202 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



bodies situated one on either side, near the end of the quadrate 

 cartilage, and the thyroid glands are similar structures, two or 

 more in number, placed near the posterior hyoid cornua. 



The cavities of the heart and blood-vessels are lined by endo- 

 thelium, which alone forms the capillary walls (Fig. 59). The 

 large lymph-cavities are similarly lined, and patches of granular 

 germinal epithelial cells occur in them, from which colourless 

 corpuscles are budded off. The heart is mainly composed of 

 unicellular, spindle-shaped muscular fibres (sometimes branched), 

 which are transversely striated, but possess no sarcolemma 



Fig. 59. CAPILLARIES (from Landois and Stirling), much enlarged. 

 The outlines and nuclei of the endothelial cells making up their walls 

 are clearly shown. 



(Fig. 57). The veins and arteries possess three coats outside 

 their endothelial lining (Fig. 58). These are, beginning from the 

 inside (1) a membrane formed by elastic connective tissue; (2) 

 a muscular coat made up for the most part of unstriated muscle- 

 fibres arranged circularly ; and (3) a connective-tissue sheath. 

 These coats are thicker in arteries than in veins, and in large 

 vessels than small. They gradually thin out as capillaries are 

 approached, till at last only endothelium is left (Fig. 59). 



Circulation. The heart receives oxygenated blood from the 



