Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/250

232 DIGESTIVE ORGANS hepatic omentum between its two layers, having the hepatic artery to its left, and the portal vein behind it. It then inclines behind the duodenum to the inner side of its descending part, where it conies into relation with the pancreatic duct. The two ducts then run together in an oblique direction through the wall of the duodenum, and open on the summit of a papilla, by a common orifice, about the junction of the descending and transverse portions of the duodenum. ANCBEAS. The PANCREAS is an elongated gland which lies in relation to the posterior wall of the abdomen, in front of the first lumbar vertebra, and extends obliquely from the right lumbar region through the epigastrium into the left hypochondriac region. It is from 6 to 8 inches long, and whilst its dilated right extremity, or head, occupies the horse-shoo curve of the duodenum, and is attached by areolar tissue to the descending and transverse portions, its attenuated left extremity, or tail, is in relation to the spleen. A prolongation of the gland, named the accessory or lesser pancreas, usually surrounds the superior mesenteric artery at its origin. Structure. The pancreas is one of the compound racemose glands, and resembles generally in structure the mucous and salivary glands of the mouth and the glands of Brunner (fig. 5). It is sometimes called the ab dominal salivary gland, and its secretion flows into the duodenum, and assists in the process of chylification. It has a yellowish creamy colour, and is divided into distant lobules by septa of connective tissue. The excretory duct, or duct of Wirsung, is completely surrounded by the lobules, and extends from the tail to the head of the gland, receiving in its passage the numerous secondary ducts, and increasing gradually in size. It leaves the head of the gland, comes into relation with the common bile duct, and with it pierces obliquely the posterior wall of the descend ing part of the duodenum, to open by a common orifice about the junction of the descending and transverse portions. Sometimes the duct from the accessory part of the pancreas opens independently into the duodenum, a little above the common hepatico-pancreatic orifice. The finest ducts within the gland terminate in the acini, or gland-vesicles, of the lobules. These acini contain the secreting cells, which have a somewhat cubical form. The ducts are lined by a columnar epithelium, and mucous glands are situated in the mucous membrane lining the duct of Wirsung. The pancreas receives its supply of blood from the splenic, superior mesenteric, and hepatic arteries. Its veins join the splenic and superior mesenteric veins, and through them contribute to the formation of the portal vein. Its blood capillaries are abundantly distri buted on the walls of the gland vesicles. Lymph vessels are found in the connective tissue between the lobules. The nerves are derived from he solar plexus, and accom pany the arteries. BETH, THE TEETH. The teeth are calcified organs developed in connection with the mucous membrane of the mouth. Their primary use is that of biting and grinding the food ; but in man they serve as aids to speech, and in many animals act as instruments of offence and defence. Arrangement and Form of the Teeth. Teeth are present in the greater number of the Mammalia, in which class they are implanted in sockets in the alveolar arches of the bones of the upper and lower jaws, and form only a single row in each arch. In a few mammals, as the toothed whales and the sloths, only one generation of teeth is produced, and when these drop out they are not replaced by successors ; these animals are called Monophyodont. In the majority of the Mammalia, however, there are two generations of teeth, a temporary or milk set, which are deciduous, and are replaced by a permanent or adult set ; these animals are called Diphyodont. But in speaking of two generations of teeth it is not to be supposed that all the teeth in the adult jaw have had temporary predecessors, for the molar or back teeth have only a single generation. A few mammals, as the toothed whales, have the teeth uniform in size, shape, and structure, and are named Homodont ; but, in the majority of the Mammalia, the teeth in the same jaw vary in size, form, and structure, an:l they are therefore called Ileterodont. In every Heterotlont mammal, possessing a complete dentition, four groups of teeth are found, which are named incisor, canine, premolar, and molar teeth. Each of these teeth possesses a crown, which projects into the cavity of the mouth, and a fang lodged in the socket in the jaw ; at the junction of the crown and fang there is usually a constriction named the neck of the tooth. In man the dentition is FIG 12 ^ A hun , an , lpper inci;0r Diphyodont and Heterodont. tootii. <-, the crown; n, neck;/, mi i t . .1 r, t le fng. 2i a section through a Ihe single row of teeth in each m , lar t o 0th . <,, cap of enamel; c, alveolar arch of the human cement; d, dentine; # pulp cavity. jaw is characterized by the crowns of the teeth being of almost equal length, and by the absence of any great interspace, or diastema, between the different teeth, or of irregularities in the size of the interspaces, so that the teeth form, an unbroken series in each jaw. The span of the upper dental arch is slightly bigger than that of the lower, so that the lower incisors fit within the upper, and the lower molars, being inclined obliquely upwards and inwards, are somewhat overlapped by the upper molars. The upper and lower dental arches terminate behind in line with each other, and the teeth are equal in number in the two jaws. Man possesses 32 teeth in his permanent dentition, arranged in four groups, viz. 8 incisors, 4 canines, 8 pre- molars or bicuspids, and 12 molars. The number and arrangement of the permanent teeth in the two jaws is expressed in the following formula : pm. 2 pin. 2 , = 32. Man possesses only 20 teeth in his milk or temporary dentition, and their arrangement is expressed in the follow ing formula : In. 2 ,-20. If the temporary and permanent formula; be compared with each other, it will be seen thut, while the incisors and canine teeth correspond in numbers in both dentitions, in the temporary dentition there, is an absence of premolars, and the molar teeth are only eight, instead of twelve, in number. The characters of the permanent teeth will now be considered. The incisor teeth, eight in number, are lodged in the front of the jaws, two on each side of the mesial plane. The upper incisors project downwards and forwards, the lower are directed almost vertically upwards. The oblique direction of the upper incisors in the Negroes, Kaffres, and Australians adds to the prognathic form of the face possessed by these races. The central pair of upper incisors are larger than the lateral ; whilst the lateral pair of lower incisors are larger than the central pair, which are the smallest incisor teeth. The crowns of the incisor teeth are chisel-shaped, and adapted for biting and cutting the food. When the crown is first erupted the cutting edge ie minutely serrated, but the serrations soon wear down by

use. The fangs are long and simple, being in the upper