STRUCTURE OF THE SPLEEN. BLOODVESSELS. 357 



one hand by numerous processes with the connective tissue of 

 the capsule, and on the other hand with the tunica adventitia 

 enveloping the capillaries and rootlets of the veins. 



The cells and intercellular substance of the pulp are not so 

 closely compressed as are those of the Malpighian bodies ; on 

 the contrary, they frequently leave rounded or lacuniform spaces 

 between them, in which, in spleens recently removed from the 

 animal after ligation of the vessels and exposure to the action 

 of chromic acid at Cent., coloured blood corpuscles constantly 

 occur. 



BLOOD VESSELS OF THE SPLEEN. Several arterial and venous 

 trunks usually penetrate into the interior of the spleen at the 

 hilus. Both sets of vessels, invested with their sheaths, run for 

 some distance in proximity to each other, branching like a tree 

 as they proceed. When they have diminished to a diameter 

 varying from 0*3 to 0'2 millimeter, the arteries separate from 

 the veins. Their mode of branching continues to be tree-like 

 without the occurrence of anastomoses between the branches. 

 In this course the arteries give branches to their investing 

 sheaths which break up into a capillary network, presenting 

 few and wide meshes. This capillary plexus is richer in the 

 Malpighian corpuscles, the meshes being particularly small near 

 the periphery. The calibre of these capillaries, as a rule, is 

 moderately small, but frequently unequal, and the structure of 

 the wall also exhibits varieties, sometimes presenting the 

 characters of fully developed and sometimes of embryonic 

 capillaries (Huxley, W. Miiller). At the surface of the Malpi- 

 ghian corpuscles the capillaries either open into the intermediate 

 blood passages or into the rootlets of the veins. No proper 

 veins accompany the arterial sheaths from the point at which 

 they become cytogenous. 



The arteries, as is usual amongst the Mammalia, quickly 

 divide into numerous capillaries, that run a long course, and 

 are invested by a delicate tunica adventitia composed of con- 

 nective tissue. Generally speaking they exhibit the structure 

 of fully developed capillaries, but in some places they present 

 for a considerable distance, walls composed of separate cells 

 rich in protoplasm, constituting the transitional vessels of 



