74 ELEMENTS OF HISTOLOGY. [Chap. x. 



diaphragm). Here the longitudinal striation ap- 

 pears very distinct, .but these differences are not 

 constant in other muscular fibres of other animals 

 (E. Meyer). 



94. Briicke has shown that striped muscular fibres 

 are doubly refractive, like uniaxial positive crystals 

 (rock crystal), the optical axis coinciding with the 

 long axis of the fibres. The lateral disc and inter- 

 stitial substance are isotropous, the sarcous elements 

 (Briicke) and Krause's membrane (Engelmann) being 

 doubly refractive. The sarcous elements are, how- 

 ever, not the ultimate optical elements, but must be 

 considered as composed of 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 

 ivall (Fig. 44), Underneath the pericardium and en- 

 docardium is a loose connective tissue, called the sub- 

 pericardial and subendocardial tissue 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 



