422 MANUAL OF ZOOLOGY SECT. 



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 aortae. 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, 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 lymphatic system is very well developed, and is re- 

 markable for the dilatation of many of its vessels into im- 

 mense lymph sinuses. 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. 



The brain (Fig. 239) has a very small cerebellum (ffff), 

 large optic lobes (MH\ a well-developed diencephalon 

 (ZH) and large hemispheres ( VH] and olfactory lobes (L. ol.), 

 the latter fused in the median plane. 



The first spinal nerve performs the function of the 

 hypoglossalQKZ. 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 terminale. In 

 correspondence with the number of vertebrae 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. 



The olfactory sacs have each two openings : the anterior 



