68 PHYSIOLOGICAL SERIES. 



ELA8MOBRANOHII. 



Miklucho-Maclay, Beit nige z. vergl. Neurol. 1870. 

 Kdinger, Arch. mikr. Anat., Bd. Iviii. 1901, p. 661 (Cere- 

 Mlum). 



The brain of the Elasmobranchs only partially fills the cranial 

 cavity. It has generally an elongated narrow form, with the 

 several regions lineally arranged and, except in the case of the 

 cerebellum, with but little overlapping of parts. 



It is chiefly remarkable for the great development of the centres 

 in connection with the sense of smell olfactory bulbs and 

 peduncles, and cerebrum. The lamina terminalis is more or less 

 thickened excessively so in Rays and frequently is indented 

 in the mid- line to form small lateral ventricles. The optic lobes 

 arc well marked, as are also the several parts of the hypo- 

 thalamus (inftindibulum, lobi inferiores, saccus vasculosus and 

 saccns infundibuli). The cerebellum, as in Bony Fish and Birds, 

 i- M rongly developed ; it overlaps the optic lobes and medulla to 

 varying degrees and frequently shows complex transverse folding. 

 Posteriorly it is continued into the convoluted margins of the 

 rhomboid fossa (medullary auricles). In Sharks the medulla is 

 usually long and without definite separation from the cord, but 

 i- much shortened in Rays. It owes its large size mainly to the 

 great development of the nuclei of the cranial nerves, especially 

 those in connection with the respiratory nerves (x.) and the 

 sensory nerves of the skin (v. and the lateral- line nerves). 

 It also contains great longitudinal motor tracts (fasciculi longi- 

 tudinales posteriorcs) in its floor, besides less developed tradi in 

 connection with the optic lobes (fillet) and cerebellum. The 

 walls of the brain are nervous except for the vascular epithelial 

 roof of the rhomboid fossa and third ventricle. There is no >i^n 

 of cortical formation in the cerebrum, but its walls consist of a 

 relatively thin outer molecular stratum and a thick cellular layer 

 surrounding the ventricles. In its base definite cell-masses give 

 rise to the anterior commissure and to longitudinal tracts that 

 pass to a large ganglion (nucleus rotundas) in the thalamus, which 

 in turn giv.-s off fibres to the hinder |>;irts of the brain. A 

 second pair of tracts pass from the cerebral roof (pallium) into 

 the base of the thalamencephalon, and decussate behind the optic 

 chiauna in close contiguity with tracts from the base of the 



