STOMACH AND INTESTINE. 



295 



The reader will, however, observe, that the 

 title of the following article does not announce 

 an essay on the process of digestion, or the 

 various organs which effect it; but limits itself 

 to two portions of the alimentary canal, 

 hitherto un described in this work. But it is 

 impossible to treat of the functions of the 

 stomach and intestine except in connection 

 with the entire process in which they take so 

 large a share. While the numerous observa- 

 tions and researches which have been- made 

 since the appearance of the earlier article 

 DIGESTION require some notice in the 

 Supplement of which the present essay forms 

 a part. For these reasons the author has felt 

 it advisable not to confine himself too strictly 

 to the exact limits which the heading " Sto- 

 mach and Intestine" might seem to imply. 

 Hence, though the following essay will treat 

 chiefly of the above segments of the alimentary 

 canal, it will also comprise a very brief account 

 of whatever is at present known concerning the 

 whole digestive act. Commencing by a routrh 



O J 



sketch of the anatomy of these parts in the 

 animal kingdom, we shall successively consider, 

 their structure and functions in the human 

 subject ; their relation to digestion and nutri- 

 tion ; and finally, their appearances in disease. 



COMPARATIVE ANATOMY. In the Infuso- 

 ria, whose minuteness places them at the 

 lowest extremity of the animal kingdom, the 

 organ of digestion has already attained such a 

 development as to form the chief basis of 

 their nomenclature. 



One or two genera present us with a rare 

 and exceptional condition : viz. the absence 

 of all traces of digestive cavity. Such are the 

 parasitic Gregarina and Opalina ; in whom, 

 as in some of the Entozoa, digestion and 

 absorption appear reduced to a simple phy- 

 sical process of endosmose, which carries 

 the nutritious substances dissolved in the 

 fluid medium they inhabit at once into the 

 mass of their corporeal juices. 



The Pulj/grtstria possess a plurality of 

 stomachs or internal sacs ; and the relations 

 of these to the intestine, together with the con- 

 dition of the latter tube, subdivide this group 

 into numerous families and genera. Thus 

 many are named "anenterous," because they 

 appear to be devoid of intestine. Of these the 

 Fionas tcrmo which has four or five globular 

 stomachs, of aurnro*' 1 or " an mcn m diameter, 

 appended immediately to its mouth may be 

 taken as the type. Others possess similar 

 sacs in connection with a simple Intestine ; 

 and are chiefly distinguished by the straight, 

 curved, or wavy course of this canal, or by 

 the single or double character, and lateral or 

 terminal position, of its apertures. Most of 

 them devour a living prey of kindred Infuso- 

 ria; prehension being often visibly effected 

 by cilia, the voluntary action of which 

 carries a current of food into the mouth, or 

 removes egesta by a simple reversal of the 

 stream. And sometimes this act of ingestion 



ordinarily sufficient, are capable of being locally 

 exhausted by the excessive demands of a particular 

 class or species, and renewed by an artificial supply. 



is aided by a dental apparatus, in the shape 

 of a hollow cylinder enclosing long teeth, 

 as in the genus Napula. 



The Rotifera are so named from the cur- 

 rents produced by their prehensile cilia ; 

 which are here limited to groups surrounding 

 the mouth of the animal. 



Many of them have an organ of mastica- 

 tion. This usually consists of three pieces : 

 each of the two facets of a kind of anvil being 

 worked upon by the rough or toothed terminal 

 surface of a recurved jaw, the longer limb of 

 which receives a muscle at its extremity. 



The intestinal canal generally exhibits a 

 pharyngeal enlargement, which is followed 

 by a narrow "oesophagus," of varying length, 

 ending in a wider " intestine." In the 

 Gasterodeln a dilatation, called a stomach, pre- 

 cedes the intestine. In the Rotifer vu/garis 

 and others, an almost globular enlargement of 

 the narrow canal is so immediately followed 

 by the constricted cloaca, as to have been com- 

 pared to a large intestine. The organ of 

 digestion is also often complicated by the 

 presence of blind tubes ; which vary, not 

 only in number and size, but also in posi- 

 tion, and possihly in import. Thus they may 

 open, either into an uniform and narrow canal, 

 or into the commencement of the intestine, 

 or into the presumed gastric dilatation ; or, 

 finally, as in the Diglcena lacustris, a set of such 

 tubes may occupy both of these latter situa- 

 tions. 



The various members of the order Entozoa 

 are grouped together in obedience to a classi- 

 fication which is here and there arbitrary and 

 anomalous, but in the main both natural and 

 useful. It offers three chief varieties of the 

 digestive organ, all of which are very inte- 

 resting. 



In many as in the Ech'mococd 



a. 



and their congeners no trace of a special 

 digestive cavity is present. Without mouth, 

 stomach, or intestine, the creature floats free 

 in the cavity of its enclosing cyst, or buries 

 its barbed head in the tissues of a living 

 habitation; whose juices, thus brought into 

 relation with its exterior, are applied to its 

 nourishment by what seems to be rather a 

 proct-ss of endosmose than of digestion pro- 

 perly so called. 



/3. In other genera belonging to the Cestoid 

 and Trematoid divisions, there is, however, 

 a canal, which is apparently related to digestion, 

 and the main features of which repetition 

 and ramification may be represented by the 

 Tiema and Distoma respectively. 



For example, in the Tape- worm, a minute 

 mouth opens into a slender tube, the bifurca- 

 tions of which reach the margins of the body 

 where this begins to assume its regular jointed 

 form. From hence onwards the canal might be 

 compared to a ladder, with rungs at the fore 

 and aft extremity of each joint, at the 

 right angles of which its longitudinal and 

 transverse branches unite. It is probable that 

 these canals possess valves. But whether 

 they have any constant or valid terminal ori- 

 fices seems doubtful. 



u 4, 



