694 SPECIAL PHYSIOLOGY. 



through the remaining vascular arches, now reduced to two in number on each 

 side, partly into the arteries of the head and neck, but chiefly into the aorta, 

 and the body generally. Lastly, the blood returning from the lungs, enters a 

 small superadded auricle, or left auricle, which becomes parted off from the 

 right ; whilst the blood which returns from the body, enters the ordinary right 

 auricle. Thus is established an incomplete double circulation, precisely the 

 form met with in the lower Reptiles. In passing to the higher Reptiles, the 

 essential change consists in the gradual rising up of a septum within the single 

 ventricular cavity, by which it becomes more and more completely divided 

 into two chambers, as seen in the Saurians, and especially in the crocodile. 

 This change, when quite complete, and constant, produces the perfectly di- 

 vided ventricles, with their entirely separate pulmonary and systemic arterial 

 trunks, as found in the heart of the Bird and Mammal. Thus are formed the 

 four-chambered heart and the completely double circulation. 



Similar changes, in the heart and great vessels, occur in the young newt or 

 water salamander ; but the external gills not being transitory, or giving place 

 to internal gills, as in the tadpole of the frog, remain throughout the larval 

 condition, becoming, at one time, very large and plumose. Like the internal 

 gills of the tadpole, they receive vessels from the bulbus arteriosus, and, as in 

 them, their disappearance is associated with the same changes in the future 

 distribution of the blood, which is henceforth sent partly to the lungs, but 

 chiefly to the body. 



Certain of the Amphibia possess, in their mature condition, both kinds of 

 respiratory organs, viz., external gills and lungs, and are hence named Peren- 

 ni-branchiata. In these, as in the Proteus, the condition of the circulation 

 corresponds with that found in the intermediate stages of the larval condition 

 of the newt. Three anterior arches from the bulb supply the branchial ar- 

 teries, and a fourth gives a vessel, which becomes the pulmonary artery of its 

 own side ; the pulmonary veins end in a rudimentary left auricle, and the sys- 

 temic veins in a right auricle ; the ventricle is single, and gives oft* the bulbus 

 arteriosus, from which arise the vascular arches just mentioned. 



In the somewhat anomalous Fish, the Lepidosiren, or mud-fish of the Gam- 

 bia, a remarkable approximation is shown towards the perenni-branchiate 

 amphibian form of respiration and circulation. It has six pairs of branchial 

 vascular arches ; of these, the first, fourth, fifth, and sixth, supply the branchiae, 

 which are filamentous, but not external, like the permanent gills of the lower 

 Amphibia. The second and third pairs, pass as simple channels into the de- 

 scending aorta, and also give off branches which unite to form a single pul- 

 monary artery, distributed to the largely developed cellulated air-bladder. 

 The single pulmonary vein ends in the sinus of the inferior vena cava. 



The preceding descriptions indicate a unity of plan in the circulating organs 

 of the Vertebrate series of animals, as marked as that which may be traced in 

 their skeleton and nervous system gradual modifications of this plan being 

 manifested in ascending from the lower to the higher animals. 



In the Amphioxus, the lowest Fish, and therefore the lowest Vertebrate 

 animal, the circulating system retains its Vertebrate type in the arrangement 

 of the principal vessels ; but it presents the singular anomaly of not having a 

 single central heart, but possessing instead, numerous contractile cavities 

 situated in the course of the chief bloodvessels. Beneath the complex branchial 

 chamber is a contractile vessel, or bulb, performing the office of a ventricle ; 

 from this, as many as fifty pairs of branchial arterial arches arise, at the root 

 of each of which is a contractile dilatation. These numerous branchial arte- 

 ries, of which the anterior ones are themselves contractile, end, as usual, in 

 the branchial veins, which are equally numerous, and combine together to 

 form the proper descending aorta, which is placed immediately beneath the 

 chorda dorsalis. The foremost vascular arch, the duct of Botal, joins the 

 aorta, without supplying branches to a branchial fringe. The aorta dis- 

 tributes branches to all parts of the body, the veins from which ultimately 

 unite to form two venous trunks, one a portal vein, a part of which is dilated 

 and contractile, and another which represents the vena cava inferior, on which 

 is found a contractile sinus, pulsating rhythmically, and propelling the blood 

 forwards into the rudimentary ventricle already described. The great num- 



