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HISTOLOGY 



int 



muscle fiber to points of attachment in all directions, especially on the 

 sides of adjacent fibers. They do not appear on all fiber ends in 

 the figure because many are but apparent ends, due to oblique cutting. 

 Considering the connecting fibrils found on the surface of smooth muscle 

 cells in other animals, it is probable that these fibrils and much of the 

 other fibrillar material lying between the muscle cells are products of 



those cells and 

 not of separate 

 connective-tissue 

 units. 



The muscle 

 nuclei are very 

 large and lie on 

 the sides of the 

 fibers. In this 

 respect they 

 differ from the 

 more familiar 

 smooth muscle 

 fibers of mam- 

 mals. They are 

 oval in form 

 and, like other 

 muscle nuclei, 

 have but little 

 chromatin and a 

 small nucleolus. 

 Outside of 

 the thick layer 

 of circular mus- 

 cle conies the layer of longitudinal muscle. It is separated from the 

 circular layer by a region of connective tissue. Where the vessel 

 traverses a cavity, either internal or external, the characteristic lining 

 of that cavity is reflected over it and forms a layer external to the 

 longitudinal muscle. The smaller vessels lose the outer layers until, in 

 the capillaries, their only wall is a single endothelium showing evi- 

 dences of its connective-tissue origin (Fig. 139). 



The blood vessels of the Arthropoda are peculiar in several ways. 

 Perhaps their most marked characteristic is a single connective-tissue 

 epithelium that forms their chief wall. Also the fact that they are lined 

 with an inner cuticle. We shall study one example in a crustacean and 

 another in an insect. 



The blood-vessel system of the lobster is a good one to examine, and 



FIG. 138. Transection of part of an artery wall of Octopus, int., 

 intiina, consisting of a thin layer of cells lying on a thick homoge- 

 neous membrane. A muscle layer consisting of circular cells 

 whose stout muscle-fibril bundles (fibers) show the peculiar, irregu- 

 lar striation found in mollusks ; mus.n., muscle cell nucleus ; conn. 

 Ji., connective fibrils at end of a fiber ; l.mus., longitudinal muscle 

 fibers. X 700. 



