CHAP. XXVIII.] 



COATS OF AKTEKIES. 



321 



lowing tunics may be enumerated as constituting the wall of an 

 artery proceeding from without inwards. 



Fig. 192. Fig. 193. 



Epithelial particles from the 

 aorta of an ox. Magnified 400 

 diameters. 



Particles of epithelium and nuclei from the 

 aorta of a horse; some of the former exhibit 

 the elongated character. Mag. 200 diam. 



First, the external coat consisting of areolar tissue. 



Second, the circular fibrous coat consisting of a series of lamellae, 

 composed of yellow elastic fibrous tissue, the most external of which 

 are intermingled with white fibrous tissue and with circular muscular 

 fibres 



Third, the longitudinal fibrous layer, which consists of two layers, 

 a finely fibrous, and a coarsely fibrous. 



And lastly, a layer of epithelium. 



The internal layer of longitudinal fibres which is the same as 

 that described by Henle under the name of fenestrated membrane 

 constitutes, along with the epithelium, the internal tunic so long 

 recognised by anatomists. 



The elastic reaction of arteries is evidently resident in the middle 

 fibrous coat, and in the same tunic the contractile power of the 

 artery resides. The existence of these two forces in the arterial 

 wall, the one of simple elastic reaction, the other of a slow muscular 

 contraction, is shown by the well-known experiments of John 

 Hunter. A piece taken from each of the large arteries of a horse 

 bled to death was laid open and extended on a flat surface without 

 stretching; it was then measured,, and afterwards subjected to 

 strong tension, it was then measured again ; on the removal of the 

 stretching force it failed to recover itself to its first dimensions by 

 a notable difference. When an animal has been bled to death, the 



