74 ELEMENTS OF HISTOLOGY. [Chap. x. 



diaphragm). Here the longitudinal striation appears 

 very distinct, but these differences are not constant 

 in other muscular fibres of other animals or of man 

 (E. Meyer). 



94. Briicke has shown that striped muscular fibres 

 are doubly refractive or anisotropous, like uniaxial 

 positive crystals (rock crystal), the optical axis coin- 

 ciding with the long axis of the fibres. The lateral 

 disc and interstitial substance are isotropous, the sar- 

 cous elements (Briicke) and Krause's membrane 

 (Engelmann) are doubly refractive. The sarcous ele- 

 ments are, however, not the ultimate optical elements, 

 but must be considered as composed of hypothetical 

 disdiaclasts, the real doubly refractive elements 

 (Briicke). 



CHAPTER X. 



THE HEART AND BLOOD-VESSELS. 



95. (A) THE heart consists of an outer serous 

 covering (the visceral pericardium), an inner lining 

 (the endocardium), and between the two the muscular 

 wall (Fig. 44). Underneath the pericardium and en- 

 docardium is loose connective tissue, called subperi- 

 cardial and subendocardial respectively. 



The free surface of both the pericardium and 

 endocardium has an endothelial covering, like other 

 serous membranes, i.e., a single layer of transparent 

 nucleated cell plates of a more or less polygonal or 

 irregular shape. The ground-work of these two mem- 

 branes is fibrous connective tissue, forming a dense 

 texture, and in addition there are many elastic fibres 

 composing networks. Capillary blood-vessels, lymphatic 



