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The spleen lies vertically on the left side of the cranial abdomen, under the last three [[Ribs and Sternum - Anatomy & Physiology|ribs]]. Dorsally it is broad but narrows as it extends cranially and ventrally. On rectal palpation it is located against the body wall and feels smooth with a sharp border. It is attached to the greater curvature of the [[Equine Alimentary System - Horse Anatomy#Stomach|stomach]] by the '''gastrosplenic ligament'''. The spleen is enclosed in a capsule of fibrous and elastic tissue that extends into the parenchyma as trabeculae.  

The spleen lies vertically on the left side of the cranial abdomen, under the last three [[Ribs and Sternum - Anatomy & Physiology|ribs]]. Dorsally it is broad but narrows as it extends cranially and ventrally. On rectal palpation it is located against the body wall and feels smooth with a sharp border. It is attached to the greater curvature of the [[Alimentary System - Horse Anatomy#Stomach|stomach]] by the '''gastrosplenic ligament'''. The spleen is enclosed in a capsule of fibrous and elastic tissue that extends into the parenchyma as trabeculae.  

The parenchyma is supported by a fine mesh of reticular fibres and is divided into two types of tissue, the red and the white pulp, which are separated by the marginal sinus.

The parenchyma is supported by a fine mesh of reticular fibres and is divided into two types of tissue, the red and the white pulp, which are separated by the marginal sinus.

*[[:Category:Secondary Lymphoid Tissue|Secondary lymphoid organ]]

*[[:Category:Secondary Lymphoid Tissue|Secondary lymphoid organ]]

===Erythrocytes & Platelets===

===Erythrocytes & Platelets===

In the foetus the spleen also has a role in [[Haematopoiesis - Overview|haematopoiesis]] when it becomes the main [[Erythrocytes|erythrocyte]] producing organ during the haematopoietic transitional phase. In the developed animal the red pulp is involved in the removal of aged, damaged or abnormal [[Erythrocytes|erythrocytes]] (along with the [[Equine Alimentary System - Horse Anatomy#Liver|liver]] and [[Bone Marrow - Anatomy & Physiology|bone marrow]]). As [[Erythrocytes|erythrocytes]] age they become less supple and this causes them to become damaged when they pass through the very narrow capillaries of the spleen, after which they are phagocytised by splenic [[Macrophages|macrophages]]. If a splenectomy is performed the number of aged [[Erythrocytes|erythrocytes]] in circulation increases.

In the foetus the spleen also has a role in [[Haematopoiesis - Overview|haematopoiesis]] when it becomes the main [[Erythrocytes|erythrocyte]] producing organ during the haematopoietic transitional phase. In the developed animal the red pulp is involved in the removal of aged, damaged or abnormal [[Erythrocytes|erythrocytes]] (along with the [[Alimentary System - Horse Anatomy#Liver|liver]] and [[Bone Marrow - Anatomy & Physiology|bone marrow]]). As [[Erythrocytes|erythrocytes]] age they become less supple and this causes them to become damaged when they pass through the very narrow capillaries of the spleen, after which they are phagocytised by splenic [[Macrophages|macrophages]]. If a splenectomy is performed the number of aged [[Erythrocytes|erythrocytes]] in circulation increases.

The red pulp also acts as a storage site for [[Erythrocytes|erythrocytes]]. The degree of storage is particularly notable in horses which, during exercise under sympathetic activity, can contract their spleen to increase the concentration of circulating [[Erythrocytes|erythrocytes]].  

The red pulp also acts as a storage site for [[Erythrocytes|erythrocytes]]. The degree of storage is particularly notable in horses which, during exercise under sympathetic activity, can contract their spleen to increase the concentration of circulating [[Erythrocytes|erythrocytes]].  

===Lymphoid===

===Lymphoid===

Blood flows through the marginal sinus. This means that most antigens present in the blood come into contact with the [[Lymphocytes#B cells|B lymphocytes]] and dendritic cells in the spleen. Dendritic cells in the marginal sinus and red pulp take up antigens from the blood and transport them to the primary follicles in the white pulp. If the antigen activates the [[Lymphocytes#B cells|B lymphocytes]] then a germinal centre will form in the primary follicle and this is called a splenic nodule. Antibody producing cells then migrate to the red pulp and marginal zone. Following splenectomy this doesn’t occur and animals are predisposed to septicaemia and infection with blood [[Protozoa|protozoa]].

Blood flows through the marginal sinus. This means that most antigens present in the blood come into contact with the [[Lymphocytes#B cells|B lymphocytes]] and dendritic cells in the spleen. Dendritic cells in the marginal sinus and red pulp take up antigens from the blood and transport them to the primary follicles in the white pulp. If the antigen activates the [[Lymphocytes#B cells|B lymphocytes]] then a germinal centre will form in the primary follicle and this is called a splenic nodule. Antibody producing cells then migrate to the red pulp and marginal zone. Following splenectomy this doesn’t occur and animals are predisposed to septicaemia and infection with blood [[Protozoa|protozoa]].