The auricular muscle consists of an outer 

 coat common to both auricles, the fibres of which have a transverse 

 or somewhat oblique direction, and of an inner coat, independent for 

 each auricle, the fibres of which are longitudinally disposed. Bun- 

 dles of these fibres stand out as ridges along the inside of the auricles. 

 They are more strongly developed in the right than in the left auricle. 

 Between the two coats bundles of muscle fibres are frequently found 

 which run in various directions. 



The disposition of the muscle tissue of the ventricles is much more 

 compHcated. It is usually described as composed of several layers, 

 the fibres of which run in dift'erent directions. The meaning of 

 these fibre layers becomes apparent when we study the arrangement 

 of the fibres in embryonic hearts in which the connective tissue has 

 been broken down by maceration. Thus dissected, the muscle of 

 the ventricles is seen to consist mainly of two sets of fibres, a super- 

 ficial set and a deep set. These run at approximately right angles 

 to each other. Both sets of fibres begin at the auriculo-ventricular 

 rings. The superficial fibres wdnd around both ventricles in a spiral 



