BULLFROG AS TYPICAL VERTEBRATE ANIMAL 71 



The capillaries are very small vessels, the walls of which are made 

 up of endothelium continued from the linings of arteries and veins. 

 They connect the distal ends of the arteries with the proximal ends 

 of the veins, but in so doing they branch extensively and anastomose 

 to form fine networks in the tissues invaded. Through their thin 

 walls, acting as semipermeable membranes, food products brought 

 by the arterial blood pass into the tissues, oxygen is unloaded from 

 the red blood corpuscles, and carbon dioxide and waste products 

 are taken up to be conducted into the veins. Leucocytes are able 

 to get out of the capillaries, squeezing their way between the cell 

 walls, and thus become free to engulf bacteria or other harmful 

 objects. 



The arteries are large vessels with elastic walls and carry blood 

 from the heart to the capillary networks in the various organs and 

 tissues of the body. The arteries arise from the conus arteriosus 

 which divides just above the auricles into a right and left truncus 

 arteriosus. Each of these trunks splits into three arches going to 

 each side of the body, the anterior carotid arch, the middle systemic 

 arch, and the posterior pulmocutaneous arch. 



The Carotid Arch. — Each carotid arch divides into two branches. 

 The more ventral, lingual arteiy, or external carotid, passes forward, 

 giving branches to the thjrroid, pseudothyroid, muscles of the hyoid 

 and tongue, and then extends along the edges of the lower jaw. 

 The internal branch is larger and is called the internal carotid. It 

 has at its base a spongy enlargement known as the carotid gland 

 which by its structure serves to steady the pressure of blood passing 

 into the artery. This artery goes to the base of the skull, giving off 

 the palatine artery to the roof of the mouth, the cerebral carotid 

 which enters the skull and supplies the brain, and the ophthalmic 

 artery to the eye. 



The Systemic Arch. — The systemic arch soon after it leaves the 

 truncus supplies a small laryngeal artery to the larynx and mus- 

 cles of the hyoid. It then curves downward and around the esoph- 

 agus on each side. It gives off an occipitovertehral artery which 

 sends a small artery to the dorsal side of the esophagus, then branches 

 at the spinal cord into the occipital artery, running anteriorly on 

 the dorsal side of the skull to the orbit and tympanum, and the 

 vertebral artery, turning posteriorly along the spinal column. Imme- 

 diately posterior to the occipitovertehral artery the large subclavian 



