v. 15 BRAIN OF ELASMOBRANCHS 169 



between-brain, the hypothalamus, is, however, as well developed 

 (relatively) in these animals as in mammals. Its hind part (inferior 

 lobes) receives olfactory impulses via the forebrain (the 'fornix' of 

 higher vertebrates) and gustatory pathways from the medulla. Its 

 efferent fibres run to reticular centres. The more anterior part of the 

 hypothalamus lies above the pituitary and contains the supraoptic 

 nucleus, whose axons form the hypophysial tract, ending in the inter- 

 mediate lobe. The supraoptic cells of all vertebrates are large and 

 contain granules of neurosecretory material that is probably passed 

 down the axons and liberated in the pituitary. The anterior hypo- 

 thalamus is a higher centre for visceral control, regulating, for 

 example, circulation, respiration, and many metabolic activities. 

 Attached to the hind end of the hypothalamus of fishes is a peculiar 

 organ, the saccus vasculosus, with folded, pigmented walls. It has 

 been suggested that this acts as a pressure receptor, since it is well 

 developed in deep-sea fishes. It is one of the characteristic features 

 that the sharks and bony fishes have in common. 



The midbrain, as in cyclostomes and teleosteans, is very large and is 

 perhaps the dominant centre of the brain. The optic tracts end in its 

 roof (tectum opticum) after complete decussation below the brain. 

 The cells of the tectum are arranged in a complicated pattern of 

 layers. Other sensory centres that send tracts to the optic lobes are 

 the olfactory (cerebral hemispheres), acustico-lateral, cerebellar, 

 gustatory, and probably also the general cutaneous centres of the 

 spinal cord. Efferent tracts leave the midbrain roof to the base of 

 the midbrain and extend backwards into the medulla, perhaps into the 

 spinal cord. The efferent midbrain fibres have direct influence on 

 the spinal cord, and electrical stimulation of points on the tectum 

 opticum produces various movements of the fins, suggesting a system 

 of control similar to that exercised over spinal centres by the cerebral 

 cortex of mammals through the pyramidal tract. Various forced 

 movements follow injury to the midbrain. 



The cerebellum is a very large organ in clasmobranchs, as in all 

 animals that move freely in space. Its main source of sensory fibres is 

 from the ear and from the organs of the lateral line system, whose 

 afferent fibres enter through the seventh, ninth, and tenth cranial 

 nerves. The internal structure of the cerebellum is very uniform and 

 essentially similar in all vertebrates. Removal of portions of it from 

 dogfishes produces aberrations of swimming. 



The medulla oblongata is the region from which most of the cranial 

 nerves spring and especially those that regulate the respiration and 



