138 INTRODUCTION. 



lous membrane, often disposed in deep folds, or valvulae, and from which 

 exudes a lubricating, or mucous fluid, for its protection ; externally, 

 like the stomach, they are covered with a smooth, serous membrane. 

 In many of the Rodentia the duodenum is sacculated, or pyriform, at 

 the commencement. The small intestines are often highly convoluted, 

 generally longer than the large intestines, and bound to the spine 

 by a membrane, termed the mesentery, which enfolds them through- 

 out their length. In Man, and some of the lower Mammalia, the ex- 

 tent of the inner surface of the small intestines is increased by a regu- 

 lar series of transverse folds of the villous membrane, termed valvulae 

 conniventes : they are wanting in the Orang. Various mucous follicles 

 (glandulae aggregatae) are also disposed on their inner surface. 



It is in the small intestines, after the mixture of the chyme with the 

 biliary and pancreatic secretions, that the formation of chyle takes 

 place : the mode of this change is imperfectly understood. By the 

 peristaltic action of the muscular fibres of the small intestines, the 

 digested pulp is moved gradually onward ; and a series of vessels, 

 termed lacteals, the absorbing mouths of which open on the lining mem- 

 brane, take up the chyle, in appearance like milk : these tubes traverse 

 the mesentery, where a system of conglobate glands (the mesenteric) 

 lie in their course, through which they pass, and at length merge into 

 the dilated origin of the thoracic duct, termed receptaculum chyli, 

 situated (in Man) on the first vertebra of the loins, to the right of 

 the aorta. This duct runs between the inferior muscles of the dia- 

 phragm (or muscular partition between the chest and abdomen), to 

 the right of the aorta ; it then ascends between the aorta and a vein 

 termed vena azygos ; it next crosses obliquely over to the left side, 

 behind the oesophagus and great curve of the aorta ; passes behind the 

 left carotid artery, and bends down, dividing into two branches, of which 

 the superior receives a large lymphatic vessel from the cervical glands ; 

 the two branches now unite, and the duct continues its course toward 

 the internal jugular vein, behind which it descends, and enters, at the 

 left side of this vein, into the left subclavian vein, through a valvular 

 orifice, mingling its contents with the blood, which is immediately trans- 

 ferred to the right cavities of the heart. 



The coats of the lacteal duct, and those, also, of the lacteal vessels, are 

 thin and transparent : the former is provided with valves, so constructed, 

 as to allow the free passage of the chyle, but preventing its reflux. 

 If an animal be suddenly killed, an hour or two after a meal, and imme- 

 diately opened, the peristaltic action of the intestines will be seen in full 

 operation, and the lacteals will appear filled with their milky fluids : 

 these vessels, however, form only one part of the great absorbent 

 system of animals. Absorption takes place on the surface of the body, 



