284 ZOOLOGY SECT. 



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

 blood having been mostly driven to the lungs, will 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 red blood-corpuscles are, like those of Fishes, oval, nucleated 

 discs. The lymphatic system (Fig. 934) is very well developed, 

 and is remarkable for the dilatation of many of its vessels 

 into immense lymph-sinuses. Between the skin and muscle 

 are large subcutaneous sinuses (Fig. 929, v. ly. s.), separated 

 from one another by fibrous partitions, and the dorsal aorta 

 is surrounded by a spacious subvertcbral 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. 935) has a very small 

 cerebellum, large optic lobes, a well-developed diencephalon, 

 and large hemispheres and olfactory bulbs, the latter fused in 

 the median plane. The corpora striata, or basal ganglia of the 

 cerebral hemispheres, are connected together, as in all Vertebrates, 

 by an anterior* commissure (com, below, lower line), above which 

 is another commissure (com, below, upper line) partly^rp*^senting 

 the hippocampal commissure of the brain of Reptiles and Mammals. 

 The metaccele is covered by a thick choroid plexus : the mesocoele is 

 divisible into a median passage or iter (i.\ and paired optocceles (opt.v.) 

 in the optic lobes : the paracosles are large cavities each communi- 

 cating with a rhinoccele in the corresponding olfactory bulb. The 

 pineal body is vestigial in the adult, a lobe of the anterior choroid 

 plexus, with a vestige of the stalk (pin), taking the position which 

 it usually occupies : in the larva it is found outside the skull and 

 immediately beneath the skin. 



The first spinal nerve performs the function of the hypoglossal 

 (Fig. 935), 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 terminals. 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 lumbo-sacral plexus giving off 

 the nerves to the hind-limb. 



Sensory Organs. The olfactory sacs have each two openings : 

 the anterior naris or external nostril and the posterior naris 

 (Fig. 929, p. na.) 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 



