LYMPH 373 



by two pairs of small contractile sacs known as lymph hearts. 

 One pair of these lies below the scapulae and opens into the 

 subscapular veins ; the other lies at the end of the urostyle and 

 opens into the femoral veins. The fluids in the coelomic (pleuro- 

 peritoneal and pericardial) cavities are lymph. The pleuro- 

 peritoneal cavity communicates with the dorsal lymph sacs 

 by minute openings in the membrane which separates it from 

 them. 



BLOOD VESSELS 



From the truncus arteriosus there arise on each side three 

 arteries, which are for some distance bound together, so that 

 they seem to be a single vessel. The hindermost of these is the 

 pulmocutaneous arch, the middle the systemic arch, the foremost 

 the carotid arch or common carotid artery. After separating, 

 the three arches continue to run outwards, diverging as they go. 

 The pulmocutaneous arch divides into the pulmonary artery 

 for the lung and the cutaneous artery for the skin and mouth. 

 The carotid arch gives a lingual or external carotid artery to 

 the muscles of the tongue and hyoid, and then becomes the 

 internal carotid artery which bears a round swelling due to the 

 fact that it here breaks up into a number of small vessels which 

 reunite. This swelling is the carotid labyrinth, often inappro- 

 priately called the carotid gland. The friction of the blood against 

 the large surface provided by its numerous small vessels is the 

 cause of the high pressure in the carotid arch. Beyond the carotid 

 labyrinth the artery runs forwards and upwards towards the head, 

 where, after an orbital (stapedial) branch to the orbit and roof 

 of the mouth, it passes into the skull and supplies the brain. The 

 systemic arch curves upwards and backwards round the oeso- 

 phagus to join its fellow in the middle line below the backbone. 

 On its way it gives off an oesophageal artery to the oesophagus, 

 an occipitovertebral artery to the head and backbone, and a 

 large subclavian artery to the arm. Just before joining its fellow, 

 the left systemic arch gives off backwards the large coeliaco- 

 mesenteric artery. This divides into an anterior mesenteric, to 

 bowel and spleen, and a coeliac, which supplies the stomach 

 after giving a hepatic branch to the liver. The vessel formed by 

 the junction of the systemic arches is the dorsal aorta. It runs 

 backwards immediately below the backbone, giving off paired 

 renal arteries to the kidnej^s, ovarian or spermatic arteries to 



