132 



MICROSCOPIC ANATOMY OF THE ORGANS. 



The nerves are derived from the cardiac plexus, the vagus, 

 and the sympathetic system. They are both medullated and 

 non-medullated, partly motor and partly sensory nerves. Small 

 ganglia are present at various places. Concerning the mode of 

 ending of nerves in the heart, see under Nerve-endings. 



2. LYMPHATIC SYSTEM. 

 (a) Lymph-vessels. 



The lymph capillaries are not. like the blood capillaries, 

 intermediate structures situated between two other systems. 

 They form the beginning of a great lymphatic system which 

 empties finally into the blood vascular system. The walls of 

 the lymph capillaries consist of a flat endothelial tube, the 



Eixiothciutl <v//.s 



Piece of a lymph-vessel of a rabbit's mesentery. The boundaries of the endothelial cells 

 are made visible by silver nitrate. X 235. 



boundaries of whose cells are irregular. The capillaries form 

 networks that have a characteristic appearance on account 

 of the uneven ness in calibre of the vessels. There are many 

 dilatations and constrictions, and in many places valves are 

 present (Fig. 93). 



The walls of thicker lymph-vessels resemble in structure 

 those of veins. There is an endothelial lining, an intirna con- 

 taining elastic fibres, a media consisting largely of smooth 

 muscle, and an adventitia. The latter is made up of longitudi- 

 nal connective-tissue bundles which contain elastic fibres and 

 longitudinally disposed smooth muscle bundles. 



