STRUCTURE OF THE INTESTINES. 1011 
arrangement of this coat will be given with the description of each division of the 
intestinal tube. 
2. Muscular Coat (tunica muscularis).—This consists of unstriped muscle arranged 
in two layers—an outer, in which the fibres run longitudinally, and an inner, in which they 
are circularly disposed. The muscular coat is thicker in the duodenum than in any other 
part of the small intestine, and it gradually diminishes in thickness until the end of the 
ileum is reached. On the other hand, in the large intestine, it is thickest in the rectum 
and thinner towards the beginning of the colon. 
The longitudinal layer (stratum longitudinale) of this coat is much thinner than the 
underlying circular layer. In the small intestines it forms a complete sheet, continuous 
ull round the gut (Fig. 683), but thickest at its free margin ; whilst in the large intestine 
it is divided up into three longitudinal bands (Fig. 692), known as the teend@ coli, which 
will be more fully described in connection with the colon. 
The cireular layer (stratum circulare), much thicker than the longitudinal layer, is 
composed of bundles of muscular fibres arranged circularly round the tube (Fig. 683), and 
forming in all parts a continuous sheet. Unlike the longitudinal fibres, those of the 
circular layer take part in the formation of the pyloric and ileo-czcal valves. 
3. Submucous Coat (tela submucosa).—This is a loose but strong layer of areolar 
tissue connecting the muscular and mucous coats, on which chiefly depends the strength 
of the intestinal wall. In addition to forming a bed in which the vessels break up before 
entering the mucous 
coat, it contains the 
glands of Brunner in 
LIEBERKUHNS GLANOS : 
= TARY Xe, 
Lieberkiihn’s 
ANA ae 
7: PARABLE ARIA? gland 
the duodenum (Fig. ae ae Masoulanis 
684), and in both = ~  TUCOseE 
: ; : =e 4 @ Brunner’s 
small and large in- Submucosa — == — glands 
testine the bases of 
the solitary olands lie Circular a Circular ae 
fae heat “684 muscular fibres muscular fibres 
Mm ii a Es t Longitudinal SI = Bie ——— pone iviaina) 
5 muscular fibres — ee ——<— = muscular fibres 
* eo ani ° 7 LARGE INTESTINE © IS DUODENUM x12. Peritoneum 
(cumica mucosa)— ;Feritoneum™: 
The mucous mem- 
brane constitutes the 
inner coat of the 
intestine, on which 
its digestive functions 
depend. It is every- 
where composed (Fig. 
684) of the following  S"bmucoss= 
parts :—(1) A layer S 
of striated, columnar, Geeaies 
epithelial cells, rest- muscular fibres” 
ing on (2) a basement Tongitedieel 
membrane. Outside muscular fibres 
this lies (3) a layer of ares 
| retiform tissue, eal Fig. 684.—Dr1acram to show the structure of the small and large intestine 
taining a considerable and the duodenum. 
number of scattered , 
lymphoid cells. This layer is limited towards the submucosa by (4) an extremely thin 
sheet of unstriped muscle, the muscularis mucosa, which is not visible to the naked eye. 
The mucous membrane is extremely vascular, particularly in the small intestine. It is 
thicker in the duodenum than in the jejunum, and thicker in this latter than in the 
ileum. 
Throughout both the small and large intestines the substance of the mucous membrane 
is closely beset with innumerable small (microscopic) tubular glands, known as the glands 
or follicles of Lieberkiihn (glandul intestinales). In shape they are minute straight 
tubes—like diminutive test-tubes—with their mouths opening on the surface, their closed 
ends lying in the deeper part of the mucous coat, and their cavities lined by columnar 
epithelium. They open on the surface between the villi of the small intestine, and 
are present also on the valvulze conniventes. In the large gut their orifices are found all 
over the surface of the mucous membrane. 
Villi 
Lieberkiihn’s 
gland 
Blood-vessels 
- forming net- 
~~ work in sub- 
_ mucosa 
~ Blood-vessel 
SMALL INTESTINE X 20 
Certain special developments of the mucous coat, found in particular regions of 
