Progressive development of the liver in the animal series.—The liver in its simplest condi tion is a mere inflection of the mucous lining of the alimentary canal, forming a small ccecal recess or follicle. The capillary vessels rami fying upon the parietes of this follicle pour their secretion upon its internal surface, and it is thence conveyed to the alimentary canal to be mingled with the ingesta. In this its most rudinientary form the liver would appear to be present in the Laginella, a small cilio-brachiate polypus described and figured by Dr. Arthur Farre.* Upon the stomach of the Laginella are seen several minute caeca which open into its cavity ; they are usually empty when the animal has been for some time without food, but become filled with a brownish fluid after a meal. The next most elementary form of the hepatic ccecurn is seen in the single lengthened follicle discovered by Owen in the ascaris ha licoris. This follicle opens into the alimentary canal at about one-third from its oral extremity. Among the Annelida, as in the medicinal leech (fig. 69, vol. i.) the liver is represented by numerous simple ccecal pouches appended to each side of the digestive canal. The next step in the complication of the organ is ob served in the lengthened filiform tubuli which are connected with the sides of the canal in the Aphrodita. These are narrow and constricted at their commencement, dilating gradually as they proceed farther from the intestine, and terminating by a small oval sac. In other species of the same genus and in the Areni cola (fig. 70, vol. i.) they display a tendency to ramify, by developing small ccecal pouches from their sides. In these terminal sacculi Pallas discovered a " bitter fluid, of an olive brown or greenish-black colour," which he conceived to be the juices of marine plants which had gained admission into the tuhuli through their openings of communication with the intestine, but which, it is more than pro bable, was the proper biliary secretion of the tubes themselves. In the class Insfeta the hepatic coca vary in progressive development from the simple vesicular dilatations observed upon the digestive canal of the Lampyrus splendidula, or the simple ccecal tubulus of the carnivorous Cicindela, to the numerous ccecal follicles of the Dytiscus, or to the more length ened tubuli of the Blatta orientalis. Throu;11 out the whole of the class the character of the liver is tubular, the development and extent of the tubuli depending upon peculiarities in the food or habits of the animals. In Arachnids, the ccecal follicles are short, and terminate at their extremities in a cluster of numerous rounded vesicles, which give' to the organ a lobulated appearance. They are seen in the Scorpion, in fig. 83, c, c, page 204, vol. i. In the classCrustacea, the hepatic organ assumes a higher and more complicated character; the simple ccecal follicle of Insecta becomes branched and ramified, of which we have a good example in the Argulus foliaceus, deli neated by 117iiller. In the Astacus fluviatills (fig. 214, page 483, vol. i.) the hepatic fol licle is more branched than in the Argulus; and in the Pagurus striatus (fig. 215, page 484, vol. i.) the liver is composed of an extraordinary assemblage of ramified follicles. In the hepatic organ of the Squilla mantis we perceive a remark able transition from the simple branched and miffed follicle of the lowerCrustacea to the forms of the organ in the inolluscous el Upon the exterior it is lobulated, and ea lobe is composed of a congeries of minor 1 bules which appear like granulations upon its surface. Examined in its interior it presents a primary dilated sac of considerable size, from which branch off a number of secondary sa of smaller dimensions, and these latter studded over every part of their surface w minute meal follicles of a rounded form. the subregnum illollusca the liver is of la size, and approaches in external form to t solid and lobulated organ of vertebrata. In internal conformation we may still trace amon the lower classes a close analogy with the ran fled tubuli of Articulata. Thus in the cla Gasteropoda the gland is composed of meal pouches, which divide and subdivide into smaller and smaller follicles and terminate in small dilated sacs. They may be compared in their disposition to the stem, branches, twigs, and fruit of a cluster of grapes. A liver of this kind is seen in the helix pomatia. In the Murex triton the follicular structure of the organ would appear to be lost. The ex ternal surface presents a lobulated form, but the interior is composed of a delicate spongy tissue, consisting of larger and smaller ails, which may all be inflated from the excretory duct. This seeming difference in the structure of the organ is, however, more apparent than real, for the numerous cells may he considered as so many follicles from which smaller fol licles are developed. The cellular character of the organ depends upon the more extensive subdivision of the follicles, their assemblage in greater numbers, their consequent compres sion, and the adhesion of their parietes. In the Sepia family the spongy structure of the hepatic organ is still more distinct. It is chan nelled into numerous canals, from which smaller canals branch off in various directions; from these branches cells are developed, and the parietes of the cells are every where surrounded by smaller and smaller cells, the entire texture being very similar in arrangement to the cel lular lung of the higher reptilia.
