xni PHYLUM CHORDATA 263 



It will be perceived that the blood poured into the right auricle 

 is mostly impure or venous, that poured into the left fully aerated 

 or arterial. When the auricles contract, which they do simultane- 

 ously, each passes its blood into the corresponding part of the 

 ventricle, which then instantly contracts, before the venous and 

 arterial bloods have time to mix. Since the conus arteriosus 

 springs from the right side of the ventricle, it will at first re- 

 ceive only venous blood, which, on the contraction of the conus, 

 might pass either into the bulbus aortaa or into the aperture of 

 the pulmo-cutaneous trunks. But the carotid and systemic 

 trunks are connected with a much more extensive capillary system 

 than the pulmo-cutaneous, and the pressure in them is propor- 

 tionally great, so that it is easier for the blood to enter the pulmo- 

 cutaneous trunks than to force aside the valves between the conus 

 and the bulbus. A fraction of a second is, however, enough to get 

 up the pressure in the pulmonary and cutaneous arteries, and in 

 the meantime the pressure in the arteries of the head, trunk, &c., is 

 constantly diminishing, owing to the continual flow of blood towards 

 the capillaries. Very soon, therefore, the blood forces the valves 

 aside and makes its way into the bulbus aortse. Here again the 

 course taken is that of least resistance : owing to the presence of 

 the carotid gland the passage of blood into the carotid trunks is 

 less free than into the wide, elastic, systemic trunks. These will 

 therefore receive the next portion of blood, which, the venous blood 

 having been mostly driven to the lungs, w r ill be a mixture of 

 venous and arterial. Finally, as the pressure rises in the systemic 

 trunks the last portion of blood from the ventricle, which, coming 

 from the left side, is arterial, will pass into the carotids and so 

 supply the head. 



The reel blood -corpuscles are, like those of Fishes, oval, nucleated 

 discs. The lymphatic system is very well developed, and is remark- 

 able for the dilatation of many of its vessels into immense lymph 

 sinuses. Between the skin and muscle are large subcutane-ous 

 sinuses (Fig. 876, v. ly. s.), separated from one another by fibrous 

 partitions, and the dorsal aorta is surrounded by a spacious sub- 

 vertebral sinus. The lymph is pumped into the veins by two pairs 

 of lymph-hearts, one situated beneath the supra -scapulae, the other 

 beside the posterior end of the urostyle. 



Nervous system. The brain (Fig. 881), has a very small 

 cerebellum (HH), large optic lobes (MH), a well-developed 

 diencephalon, and large hemispheres and olfactory lobes, the latter 

 fused in the median plane. The optic thalami are connected with 

 one another by anterior and posterior commissures (co. a., co. p.}, and 

 above the former is a transverse band of fibres (co.s.) which prob- 

 ably represents the hippocampal commissure of the mammalian 

 brain. The metaccele ( V. iv.) is covered by a thick choroid plexus : 

 the mesoccele is divisible into a median passage or iter (Aq. Syl.) 

 and paired optocceles in the optic lobes : the paracoeles are large 



