68 THE AMERICAN MONTHLY [April, 



members of the muscular layer. The connective tissues which are present in 

 all the organs of complex histology is a most difficult tissue of which to study 

 the cellular structure, and we can very properly set it aside, for the present, to 

 examine it in a place where it can be studied more favorably than in the 

 present situation. 



5. The muscles of the mucous lining will detain us but very shortly. 

 They will be found, by examination of the mucous lining, scattered through 

 the sub-mucous layer in no very definite pattern, except that they seemingly 

 pull the mucous lining this way or that. They are shown in both fig. 2 and 

 fig. 4, and are of the same histological structure as the inner muscle coat, 

 namely, transversely and longitudinally striped bands. They further seem, in 

 some places, to run into and join the bundles of the longitudinal layer. 



Summary . — We have now reached a position where we can see how^ the 

 statement as to the greater histological complexity "of the intestine, as compared 

 with the liver, is true. Examination of the liver, and thoughtful consideration 

 of its mode of composition, shows it to be made by the repetition of one kind 

 of cell, varying in shape, but of one kind. If the intestine be thought on, it 

 is found to be constrvicted by the repetition of four kinds of cells, all very dif- 

 ferent, viz., epithelium cells, striped muscle cells, unstriped muscle cells, and 

 connective tissue cells. Further reflection will show that the epithelium 

 of the intestine corresponds in position as well as in general in shape with 

 that of the liver, being situated upon a basement membrane which intervenes 

 between the cells and the blood and lining a cavity which opens to the out- 

 side of the body. But a section at the proper place would further prove that 

 the epithelium of the liver duct is continuous without a break anywhere with 

 that of the intestine. We, therefore, are justified in regarding the mucous 

 membrane of the intestine as a gland strictly comparable with the liver or the 

 green gland, while the other parts of the intestine are additional to anything 

 present in the other two organs. It is for this reason that the intestine is his- 

 tologically more complex. The other parts are to cause movements of the 

 glandular part, or to keep the contents of the intestine from standing still, 

 thus bringing all parts of the food in contact with the glandular part. 



We may now revise our definition of a tissue as a structure composed of an 

 assemblage of cells similar in structure and use, and ^ve find the mucous mem- 

 brane a tissue as tried by this definition, and the outer muscular coat a second, 

 and the longitudinal muscular coat a third. If we made a complete enumer- 

 ation of the tissues which work together for a common purpose in this organ, 

 the intestine, we should include the parts of the nervous tissues. They are 

 present in the intestine, but not visible by our mode of treatment, and will be 

 adjourned for the present. 



We have now studied two kinds of tissue, the glandular and the muscular ; 

 several others yet remain, the nerves and the special apparatus, end-organs 

 for the special senses, the skin and protective tissues, the connective tissues, 

 and the reproductive tissues. Some or all of these we hope to be able to con- 

 sider in subsequent chapters. 



{To be continued.) 



Explanation of Plate in February Number. 

 Fig. I.— Cross section of intestine magnified x 50 di- ^^^ outlines drawn -with camera lucida. 



ameters. d d ► i. 



Fie 2 — Section showing mucous membrane with cuti- c. m. Basement membrane. 



cle and mucous muscles and sub-mucous connec- c. Cuticular part of mucous lining. 



live tissue cells. Camera lucida magnified x 370 C. M. Circular muscle layer. 



diameters *--• ■'■ ■ Connective tissue cells. 



Fie Q —Semi-diagrammatic camera lucida drawing of L. m. Longitudinal muscular layer. 



mucous membrane highly magnified (Zeiss F) L. w. Side wall of mucous epithelium cells. 



showing cuticle removed from the cells. Magni- M. ra. Mucous membrane. 



fied X 690 diameters. M- "uc. Muscles of mucous layer. 



Fig. 4.— Longitudinal section of entire intestine wall, M. nuc. Muscle nuclei. 



showing outer muscle cells endways and inner coat S. m. Sub-mucous layer. 



longitudinally. Magnified by zoo diameters. 



