58 COMPARATIVE ANATOMY 



to the distal row of semi-lunar valves, that so long as the 

 pressure in the carotid arches is greater than that in the 

 systemic and pulmonary arches, the edges of the semi-lunar 

 valves are kept pressed together so as to bar the passage 

 to the carotids. 



We have seen how the arterial blood in the carotids is 

 distributed to the head, there to pass into the capillaries of 

 the tissues, to part with its oxygen, take up carbonic acid, 

 and eventually to return by the superior vena cava to the sinus 

 venosus. Similarly, the mixed blood in the systemic arches is 

 distributed to the trunk, viscera, and hind-limbs by the dorsal 

 aorta and its branches, and is collected and returned by the 

 veins in the manner already described. The venous blood 

 which enters the pulmonary artery goes to the lungs, takes up 

 oxygen, parts with its excess of carbonic add, and is returned 

 to the left auricle by the pulmonary veins. 



The frog, therefore, has a greater and a lesser (pulmonary) 

 circulation, the blood in the former case making its circuit 

 through the tissues of the body, in the latter through the 

 shorter circuit of the lungs. The two circuits, however, are 

 not closed off" from one another, as they are in the higher 

 vertebrates, but have a common starting-place in the single 

 ventricle of the heart. 



There remains but one set of organs and tissues for us to 

 consider, the nervous system and the organs of special sense 

 associated with it. 



The nervous system consists of the cerebro-spinal axis and 

 its nervous branches, and the sympathetic ganglion chain. 

 The cerebro-spinal axis lies, as we have already seen, in the 

 cranium, and in the canal formed by the neural arches of the 

 vertebral column. That part which lies in the cranium is the 

 brain; and that part which lies in the neural canal of the 

 vertebral column is the spinal cord. 



The spinal cord is a cord of nervous tissue extending from 

 the foramen magnum into the urostyle. It is not circular in 

 section, but somewhat flattened dorso-ventrally, and is traversed 

 both dorsally and ventrally by a median longitudinal narrow 

 cleft or fissure — the dorsal and ventral fissures, — but neither 

 of these extend to its most posterior part. Nor is the cord 

 of the same diameter throughout, but it is swollen in the 

 regions of the second and third, and again in the regions of 



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