VASCULAR SYSTEM. 287 



ditional resistance to the entrance of blood into the carotid arteries, so 

 that no -blood enters them until the systemic arteries are full. Lastly, 

 we "must mention the fact that there is in each systemic aorta, at the 

 point where it diverges from the carotid and pulmonary, a small semi- 

 lunar valve, so arranged as to interpose resistance to the flow of blood 

 into- the aorta. It is difficult to see how this paradoxical valve (valvula 

 paradoxa as it has been called by Gaupp) assists in sifting the blood of the 

 cavum aorticum, but it may be of use, as probably the carotid gland is 

 of use, in preventing the venous blood which enters the conus in the first 

 phase of the ventricular systole from passing into the systemic aortae. 

 To summarize the wholo matter : the first blood which enters the conus 

 purely venous and passes mainly into the pulmonary arteries in which 

 the resistance is less than in the systemic and carotids, partly because 

 the pulmonary arteries are empty and ready to receive it and partly 

 because of the resistance of the valvtila paradoxa and carotid f glands ; 



FIG. 161. The arterial arches of a larval salamander slightly diagrammatic (after Boas). 

 la-3a the three branchial arteries (afferent branchial vessels) ; lv-3v the corresponding 

 branchial veins (efferent vessels) ; 4 the fourth arterial arch ; ao dorsal aorta ; aw root of 

 dorsal aorta ; ce external carotid ; ci internal carotid ; p pulmonary artery : tr ventral 

 aorta ; x anastomosing vessels between external carotid and first afferent branchial vessel ; 

 y, z, anastomosing vessel between the afferent and efferent vessels of the second and third 

 branchial arches. 



the last blood which enters the conus is unable to pass into the pulmonary 

 because the cavurn pulmo-cutaneum is entirely cut off from the ventri- 

 cular orifice by the longitudinal valve. The first of this blood together 

 with the previously-arrived venous blood, i.e. mixed blood, enters the 

 left systemic arch because this is easiest of access, being in a straight line 

 with the conus and ventral aorta. This blood is distributed mainly to 

 the visceral arteries (see below). The last blood, i.e. the most purely 

 arterial, will enter the right arch and the carotid arteries which leave 

 the right arch. This blood is distributed to the head and the posterior 

 part of the body (exclusive of the viscera). The valvula paradoxa being 

 a small single valve will cease to act as a serious obstruction as soon as 

 the artery becomes sufficiently distended to allow of the blood passing it. 

 If this account is correct it is clear that the systemic arches are differen- 

 tiated functionally in the Anura as they are in the Reptilia. The left arch 

 receives mixed blood which is mainly sent to the viscera, the right arch 

 receives purely arterial blood which is sent to the head and posterior 



