xii PHYLUM CHORDATA 425 



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 lymphatic system is very well developed, and is 

 remarkable for the dilatation of many of its vessels into 

 immense lymph sinuses. The lymph is pumped into the 

 veins by two pairs of lymph-hearts, one situated beneath 

 the supra-scapuke, the other beside the posterior end of the 

 urostyle. 



The brain (Fig. 255) has a very small cerebellum {H H), 

 large optic lobes (M H), a well-developed diencephalon, 

 and large hemispheres and olfactory lobes, the latter fused 

 in the median plane. 



The first spinal nerve performs the function of the 

 hypoglossal, one of the cranial nerves of higher Vertebrates 

 supplying the muscles of the tongue : it passes out between 

 the first and second vertebrae. The spinal cord is short, 

 and ends in a delicate filament, the filum terminate. In 

 correspondence with the number of vertebras there are only 

 ten pairs of spinal nerves, of which the second and third 

 unite to form a brachial plexus giving off the nerves to the 

 fore-limb, while the seventh to the tenth join to form a 

 lumbosacral plexus giving off the nerves to the hind-limb. 



The olfactory sacs have each two openings: the anterior 

 naris or external nostril and the posterior naris (Fig. 251, 

 /. 11a), or internal nostril, which opens into the mouth 

 immediately external to the vomer. 



The eye and the auditory organ have the usual structure, 

 but in connection with the latter there is an important 

 accessory organ of hearing not hitherto met with. Bounded 

 externally by the tympanic membrane, and internally by the 

 outer wall of the auditory capsule, is a considerable space, 



