110 NORMAL HISTOLOGY. 



the auriculo-ventrictilar orifices and the aortic orifice, this fibrous 

 tissue is reinforced by a mass of fibro-cartilage. 



The cavities of the heart are lined by the endocardium, consisting 

 of endothelium resting on areolar tissue. The deeper portions of 

 the epi- and endocardium merge with the areolar tissue of the body 

 of the heart. Smooth muscle-fibres are of occasional occurrence in 

 the deeper layers of the endocardium. 



The auricles and the basal third of the ventricles contain ganglia, 

 connected on the one hand with the nerves received by the heart 

 from the cerebro-spinal and sympathetic systems, and on the other 

 hand with a nervous plexus which penetrates the substance of the 

 heart and gives off minute nervous fibrilla? to the individual cells 

 of the cardiac muscle. These fibrillse end in minute enlargements 

 connected with the surfaces of the muscle-cells. Many of the gan- 

 glia lie beneath the epicardium or in the areolar or adipose tissue 

 situated in its deeper portions. 



The valves of the heart are composed of fibrous tissue, con- 

 tinuous with that forming the rings around the orifices. Their 

 surfaces are covered by extensions of the endocardium, except the 

 outer surfaces of the pulmonary and aortic valves, which are cov- 

 ered by extensions of the not dissimilar inner coats of the pul- 

 monary artery or aorta. The fibrous substance of the valvular 

 pockets of those two valves are further strengthened by tendinous 

 strips of fibrous tissue at their lines of contact when the valves are 

 closed. The curtains of the auriculo-ventricular valves are also 

 reinforced by fibrous tissue derived from fan-like expansions of the 

 chordae tendinese. 



2. The Arteries. It will be best to consider first the structure of 

 the smaller arteries, because the individual coats are less complex in 

 these than in the larger arteries. 



The arterial wall consists of three coats : the intima, or internal 

 coat ; the media ; and the adventitia, or external coat (Fig. 96). 



The intima consists of three more or less well-defined layers. 

 These are, from within outward : 1, a single layer of endothelium ; 

 2, a layer of delicate fibrous tissue containing branching cells ; 3, a 

 layer of elastic fibrous tissue. The endothelial layer consists of 

 cells, usually of a general diamond shape, with their long diago- 

 nals parallel to the axis of the vessel they line. When the vessel 

 expands these cells broaden somewhat and appear very thin. When 



