This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1928). The stomach of the Crocodile is remarkable as affording another among the innumerable instances that might be adduced of that gradual transition everywhere observable as we pass from one class of animals to that which next succeeds it in the series of creation. The Crocodile is the connecting link between Reptiles and Birds, and in almost every part of its body it presents a type of structure almost intermediate between the two.
Fig. 339. Stomach of the Crocodile.
(1929). The stomach of this creature (fig. 339) might, in fact, be almost mistaken for the gizzard of a rapacious bird. The oesophagus (c) terminates in a globular receptacle, the walls of which are very muscular; and the muscular fibres (a) radiate from a central tendon (6) precisely in the same manner as those of a bird. The pyloric orifice is closely approximated to the termination of the oesophagus, and the commencement of the duodenum dilated into a round cavity (d), - an arrangement which, as we shall see in the next chapter, exactly resembles that met with in the feathered tribes.
(1930). In the neighbourhood of the pylorus, the walls of the stomach in all the Reptilia become perceptibly thickened; the intestine is generally short, and usually divided into two portions, representing the small intestines and the colon, the division between the two being marked by a prominent valve analogous in function and position to the ilio-colic valve in the human subject; and sometimes, moreover, as for instance in the Iguana, there is a distinct caecum developed at the commencement of the large intestine.
(1931). The auxiliary secretions subservient to digestion in the class before us are the salivary, the hepatic, and the pancreatic.
(1932). The salivary glands are of very peculiar construction *. In the Cheloistan, the Saurian, and the Batrachian orders, the substance of the tongue seems to be principally made up of a thick glandular mass, formed by a multitude of little tubes united at their bases; but, becoming separate towards the surface of the tongue, they give the whole organ a papillose or velvety appearance. This glandular apparatus rests immediately on the muscles of the tongue, and upon its sides a multitude of pores are visible, through which the salivary secretion exudes.
(1933). In the Ophidian Reptiles, from the manner in which they swallow their prey, the bulk of the tongue is necessarily reduced to the utmost extent; the whole organ seems converted into a slender bifid instrument of touch, and is covered with a delicate membrane. Instead of the salivary apparatus described in the last paragraph, two glandular organs (fig. 337, s s), placed immediately beneath the skin of the gums, surround the margins both of the upper and lower jaws; and from these an abundant salivary secretion is poured into the mouth, through orifices situated externally to the bases of the teeth.
(1934). The liver of Reptiles (fig. 340, h) requires no particular description: its secretion, as well as that of the pancreas (fig. 340, o), is poured into the intestine in the usual manner, at a little distance from the pylorus.
(1935). The spleen and system of the vena portce are disposed in the same manner as in other Vertebrata. The spleen (fig. 340,1) is generally more or less closely connected with the stomach; and the large vein derived from it, being joined by those proceeding from the other viscera of the abdomen, forms the trnnk of the portal vein (m), which soon divides again into numerous branches that ramify in the substance of the liver.
* Cuvier, Lecons d'Anatomie Comparee, iii. p. 223.
(1936). The lymphatic and lacteal systems are very important parts of the economy of these creatures; and, from the large size of the absorbent vessels, their disposition is more easily traced in the class before us than in any other. The principal trunks surround the aorta and other large blood-vessels, and communicate very extensively with the veins in different parts of the body. From the imperfect condition of the valves in their interior, the lacteals of many tribes may be readily injected from trunk to branch, and when thus filled with mercury, they are found to spread out between the coats of the intestines like a dense network of silver.
(1937). But the most remarkable circumstance connected with the absorbents of this class of animals is the discovery, made by Professor Muller of Berlin *, of a system of lymphatic hearts destined to propel the products of absorption from the chief lymphatic trunks into the veins. In the Frog, four of these pulsating cavities are easily displayed by simply raising the skin covering the regions of the body where they are situated. The posterior pair of hearts are appendages to the lymphatic trunks which convey the absorbed fluids derived from the hinder extremities into the ischiadic veins; they are situated on each side midway between the extremity of the long bone which represents the os coccygis and the hip-joint, and are placed immediately beneath the integument. They each consist of a single cellular cavity, and pulsate regularly; but their pulsations are quite independent of those of the heart, neither are the contractions of the two lymph-hearts synchronous with each other.
(1938). Another pair of these contractile cavities is situated beneath the posterior margin of the scapula, close to the transverse process of the third vertebra: this pair forces the contents of the lymphatics of the anterior portions of the body into the jugular veins.
(1939). Fishes respire water by means of gills. Reptiles, breathing a lighter medium, are provided with lungs - membranous bags, into which the external element is freely admitted, and again expelled in a vitiated condition, its oxygen having been employed in renovating the blood, which circulates in an exquisite network of delicate vessels that ramify in rich profusion over the walls of the pulmonary chamber.