The liver in Vertebrata is more close and complex in its structure and less amenable to the observations of the anatomist than in the inferior series. We ohserve nothing, even in
the lowest fishes, which hears any direct com parison with the cellular structure of the liver of Cephalopoda. The general character of the organ in fishes is loose• and flabby, shewing that, although difficult to demonstrate, its in ternal texture evidently contains numerous tu buli. If the efferent duct of the liver of a fish be inflated, the whole organ appears dis tended ; hence we might infer that the primi tive structure of the organ is precisely the same, consisting in the ramifications of the hepatic tubuli or ducts, the increased wants and higher position of the animal demanding an augmented extension of surface. This is the great principle in the development of all glandular organs—extension of surface. The simple follicle is sufficient for an animal so low in the scale as a cavitary entozoon, but as the functions of the animal increase, its simple fol licle must be extended to a greater length, or branched or ramified ; and as high in the ani mal scale as the Vertebrata these subdivisions have attained so great a degree of minuteness that they are demonstrable to the practised eye only through the aid of the highest microscopic powers.
Muller arranges the glandular system into simple and compound glands. The former he divides into two groups: 1. " simplest glands," which " are mere recesses of greater or less dimension in the surface of a membrane;" and 2. " more complicated forms," in which se veral of the recesses are assembled together and open by so many distinct mouths, or they unite and form a common duct which termi nates by a single opening. The " compound glands" he likewise subdivides into two groups: 11. those which " ramify with a certain degree of regularity, the principal trunk giving off branches laterally at certain intervals, these sending out in the same way side branches, which in their turn afford a third set." This disposition constitutes lobulated glands, and is the type of conformation of the liver in Inver tebrata. 2. " The second group of the glands with ramified secreting tubes consists of those in which the ramification is irregular, and in 'which there is no division and subdivision of the gland into" secreting " lobules. The liver of Mammalia belongs to this group." The form of the liver in _Fishes corresponds with the direction of the long axis of the body ; thus, for instance, it is elongated, and con sists of a single lobe in the eel, while in the skate it is broad and extends into each lateral half of the abdominal cavity. In other fishes it is variously divided into lobes, and is often placed altogether on the left side of the body. In the class Aniphibia, the liver-efts° corresponds with the form of the body of the animal: in the frog it is short and divided into two primary lobes and several lobules ; in the lengthened forms it is long and less divided. In the class Reptilia the liver is large, and bears an equal relation to the form of the visceral cavity. It is long and undivided in Ophidia, and short and divided into a right and a left lobe in Sarnia and Che lonia, the two lobes being spread out over the intestines. In Birds there is great uniformity in the form and size of the liver. It is smaller in proportion to the bulk of the body than in Reptilia and Fishes, and larger than in Marn malia. It is situated in the middle line of the visceral cavity, and receives the heart into a depression upon its under surface. In the class illammalia the liver is very much reduced in size, and is more compact and firm than in the lower vertebrata. In animals with simple stomachs it is situated in the middle line of the abdomen. In others, with large or compound stomachs, it is pressed towards the right side. The number of lobes does not depend upon a greater or less division of the liver into parts in accordance with the activity and mobility of the animal, but obeys a law in the animal economy, by which new parts are superadded in proportion to the increase of the wants of the creature. PrIan is placed at the foot of the scale in the progressive complication in exter nal form of the liver of vertebrata ; the entire organ may be considered in him as a central lobe, the lobos Spigelii being the rudiment of a second or right lobe. The liver of the ourang offers the same character. Ruminants have also a liver which presents the most rudimen tary form of division. The liver of man is the type of the central or principal lobe, to which are added upon each side, in the animal scale, a right and a left lobe, and from these latter are developed a right lobule and a left lobule. This most complicated form of liver, consisting of five lobes, is met with among Carnivore and Rodentia; and throughout Mammalia, the suc cessive additions and subtractions from this normal type form a constant and generic cha racter. Besides this real division of the liver into five lobes, fissures of various depth are constantly met with, as in man, which give the appearance of a much greater subdivision. These secondary portions are to be looked upon as the mere results of separation, and have no relation with the primitive type. A most ex traordinary form of liver is met with in a small rodent animal from Cuba, the Caprornys, in which the whole surface is divided by deep fissures into small masses of a triangular and quadrangular form, like the kidney of a bear.