xxix NERVOUS SYSTEM 39? 



tissues and pass it into the veins. The fluid they contain, 

 called lymph, is practically blood, minus its red corpuscles; 

 its leucocytes are formed in structures called lymphatic glands^ 

 which occur in the course of the vessels. The lymphatics 

 of the enteric canal are called lacteals ; they take an im- 

 portant share in the absorption of fats. 



The nervous system, like the circulatory organs, is vastly 

 in advance of anything we have yet met with. The central 

 nervous system consists of a brain (Fig. 103), contained 

 in the cranial cavity, and continuous posteriorly with a 

 spinal cord (sp. c] contained in the neural canal of the back- 

 bone. Thus the central nervous system is exclusively dorsal 

 in position, and is not traversed by the enteric canal as in 

 Polygordius, the crayfish, and the mussel. 



Another characteristic feature of the dogfish's nervous 

 system is that it is not solid, like that of Polygordius and 

 the crayfish, but is tubular, being traversed by a longitudinal 

 canal, the neurocxle (Fig. 99, N. Cos}, lined with epithelium. 

 In the spinal cord the neuroccele has the form of a narrow 

 central canal ; in the brain it expands into a fairly capacious 

 system of cavities, the cerebral ventricles. 



The brain or anterior expansion of the nervous system is 

 a complex structure divisible into several parts, The hind- 

 most division, continuous with the spinal cord, is the medulla 

 oblongata (Figs. 103 and 105, NH}, and has above it the cere- 

 bellum (Hff). Immediately in front of these two divisions is 

 the mid-brain, produced above into paired elevations, the optic 

 lobes. In front of the mid-brain is a small section called 

 the diencephalon (ZH), and anterior to this again a large 

 prosencephalon (Vlf], corresponding with the cerebral 

 hemispheres of the higher Vertebrata, and giving off in front 

 paired olfactory lobes (L. ol). All these divisions of the 

 brain contain ventricles (F. rho\ varying considerably in 



