The Digestive System 169 
It is composed of a fairly dense mass of connective 
tissue, mainly elastic fibers, through which are 
scattered small blood-vessels, bv, and small dark 
areas, mb, that are appar- 
ently longitudinal bundles 
of involuntary muscle fib- 
ers. These few and scat- 
tered fibers probably 
represent the muscularis 
mucosa that is so well 
developed in the poster- 
ior region of the cesoph- 
agus. 
Outside of the mucosa 
is a thick circular layer of 
involuntary muscle fibers, 
cm, the fibers being col- 
lected into irregular bun- 
dles, between which are 
narrow spaces filled with 
connective tissue that con- 
tains a few small blood- 
vessels. 
Surrounding the circu- 
Fic. 41. A transsection 
through the anterior region of the 
cesophagus of the hibernating 
animal under low magnification 
bv, blood-vessels; mb, muscle 
bundles; other letters as in Fig- 
ure 42. 
lar layer is a thinner and less clearly defined layer 
of longitudinal muscle fibers, Im. The muscle 
bundles are more definite than in the circular layer 
and are separated from each other by a consider- 
able amount of connective tissue with a few small 
blood-vessels, 
