500 



THE TISSUES. 



exist on the muscular substance of the heart beneath the pericar- 

 dium; but whether they are present in its substance and in the 

 pericardium, is not determined. The nerves (from the pneu mo- 

 gastric and sympathetic) contain, except the largest, only pale and 

 fine_ fibres. Ganglia also exist in the substance of the .heart. Dr. 

 Lee, of London, however, mistook for ganglia mere thickenings of 

 the perineuria. (Kolliker.) How the nerve-fibres terminate, is 

 unknown. 



IT. THE BLOODVESSELS. 



The bloodvessels are divided into the arteries, the capillaries, 

 and the veins. 



A. The Arteries. 



The arteries* have three tunics, the external (adventitia), the middle 

 (media), and the internal (intima). (Fig. 332.) Each of these may, 



Fig. 332. 



Transverse section of the aorta below the superior mesenteric artery. 1. Inner tunic. 2. Middle 

 tunic. 3. External tunic (adventitia). a. Epithelium, b. Striped lamellos. c. Elastic membrane 

 of the inner coat. d. Elastic lamellae of the middle tunic, e Its muscular fibres and connective 

 tissue. /. Elastic networks of the external tunic. From man, treated with acetic acid. Magnified 

 30 diameters. (Kblliker.) 



however, be subdivided, as will be seen. In general, the external 

 coat consists of areolar tissue; the middle of elastic tissue, with 

 more or less smooth muscular fibres admixed ; and the internal of 

 an elastic network (fenestrated layer), supporting an epithelium. 



1. The external coat of the (1st) larger arteries (above 2 or 3 lines 

 in diameter) is both relatively and absolutely thinner than in the 

 smaller being g ^ to 3^0 of a line and presents the same struc- 

 ture as in the smaller arteries on the whole ; it being composed of 



1 Meaning "air-holders" (p. 405, note) ; since the ancient anatomists observed 

 that they usually contained air after death. 



