98 BIOLOGY OF THE LABORATORY MOUSE 



According to Knisley (63) who observed the circulation of the Kving 

 mouse spleen by transillumination, the branches of the penicilli divide into 

 arterial capillaries. Some of these capillaries after a somewhat curved,, 

 unbranched course connect directly with venules. Others after a short 

 course connect with the afferent ends of venous sinuses. Some of the 

 sinuses intercommunicate, forming multiple sinus routes; others form a single 

 sinus route. Both routes finally open into venules. The tissue between the 

 sinuses forms the splenic cords. According to the same author, in the 

 unstimulated spleen few erythrocytes are present in the splenic cords. 

 These leave the closed vascular system by individual penetration of the 

 walls. In traumatized and dying spleens rapid changes occur which result 

 in the passage of large numbers of erythrocytes into the pulp tissue. 



The red pulp fills the spaces between the terminal venous sinuses, forming 

 the splenic cords. The framework is formed by reticular fibers, primitive 

 reticular cells and fixed macrophages. In addition to the lymphatfc ele- 

 ments and elements of the circulating blood, free macrophages, small groups 

 of myelocytes, erythroblasts and plasma cells are present. Megakaryocytes 

 are constant constituents although their number varies considerably. As 

 the red pulp of the mouse contains many lymphocytes and few erythrocytes, 

 it is not well delimited from the white pulp. The reticular cells of the red 

 pulp almost always contain varying amounts of pigment. 



There is great variation in the size of the spleen. The distal end of the 

 organ occasionally shows bifurcation. Accessory splenic tissue in the pan- 

 creas or in fat lobules of the mesentery is often found. 



Endocrine Glands 



Hypophysis. — The hypophysis rests on a slight depression of the sphenoid 

 bone. It is attached to the floor of the third ventricle by a short stalk (96). 

 It consists of two main parts which are separated from each other by a 

 narrow cleft, the residual lumen of Rathke's pouch. The part which is 

 directly above the sphenoid bone and below the cleft is the anterior lobe or 

 pars distalis, while the parts above the cleft are the pars intermedia and the 

 pars nervosa (Fig. 41). 



The anterior lobe is formed of epithelial cells arranged in cords or alveolar 

 groups which are separated from each other by delicate connective tissue 

 septa. Small cysts lined by ciliated cells have been found occasionally. 

 The epithelial cells can be classified as chromophobe and chromophil cells. 

 The chromophobe cells (also called chief cells) have large, light staining 



