They are based on general nervous/cardiovascular systems for vertebrates, which don't differ all that much in general form. Brain, the heart and kidneys, which are shown there, as well as the large glycogen expansion/sacral ganglion of the distal spine, are essentially basic stuff. There's even the recurrent pharyngeal nerve, which even in birds as in crocs loops waaaay down the neck and around the aortic arch to come back up to the hyoid region. It is a bit trickier with the heart as the heart isn't shaped the same way in birds/crocs as it is in mammals, but the essential branches are there. Only the arterial system is shown. In mammals, the carotic arteries run down the sides of the vertebrae, but in birds and crocs, they run mostly down the ventral sides, so this is also shown -- somewhat. No other special reference was used.