HEART OF LEPIDOSIREN. 703 



atrophy of the branchial capillaries, and subsequently of the whole bran- 

 chial apparatus, is produced. 



(1972.) We must in the last place, before leaving the consideration 

 of the circulating system of the B-EPTILIA, describe that of the Lepido- 

 siren, a creature so exactly intermediate between the two classes, that 

 it is really difficult to determine whether it ought most properly to be 

 called a fish provided with lungs, or a reptile with the circulatory organs 

 of a fish. 



(1973.) The heart resembles that of a fish, and consists of a single 

 auricle (fig. 347, a), a ventricle (6), and bulbus arteriosus (c). The vena 

 cava (e), bringing the vitiated blood from the system, terminates at once 

 in the auricle, which is represented in the figure as laid open ; but the 

 pulmonary vein (/), whereby the aerated blood is brought from the 

 lungs (m m), passes along as far as the auriculo- ventricular opening, 

 where it empties its contents into the ventricle by a distinct orifice, pro- 

 tected by a cartilaginous valvular tubercle. 



(1974.) It is, therefore, only necessary in this case to dilate the pul- 

 monary vein previous to its termination, to make a heart with two 

 auricles ; but, as Professor Owen observes, the same advantage is secured 

 to the Lepidosiren in a different manner ; for, while it still retains the 

 dioecious type of the heart of the fish, the continuation of the pulmonary 

 vein prevents the admixture of the respired with the venous blood until 

 both have arrived in the ventricle. 



(1975.) The aorta, or, rather, the bulbus arteriosus (</), in this inter- 

 esting creature, fulfils at once the office of a systemic, a branchial, and 

 a pulmonary artery. It gives off on each side six vessels, which corre- 

 spond to the six cartilaginous branchial arches; of these arches four, 

 namely the 1st, 4th, 5th, and 6th, support gills, so that the arteries 

 belonging to them (1, 4, 5, 6) are, as in fishes, distributed over the 

 branchial fringes, and are thus true or functional branchial vessels. But 

 the 2nd and 3rd arches have no gills appended to them ; so that the 

 arteries (2, 3,) belonging to these arches do not divide, but are continued 

 round to the dorsal region, where they unite to form an aorta, as in 

 Menopoma (fig. 343) ; moreover, before their union to form the systemic 

 trunk, they give off the pulmonary arteries (I, m) by which the pulmo- 

 nary circulation is supplied. Thus each contraction of the ventricle of 

 the heart drives the mixed blood derived from the vense cavse and pul- 

 monary veins, first, to the gills ; secondly, to the aorta, through the 

 vascular trunks (2, 3) ; and thirdly, to the lungs, through the pulmo- 

 nary artery (I, m) ; so that from this arrangement, whether the creature 

 be placed in water or in air, respiration is carried on efficaciously either 

 by the pulmonary or branchial apparatus vicariously. 



(1976.) The principal difference observable between the brain of 

 Reptiles and of Fishes is the increased proportionate size of the cere- 

 bral hemispheres (fig. 347, b) but they are still extremely small when 



