SPLEEN 215 



elements are less numerous in the human spleen than in those of many 

 animals. By contraction they force blood from the pulp and cause the 

 circulation to follow more definite channels. When they are paralyzed, 

 the pulp becomes filled with the blood corpuscles. 



The nerves of the spleen, from the right vagus and the cceliac sympa- 

 thetic plexus, are medullated and non-medullated fibers, chiefly the latter. 

 They form plexuses around the blood vessels (Fig. 202) and send fibers 

 into the pulp. Besides supplying the muscles of the vessels and trabeculae, 

 some of them are thought to have free sensory endings. Lymphatic 

 vessels are said to occur in the capsule and trabeculae, but not in the pulp 

 or nodules of the spleen. 



The spleen is a large organ, without obvious subdivisions. On its surface, when 

 fresh, there is a mottled effect due to areas bounded more or less definitely by tra- 

 beculae. Such areas, about i mm. in diameter, have been described by Mall as "lobules," 

 and he states that they "can easily be seen on the surface of the organ or in sections." 

 A lobule, as he describes it, has a central artery, and its base is where the lymphoid 

 sheath of the artery terminates. It has peripheral veins, often three, enclosed in the 

 trabeculae. A lobule is composed of some ten structural (or histological) units, imper- 

 fectly separated from one another by branches of the trabeculae. Each unit contains a 

 central terminal artery (branches of the lobular artery) and has peripheral veins 

 (branches of those about the lobule). Apparently, therefore, the lobules shown in the 

 diagram, Fig. 199, except along its lower border, represent groups or pairs of Mall's 

 lobules. Stohr notes that "a division into lobules in the interior of the spleen is im- 

 possible." The arrangement of lobules at the periphery suggests an ill-defined cortex. 

 Lobes have also been described, corresponding with the main branches of the splenic 

 artery, but the lobes are not generally recognized. The spleen may present inconstant 

 subdivisions, which sometimes produce detached portions called accessory spleens. 



THE ENTODERMAL TRACT. 



DEVELOPMENT OF THE MOUTH AND PHARYNX. 



In a previous section the early development of the fore-gut or pharyn- 

 geal pocket of entoderm has been described and illustrated (Figs. 27 and 

 28). This fore-gut of the young embryo is to produce the pharynx, 

 oesophagus, and stomach of the adult. Its anterior extremity encounters 

 the ectoderm at the bottom of a depression. The ectoderm and entoderm 

 there fuse to make the oral plate (Fig. 203), which becomes thin, ruptures, 

 and disappears. Just anterior to the plate, in the median line, the 

 ectoderm sends a gland-like projection toward the brain. It branches 

 and becomes detached from the oral ectoderm, lying in the sella turcica 

 of the adult. It is known as the anterior lobe of the hypophysis, and will 

 be described with the brain, from which, the posterior lobe develops. 

 The ectoderm in front of the oral plate forms also the epithelium of the 

 lips and of the peripheral part of the mouth, including the enamel organs, 

 as has already been described. The salivary glands are also considered 



