576 KEPT ILIA. 



ventricle by a distinct orifice, protected by a cartilaginous valvular 

 tubercle. 



It is, therefore, only necessary in this case to dilate the pulmo- 

 nary 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 diccelous 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. 



The aorta, or rather the bulbus arteriosus (g), in this interesting 

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

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

 correspond 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 func- 

 tional branchial vessels. But the 2nd and 3d 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 (Jig. 257) ; 

 moreover, before their union to form the systemic trunk, they give 

 off the pulmonary arteries (/, m) by which the pulmonary circula- 

 tion is supplied. Thus each contraction of the ventricle of the 

 heart drives the mixed blood derived from the verise cavse and 

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

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

 pulmonary artery (/, 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 vica- 

 riously. 



(633.) The principal difference observable between the brain 

 of Reptiles and of Fishes, is the increased proportionate size of 

 the cerebral hemispheres (Jig. 261, 5), but they are still ex- 

 tremely small when compared with the bulk of the body. The 

 appended figure, which represents the brain of the Tortoise in 

 three different aspects, may easily be compared with that of the 

 fish already given. The olfactory lobes (c) might now be mis- 

 taken -for prolongations of the anterior extremity of the hemi- 

 spheres ; they contain distinct ventricles, and of course give origin 

 to the olfactory nerves (o, o). The hemispheres (b) are much 

 more developed than in the last class ; their surface is always 



