CIRCULATORY CHANNELS 



157 



FIG. 139. Section of a small capillary of Octopus, 

 containing the slightly shrunken blood content in 

 which two blood cells appear. X 580. 



consists of a well-defined pulsating region or heart, large carrying vessels, 

 capillary and lacunar peripheral parts as well as the easily studied blood 

 glands. We shall begin with a study of the lacunar spaces (see Fig. 67). 

 These can be found in many parts of the body, and serve to exhibit a case 

 where the blood vessel is shown in its true light, as a cleft between masses 

 of the connective tissue 

 whose cells constitute its 

 walls. 



The Leidig's connective- 

 tissue cells which border on 

 these lacunae exhibit hardly 

 a trace of differentiation. 

 The single peculiarity which 

 they show seems to be a 

 slight cuticular formation on 

 the surface which they 

 present to the blood. In 

 the arteries which carry blood into the sinuses, a difference exists 

 which represents the greatest differentiation of the connective-tissue 

 walls (Fig. 140). The contiguous lining cells are smaller and have 

 acquired a proximo-distal striation, which is thus at right angles to 



the vessel's surface. They 

 also secrete a cuticle of 

 some thickness, and this 

 cuticle most probably is 

 elastic during life. It is 

 irregular, in that it con r 

 tains thickened ribs which 

 run in the direction of the 

 vessel's course. 



Those cells lying just 

 outside of the epithelium 

 layer are also differenti- 

 ated. They have devel- 

 oped a great profusion 

 of circular connective-tissue fibrils that hide the cytoplasm and among 

 which the nuclei are, at first, difficult to see. These fibrils are either 

 elastic or their curled and twisted arrangement permits of a spring-like 

 elasticity, even if they are not elastic. Some of the veins show condi- 

 tions which are intermediate between this and the sinus. A few of 

 them, in particular positions, show a flat epithelium-like arrangement 

 of the lining cells and a development of very fine fibrils directly in this 

 first layer (see Fig. 146). 



c.i. 



conn. t.l. 



FIG. 140. Part of a transection of the wall of a lobster's 

 artery; conn.t.l., connective-tissue layer; c.l., cell lining 

 on which appears the cuticle (CM.). X 700. 



