90 EVERS's COMPARATIVE ANATOMY. 



to the left side as in the human subject. In the upper part of the 

 substance of the left ventricle of the pig, the stag", and other bisulea, 

 are two small flat bones, cruciformly disposed in the stag, they are 

 formed about the third year of the animal's life, and are but slightly 

 developed in the female. It has been generally supposed that the 

 foramen ovale and ductus arteriosus remain permanently open in 

 seals, otters, and cetaceans, and although the dissections of Cuvier, 

 Home, Blumenbach, and T. Bell, go to prove that they have 

 remained unclosed in a few instances, still we must look upon these 

 as exceptions, admitting, however, that these passages continue 

 longer pervious in these diving animals than in other mammalia. 



As regards the arterial system some remarkable peculiarities are 

 observed in the branches arising from the arch of the aor^a, thus 

 in the horse, the camel, and many of the long necked mammalia, 

 this great trunk just after its origin divides into two branches, one 

 becomes the descending aorta, the other ascends vertically and 

 divides into a right innominata, a left subclavian and a left carotid, 

 which latter appears in direction the continuation of the trunk. In 

 the elephant there is but one coronary artery, and both carotids 

 arise by a common trunk between the two subclavians. 



The dolphin has an innominata on each side, this constitutes the 

 type of the cheiroptera. In the marmot and the guinea-pig, the 

 right innominata gives rise to the two carotids, and the right sub- 

 clavian. This also is the type of the order quadrumana, and most 

 of the carnivora. Among the peculiarities in the distribution of 

 arteries we may notice the rete mirabile formed by the internal 

 carotid at its entrance into the cranium in several carnivora and 

 ruminant bisulca ; the plexifonn arrangement of arteries which 

 exists under the pleurae and between the ribs in the cetacea ; and 

 the remarkable anastomosing divisions of the arteries of the extre- 

 mities and tail in the slow moving and climbing animals as the 

 bradypus, myrmecophaga, pangolin, and stenops. This condition 

 of the arterial system doubtless has reference to the peculiar living 

 habits of the animal it exists in, thus in the cetaceans these serpen- 

 tine vessels constitute so many reservoirs for containing arterial 

 blood during the obstructions to the circulation which are almost 

 inseparably connected with the aquatic habits of these mammalia, 

 whilst in those with depending heads, the rete mirabile is admirably 

 calculated to . obviate the injurious effects of sudden influxes 

 to the brain, and in the edentata the arterial divisions alluded 

 to are no less indicated, lest the large trunks of the extremities 

 should suffer from pressure during their long continued action 

 in climbing. 



The plexiform disposition, which characterises so many parts of 

 the arterial system, is no less strikingly displayed in the venous. 

 This is well seen in the tortuous sinuses which receive the inter- 

 costal veins in the porpoise, the vena azygos being absent in this 

 animal. A beautiful distribution of veins constituting the rete 

 mirabile venosum, is met with on the foot of the horse, where these 



