VERTEBRATA. 



33 



spongy mass of minute cells 

 communicating with the ex- 

 ternal air. Over the surface 

 of these cells the blood-vessels 

 ramify, and their coats become 

 so thin as to admit the union of 

 the oxygen with the contain- 

 ed blood, the result of which 

 is an instant change of its color 

 from dark red to vivid scarlet. 

 The gills of fishes perform the 

 same office as the lungs, water 

 being used instead of air. The 

 blood thus renewed is return- 

 ed bv being carried to another 

 chamber of the heart, whence 

 it is ejected into the other set of 

 vessels, called arteries, which, 

 large at first, but branching in- 

 to innumerable ramifications, 

 convey it to every part, and at 

 last, in a manner not thorough- 

 ly understood, transfer it into 

 the extremities of the veins, 

 which, as we have seen, carry 

 it back to be renewed again. 

 This is what is meant by the 

 well-known expression, the cir- 

 culation of the blood, and which 

 is carried on not only in man, 

 but in all the other vertebrata. 

 The young are produced in 

 all cases from ova, or eggs, 

 which exist in the body of the 



J 



female. In most cases, the 

 young animal is excluded from 

 the egg while it is within the 

 body of the mother; in others 

 the egg itself is produced, and 

 the young hatched some time 

 afterward. The animals of the 

 former class are called vivipa- 

 rous ; of the latter, oviparous. 



A curious and remarkable 

 departure from both of these 

 systems is found in the marsu- 

 pialia, as the opossum, kanga- 

 1 roo, and others, which have a 

 poach under the belly, into 

 which the young, born in an 

 immature state, are received 

 and nursed till they are able 

 to take care of themselves. 



Vol. I.— 5 



Facial Nerve. 

 Brachial Plexus. 



Internal Cutaneous 

 Nerve. 



Median Nerve. 

 Cubital Nerve 



Sciatic Plexus. 



External Peroneal 

 Nerve. 



Tibial Nerve. 



THK NERVOUS SYSTEM IN MAN. 



Temporal Artery. 

 Carotid Artery. 



Aorta. 



Renal Artery. 

 Iliac Artery. 



Brain. 



Little P.rain. 

 Spinal Marrow. 



Radial and 



Musculo -Cutaneous 



Nerve. 



Intercostal Nerves 

 Femoral Plexus. 



External Peronea' 

 Nerve. 



External Saphenous 

 Nerve. 



Femoral Artery. 



Anterior Tibial 

 Artery. 



Art. Podiosa. 



Vertebral Artery. 

 Subclavian Artery. 



Axillary Artery. 

 Brachial Artery. 



('celiac Artery. 

 Radial Artery. 



Posterior Tibial 

 Artery. 



Peroneal Artery 



THE ARTERIAL SYSTEM IN MAN. 





