



Millimeters 



0.5 



Millimeters 



0.5 



B 



Figure 141. — Cross sections of the two portions of the adductor muscle of C. virginica. A — white or opaque part. B — 

 translucent part. The muscle bands of the white part are more compact and are surrounded by tougher connective 

 tissue than thase of the translucent part (right). Bouin, with formalin hemato.xylin-cosin., 



the dorsal side adjacent to the pericardiutii. This 

 portion is frequently called the vitreous or dark 

 part. The remainder is crescent-shaped and an 

 opaque milky-wdite. The fibers of this part are 

 tougher than those of the translucent portion; 

 the difference shows clearly when the nuiscle is 

 being cut or teased. 



The fibers of the adductor muscle form dense 

 bands surrounded by connective tissue. In a 

 cross section examined under a low-power micro- 

 scope (fig. 141) the bands appear as separate units 

 packed more or less parallel to one another. This 

 arrangement is less pronounced in the translucent 

 part (fig. 141, right). The tissue that surrounds 

 the muscle bands is better developed in the opaque 

 section. A layer of connective tissue separates 

 these two major parts of the adductor. 



Connective tissue provides a framework for the 

 muscle. Individual fibers do not run the full 

 muscle distance between the two valves; they are 

 anchored at one or both ends in the sheets of 

 tissue which surround the bands. A very thin 

 membrane, called endomysium, invests each mus- 

 cle cell; the sheathing around the bauds of cells is 



epimysium ; the septa which radiate fi"om the latter 

 form perimj'sium. 



The cross-sectional areas and the weight of the 

 two portions vary in different specimens. It was 

 reported by Hopkins (1930) that the ratio of 

 weight of the translucent to the white part of the 

 muscles of oysters growing near Beaufort, N.C., 

 depends on ecological conditions. In the oysters 

 found at the upper limit of their vertical distri- 

 bution near the high-water level the ratio was 1.26, 

 while in the oysters taken at a level 2.5 feet lower, 

 where they were submerged during about three- 

 quarters of the time, the ratio was 2.51. 



The entire adductor muscle is well supplied with 

 blood; wandering leucocytes are usually seen 

 between the fibers and in the connective tissue. 

 Both parts of the adductor muscle are abundantly 

 supplied with nerves. The innervation of the 

 muscle is discussed in Chapter XII. 



MICROSCOPIC STRUCTURE 



The muscle fibers of the two parts of the adduc- 

 tor differ in both size and structure. The white 

 muscles are smooth and wide, while the dark 



THE ADDUCTOR MUSCLE 



153 



