VESSELS. ARTERIES. VEIttS. CAPILLARIES, ETC. 73 



the blood contained in x the vessel, there is another 

 lamella known by the name of fenestrated membrane. 

 It is amorphous, elastic, traversed by numerous open- 

 ings which vary both in size and shape, and contains 

 elastic fibres which are disposed at right angles to 

 the axis of the vessel (PL XV. fig. III. 1, 2, 3, 4). 

 The deepest layer of the internal coat is composed of 

 fine elastic fibres, which run in the direction of the 

 length of the vessel. This layer is the thickest of the 

 three laminae which constitute the internal coat, espe- 

 cially in the larger arterial trunks (PI. XIV. fig. V. 

 1 ; PI. XV. fig. III. 5 ; fig. IV. 1). The semilunar 

 valves and the endocardium are formed by the in- 

 ternal coat. 



The middle coat is made up of elastic fibres, and Middle c^t. 

 those of non-striated muscle. The first are distri- 

 buted uniformly throughout the thickness of the 

 layer, but seem to run in no determinate direction ; 

 this is readily recognised by comparing transverse 

 and longitudinal sections under the microscope (PL 

 XIV. fig. V. 2 ; fig. VI. 1 ; PI. XV. fig. IV. 6). The 

 network formed by these fibres is closer in its meshes 

 in proportion to the calibre of the artery to which it 

 belongs, and in these meshes the muscular fibres are 

 contained. To bring the latter into view it is well 

 to treat the specimen with dilute acetic acid. In 

 transverse sections it is difficult to distinguish the 

 outlines of these fibres on account of their extreme 

 paleness; but their nuclei, club shaped, and arranged 

 perpendicularly to the axis of the vessel, are easily 

 recognised (PL XIV. fig. V. 3). In longitudinal sec- 

 tions, the outlines of the smooth muscular fibres are 



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