section_path: Other animals/Vertebrates
section_name: Vertebrates
paragraph_text: Vertebrates
  
  The size of the heart varies among the different animal groups, with hearts in vertebrates ranging from those of the smallest mice (12 mg) to the blue whale (600 kg). In vertebrates, the heart lies in the middle of the ventral part of the body, surrounded by a pericardium. which in some fish may be connected to the peritoneum.
  
  The sinoatrial node is found in all amniotes but not in more primitive vertebrates. In these animals, the muscles of the heart are relatively continuous, and the sinus venosus coordinates the beat, which passes in a wave through the remaining chambers. Since the sinus venosus is incorporated into the right atrium in amniotes, it is likely homologous with the SA node. In teleosts, with their vestigial sinus venosus, the main centre of coordination is, instead, in the atrium. The rate of heartbeat varies enormously between different species, ranging from around 20 beats per minute in codfish to around 600 in hummingbirds and up to 1200 bpm in the ruby-throated hummingbird.