72 



GENERAL ANATOMY. 



THE VASCULAR SYSTEM. 



The Vascular System, exclusive of its central organ, the Heart, is divided 

 into four classes of vessels the Arteries, Capillaries, Veins, and Lymphatics 

 the minute structure of which we will now proceed briefly to describe, referring 

 the reader to the body of the work for all that is necessary in the details of 

 their ordinary anatomy. 



The Arteries. The arteries are composed of three coats internal serous, or 

 epithelial coat (tunica intima of Kolliker), middle fibrous or circular coat, and 

 external cellular coat or tunica adventitia. 



The two inner coats together are very easily separated from the external, as 

 by the ordinary operation of tying a ligature on the artery. If a fine string 

 be tied forcibly upon an artery, either before or after death, and then taken off, 

 the external coat will be found uninjured, but the internal coats are divided in 

 the track of the ligature, and can easily be further dissected from the outer coat. 

 The inner coat can be separated from the middle by a little maceration. 



The inner coat consists of: 1. A layer of pavement epithelium, the cells of 

 which are oval or fusiform, and have very distinct nuclei. 2. This epithelium 

 rests upon a layer of longitudinal elastic fibres, in which, under the microscope, 

 small elongated apertures are seen, and which was therefore called by Henle 

 the fenestrated membrane. This layer is marked with numerous reticulations; 

 it is perfectly smooth when the artery is distended ; but when empty, presents 

 longitudinal and transverse folds. The fenestrated membrane can often be 

 separated into more than one layer. 



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An artery from the mesentery of a child, .062'", nnd b, vein .067'", in diameter (treated with acetic acid 

 and magnified 350 times), a. Tunica ndventitia, with elongated nuclei. /?. Nuclei of the contractile fibre- 

 cells of the tunica media, seen partly from the surface, partly apparent in transverse section, y. Nuclei of 

 the epithelial cells. <T. Elastic longitudinal fibrous coat. 



In arteries of less than a line in diameter, the internal coat consists of two 

 layers, as above described; but in middle-sized arteries, several lamellae, 

 composed of elastic fibres and connective tissue, are interposed between the 

 epithelial and elastic coats. In the largest arteries, the inner coat is usually 

 much thickened, especially in the aorta ; and consists of a homogeneous sub- 

 stance, occasionally striated or fibrillated, transversed by longitudinal elastic 

 networks, which are very fine in the lamellae immediately beneath the epithe- 



