The sympathetic trunk commences at the Gasserian ganglion, is connected with the vagal ganglion, and passes out with the vagus through a foramen in the exoccipital. A ramus communicans connects it with each of the spinal nerves from the first to the sixth inclusive, and a single ganglion, corresponds to each ramus. But the posterior spinal nerves, especially the tenth, give off a variable number of rami communicantes, and the ganglia vary correspondingly. The first and second sympathetic ganglia send important (accelerator) twigs to the heart: the fifth sends a twig to the caeliac plexus, which is in connection with the root of the mesenteric artery.
The sinus venosus of the heart is divided into a large right moiety receiving the two venae cavae superiores and the vena cava inferior, and a small left moiety receiving the pulmonary veins. Two valves guard the aperture into the right auricle. A thin non-muscular septum separates the two auricles: its lower free edge is adherent to the two auriculo-ventricular valves, one anterior, the other posterior. Their free edges and under surfaces are tied by chordae tendineae to the walls of the ventricles. These walls are produced into trabeculae which have a fixed direction and therefore influence the course of the arterial and venous blood-currents respectively. A conus arteriosus leads from the ventricle: its walls are yellowish, semi-transparent, and contain striated muscular fibre. From it springs the truncus aortae which is extremely short and gives origin at once to a right and left branch; its walls are whitish, somewhat opaque, and contain only smooth muscle fibres. The conus is separated from the ventricle by three valves, and from the truncus also by three valves. Of these latter, one is a large right valve prolonged as a spiral fold down the dorsal wall of the conus; the other two are small valves lying a little to the left, one dorsally, the other ventrally.
A vertical septum, placed transversely, divides the origin of the pulmonary arteries from the origins of the aortae and carotids. It is continued into the sinus of the large valve, is fixed to its free edge and to the wall of the conus between the two small valves. The cavity of the conus is consequently divisible physiologically into a dorsal portion which leads to the pulmonary arteries and a ventral from which arise carotids arid aortae. Each half of the truncus contains three vessels, carotid anteriorly, aorta in the middle, and pulmonary artery posteriorly. A fibrous band replaces a vessel, the ductus Botalli or primitive union between the carotid and aorta of each side. At the spot where the carotid splits into the lingual artery, which corresponds as in Reptilia to the external carotid of Aves and Mammalia, and into the carotid so-called in Amphibia, or internal carotid of higher Vertebrata, there is a rete mirabile, the carotid gland, formed by the development of anastomoses between the lingual or external carotid, and the first branchial artery of the Tadpole (the common stem of the lingual and carotid of the adult) where they are contiguous to one another.
The two aortae unite into a sub-vertebral aorta under the backbone, and the left arch gives off a large caeliaco-mesenteric artery before it unites with the right. The pulmonary artery divides into the pulmonary vessel proper and the cutaneous artery. It represents the fourth aortic arch of the Tadpole, the third disappearing.
Each cava superior traced upwards breaks up successively into the external jugular, the vena anonyma, and the cutanea magna, which receives the subclavian vein. The cava inferior is formed by the union of the efferent renal veins and receives the veins of the genital glands, the fat bodies, and near the heart the hepatic veins. The epigastric vein divides proximally into three branches, one for each lobe of the liver, and a third which takes up the portal and splenic veins and then enters the left liver lobe. Traced downwards, this vein receives first a cardiac vein from the conus, secondly veins from the right and left abdominal parietes, finally the veins of the allantoid bladder. It then divides into a right and left branch traceable respectively to the right and left femoral veins. Each femoral vein gives off also the reni-portal vein which passes to the outer side of the kidneys. Into each reni-portal falls a dorso-lumbar vein, and in the female the oviducal veins. Anastomoses exist between the allantoid, rectal and oviducal veins.
The spleen is a reddish-brown body attached to the mesentery at the level of the commencing rectum. A pair of lymphatic hearts opening into the subscapular veins lie posteriorly to the outer ends of the transverse processes of the third vertebra. A second pair lie one on either side the urostyle posteriorly, and opens into a vein which falls into the communicating vein between the femoral and ischiadic veins. A large lymphatic sac, cisterna lymphatica magna, lies at the back of the abdomen. Its ventral wall, formed by the peritoneum, is pierced by microscopic apertures or stomata.
There is an upper but no lower lip. On the roof of the mouth are (1) the minute apertures of the intermaxillary glands, the homologues of the internasal glands of the Urodela, immediately behind the fold of mucous membrane which protects the praemaxiliary teeth; (2) the internal nares to the outer side of the vomers; (3) the Eustachian tubes close to the articulation of the lower jaw. The broad flat tongue is affixed to the symphysis of the mandible: its free end is bifid. In the male an aperture on either side, close to the ramus of the lower jaw, leads to the croaking sac. The teeth are conical, and restricted to the upper jaw and vomers. They consist of enamel, dentine, and a bony base or pillar. New teeth are continually formed during life to replace those that are worn or broken away. A short oesophagus leads into a stomach which is at first dilated, then narrows to the pylorus, and lies on the left side of the body. It is surrounded to a great extent by a lymph sac. The duodenum is bent at a sharp angle with the stomach. The coils of the intestine lie on the right side, and end in a short median dilated rectum which opens into the cloaca dorsally to the aperture of the bifid allantoid bladder. The gall-bladder lies in the notch between the median and right liver lobes.