The spleen is essentially a lymphatic organ, its peculiar structure depending largely upon the arrangement of the blood-vessels. Compact lymphatic tissue occurs in the spleen in spherical, oval, or cylindrical collections of closely packed lymphoid cells. These masses are known as the _Malpighian bodies_, or _splenic corpuscles_. They are distributed throughout the splenic pulp. Each splenic corpuscle contains one or more small arteries. These extend near the periphery of the corpuscle and more rarely in the center. The splenic artery passes in and the splenic veins out at the hilum which is located on the concave, or attached, side of the spleen. The splenic artery, upon entering the organ, at once branches, the trabeculæ forming a support for the vessels. After the arteries have entered the hilum, as stated above, they divide into many branches which follow the septa of the connective tissue. At first the arteries are accompanied by branches of the splenic veins. Soon, however, the arteries leave the veins and the septa, and penetrate the splenic pulp. In the splenic pulp the adventitia of the smaller arteries assume the character of reticular tissue, and become infiltrated with lymphoid cells. This infiltration forms masses which are called the splenic corpuscles, or Malpighian bodies. The terminal arteries break up into capillaries, which still retain an adventitia, and empty into border spaces, or sinuses, sometimes spoken of as ampullæ. These sinuses in turn empty into cavernous sinuses of the splenic pulp. From these are finally formed the venules; and the collections of venules form the splenic veins through which the blood gains exit from the spleen. =The Abdominal and Pelvic Cavities.=—In birds the abdominal cavity is divided into two smaller cavities by a fibrous septum. The anterior cavity representing the abdominal contains the liver, and the other representing the pelvic contains the gizzard, intestines and oviduct.