408 ESSENTIALS OF PHYSIOLOGY. 



to the part played by the absence of thyroid and parathyroid tissues 

 respectively in the production of the symptoms which follow the 

 removal of the entire gland. Some consider that parathyroid tissue 

 has no distinct function of its own, but is merely undeveloped thyroid 

 tissue. They find that when the greater part of the thyroid is removed 

 the parathyroids increase in size and contain colloid material ; and they 

 regard all the symptoms which follow the removal of the thyroid and 

 parathyroid glands as the result of the removal of thyroid tissue 

 proper. 



Others believe that removal of the parathyroid glands produces the 

 acute nervous symptoms, and more especially tetany, whereas removal 

 of the thyroid gland alone brings about chronic changes in metabolism 

 and nutrition. The difficulty of determining the function of the para- 

 thyroid tissue is due to the fact that in many animals it is deeply 

 embedded in the substance of the thyroid gland. When this is the 

 case, it is difficult either to remove the thyroid gland without also 

 damaging the parathyroid tissue, or to remove the parathyroid glands 

 without serious injury to the thyroid glands. 



The balance of evidence, however, favours the view that the 

 functions of the thyroid and parathyroid tissue are distinct, and that 

 the former is concerned solely with metabolism, tetany being the result 

 of removal of the parathyroids. 



THE SPLEEN. 



The spleen is a solid organ enclosed in a capsule, which is partly 

 fibrous and partly consists of plain muscular tissue. The capsule sends 

 trabeculse, also containing unstriated muscle, into the interior of the 

 organ ; these branch to form a framework, in the interstices of which 

 lies the spleen pulp. This consists of a fine network of connective- 

 tissue fibrils, covered by endothelial cells, and containing in its meshes 

 lymphocytes, red blood corpuscles, and large cells which are amoeboid 

 and often contain partly broken-down red corpuscles. Multinucleated 

 giant cells are also sometimes present. 



The outer coat of the arteries in the spleen consists of lymphoid 

 tissue, an enlargement of which is present on each arteriole and forms 

 a Malpighian corpuscle. Some capillaries are found in the Malpighian 

 Bodies, but, with this exception, the arterioles open directly into the 

 spleen pulp, from which the blood is again gathered up to leave the 

 spleen along the splenic vein. The blood thus comes into direct contact 

 with the tissue elements of the spleen, whereas in almost every other 

 organ of the body it is separated from the tissues by a capillary wall. 



The flow of blood through the spleen is assisted by the alternate 



