294 A MANUAL OF VETERINARY PHYSIOLOGY 



its internal secretion of urea and glycogen ; the pancreas, with 

 its digestive fluid, and its invaluable internal secretion, which 

 regulates the destruction of sugar. Even the kidney, in all 

 probability, possesses an internal secretion affecting metabolism. 

 The spleen, on the other hand, would appear to possess neither 

 an internal nor an external secretion, for it has frequently been 

 removed without ill effects ; but the question must be dealt with 

 in a little more detail. 



The Spleen, in spite of the numerous observations to which 

 it has been subjected, is still a physiological enigma. Its vascular 

 arrangement is peculiar in that it is capable of holding a con- 

 siderable quantity of blood, and for this purpose readily lends 

 itself to change of size. Further, it is the only tissue in the body 

 where the cell elements are directly bathed in blood without the 

 intervention of even a capillary wall. The spleen contains a 

 considerable amount of involuntary muscular fibre and is capable 

 of movement. These movements have been carefully studied, 

 and it is established that they are of two kinds. One is a slow 

 expansion due to unknown causes, which occurs after a meal ; 

 it reaches its maximum about the fifth hour, and is followed by 

 contraction. The other is a rhythmical expansion and contrac- 

 tion occurring in certain animals, such as dogs and cats, at 

 intervals of about One minute. It is believed that the latter 

 movement is for the purpose of assisting the circulation through 

 the organ, to which the splenic pulp offers considerable resistance. 

 That the movement is brought about by the bands of involuntary 

 muscular fibre is undoubted ; the spleen is liberally supplied 

 with motor nerves carried in the splanchnic, and stimulation of 

 these leads to a reduction in the volume of the organ. It is 

 also considered that there are nerves to the spleen, which dn 

 stimulation produce dilatation. 



The use of the gland is largely based on conjecture. By some 

 it has been considered the seat of formation of red blood-corpuscles, 

 and that this is the case during intra-uterine life and shortly 

 after birth is undoubted ; but there is no evidence of this function 

 in the adult. It has been claimed to be the seat of destruction 

 of the red cells and of phagocytosis, and on this point there are 

 some telling facts ; for instance, certain large amoeboid cells 

 found in the spleen are capable of ingesting and destroying 

 worn-out blood-cells and other solid matter such as micro- 

 organisms, while the richness of the splenic pulp in iron is 

 regarded as due either to the haemoglobin of the destroyed red 

 blood-cells being stored up for future use, or to the preparation 

 of new haemoglobin. That the conservation of iron is one of 

 the functions of the spleen, would appear from the fact that 

 removal of this organ in dogs causes a distinct daily loss of iron. 



