REPTILIA. 



303 



empties itself into the principal vein of the 

 corresponding hinder extremity, i 



The lymphatic hearts are in the snakes 

 situated just above the origin of the tail. 

 They communicate with a branch of the 

 caudal vein, and receive lymphatic vessels 

 from the posterior extremity of the great 

 lymphatic reservoir. 



In the Pythons, the situation of the lym- 

 phatic hearts is external to the abdominal 

 cavity in a special chamber, which is bounded 

 anteriorly by the last rib ; each heart receives 

 the lymph by three canals that open into its 

 dorsal aspect, and it communicates with the 

 caudal veins by two orifices situated at its 

 anterior extremity. Each of these lymphatic 

 hearts is composed of three membranes ; an 

 external one, which is cellular ; a middle one, 

 which is muscular, the muscular fasciculi being 

 arranged as in the hearts of the higher ani- 

 mals ; and an inner coat, which forms valvular 

 folds, serving to prevent the escape of the 

 venous blood into the lymphatic system. 



These lymphatic hearts are without peri- 

 cardium, and adhere to the walls of the cavity 

 which contains them. In a Python Tigris of 

 seven feet long, the length of each lymphatic 

 heart was six lines, and its diameter four lines 

 and a quarter. 



Venous System. The veins of reptiles have 

 very thin walls, and exhibit no fibres in their 

 structure, except in the large trunks of spe- 

 cies of considerable size. In the Chelonians 

 and the crocodiles they are furnished inter- 

 nally with a few valves, but these, it would 

 appear, do not exist in the veins of Ophidians, 

 at least they can be injected with facility in 

 any direction. 



As in other Vertebrata, the veins of reptiles 

 are more numerous than the arteries, and 

 from the frequency of their anastomoses re- 

 present rather a net-work than an arborescent 

 arrangement. Their circulation, moreover, 

 not being confined to a determinate course 

 through the lungs, as in Mammalia and birds, 

 the venous system in the Reptilia is never 

 overloaded with blood, as must be the case in 

 the two former classes, when respiration is 

 suspended. And it is probably on this ac- 

 count that their veins appear less capacious, 

 as compared with the arteries, than in those 

 Vertebrata which possess a double circulation. 

 For the same reason they are not dilated into 

 reservoirs as they are in Mammalia and diving 

 birds, or as in fishes, where the blood has but 

 one route through the branchiae. 



The Chelonians have two posterior vence 

 cavce which traverse the liver on each side 

 and receive in their course a multitude of 

 small hepatic veins. Immediately after issuing 

 from the liver, they are each joined by an an- 

 terior vena cava of the corresponding side, 

 or by the common trunk of the jugular and 

 subclavian, all of them opening into a kind 

 of reservoir, which communicates with the 

 right auricle of the heart through a slit-like 

 orifice guarded by two valves. The two veins 

 called above posterior venae cavae are the 

 umbilical veins of Bojanus, the analogues of 



the single abdominal or median vein of the 

 Batrachian reptiles, which become confluent 

 by winding towards each other and assuming 

 a transverse direction in the isthmus that 

 unites the lateral lobes of the liver : it is into 

 this single transverse trunk that the two 

 abdominal veins open. 



Each abdominal vein communicates by a 

 pectoral branch with an intercostal vein, and 

 by this intermedium with a cervical branch 

 from the jugular. 



Each abdominal vein has, moreover, an 

 anastomosis posteriorly with the inferior 

 common intercostal ; it is essentially a con- 

 tinuation from the iliac vein, which receives 

 the blood from the femoral, from the iliac cir- 

 cumflex vein, from the ischiadic, from the 

 caudal, from the hypogastric, and from the 

 renal, through the descending trunk of the 

 vena azygos. This latter, after anastomosing 

 in front of the thorax with a cervical branch 

 from the jugular, seems to convey the blood 

 from before backwards, if we may judge from 

 the gradual dilatation of its calibre, as it re- 

 ceives the intercostal veins, the muscular 

 veins of the back, and the branches of the 

 vertebral veins. Its trunk, as it descends 

 towards the kidney, anastomoses with the 

 vein derived from the generative organs, and 

 joins the hypogastric to form the iliac : here, 

 then, we have an arrangement, of the venous 

 system which determines the direction of the 

 blood towards the liver, and makes the ab- 

 dominal vein relatively to the liver what the 

 pulmonary artery is to the lungs. A vein 

 derived from the organs of generation, which, 

 as already stated, anastomoses with the trunk 

 of the vena azygos, likewise runs to the liver, 

 traversing its right lobe like a vena cava, and 

 in the same way receives many small hepatic 

 veins and terminates immediately that it 

 issues from the liver in the great sinus, com- 

 mon to the veins of the body. 



Arterial System. In reptiles there are 

 always two distinct aortae given off' sepa- 

 rately from the heart, and a third artery 

 destined exclusively to the lungs. 



In the Chetonian order, the two aortae, 

 together with the pulmonary artery, are 

 united together for a little space ; but the 

 former soon separate to take the position and 

 character, one of the right, and the other of 

 the left posterior aortae. The right aorta 

 gives off, shortly after its commencement, a 

 considerable artery, which might be called 

 the anterior aorta, which soon bifurcates ; each 

 division again subdividing into two others, 

 namely, the common carotid, and the sub- 

 clavian. 



The subclavian gives off almost the same 

 branches as in the mammalia ; namely, 



1. An artery analogous to the inferior 

 thyroid of Mammalia, which supplies a very 

 vascular cavernous little thyroid body, situ- 

 ated at the bottom of the neck. 



2. The common cervical, which runs forwards 

 under the neck internal to, and beneath the 

 carotid, distributing branches to the muscles 

 and other organs of the throat. 



