23 



which runs forward. At the anterior end of the pericardium this 

 divides into a dorsal and a ventral branch which innervate the sur- 

 rounding muscles. The hypobranchial nerve innervates the skin 

 of the region immediately in front of the pectoral girdle, and the 

 coraco-arcualis communis, coracomandibularis, coracohyoideus, and 

 coracobranchialis muscles. 



The third, fourth, fifth and sixth spinal nerves pass backward 

 and ventrad till they reach the level of the articulation of the pec- 

 toral fin with the girdle. Here they join to form a simple brachial 

 plexus, from which arise branches proceeding to the musculature 

 of the dorsal and ventral faces of the fin. The seventh to eleventh 

 spinal nerves pass downward to the level of the fin, and then branch, 

 one portion entering the muscles of the ventral body wall, while the 

 other passes into the depressor muscles of the fin. 



The pelvic fin is innervated by eight or nine spinal nerves which 

 pass backward and downward along the medial edge of the septum 

 between the myomeres, entering the dorsal side of the fin along its 

 axis. No plexus is formed. 



OLFACTORY ORGAN (NASAL SAC). Dissect away the skin and other 

 tissues around the nostril so as to expose completely the olfactory 

 organ; this will be found to be a dark-colored, nearly spherical 

 mass, of half the diameter of the eye, firmly attached at its base. 

 By cutting away the cartilage dorsal to the nasal sac its base will be 

 exposed, and the olfactory bulb will be shown to be closely adherent 

 to a considerable part of the postero-dorsal surface of the organ. 

 Numerous short nerves can be demonstrated to pass from the olfac- 

 tory bulb into the olfactory organ; all these nerves together are 

 considered as the first cranial or olfactory nerve. Remove the 

 olfactory organ from the head; divide it by a median longitudinal 

 cut; observe the arrangement and structure of its double series of 

 internal folds (lamellae), and the complete median septum. 



VENTRAL SURFACE OF THE BRAIN. Cut the cord in two some dis- 

 tance back of the brain. Cut all cranial nerves just inside the 

 cranium and carefully lift the brain out. Parts of the ventral por- 

 tion of the brain lie in a recess beneath the mesencephalon and 

 must be disengaged very gently. 



Identify and examine the ventral parts of the brain. Note the 

 considerable lateral compression of the mesencephalon. The optic 

 nerves cross beneath the diencephalon, forming the optic chiasma. 

 From the sides of the chiasma slightly elevated optic tracts, formed 

 by the fibres of the optic nerves, can be traced into the optic lobes. 



Back of the optic chiasma the projecting ventral portion of the 

 diencephalon forms the hypothalamus. The posterior lobe of this 

 structure is the hypophysis or pituitary body. 



The oculomotor nerves emerge over the posterior end of the 

 hypothalamus. 



The ventral portion of the mesencephalon is formed by the 

 cerebral peduncles (crura cerebri) , columns of fibres passing be- 

 tween the myelencephalon and telencephalon. 



The abducens nerves arise on the ventral surface of the myelen- 

 cephalon near the midline and just back of a line connecting the 

 roots of the auditory nerves. 



The internal carotid arteries reach the brain at the sides of the 

 hypothalamus. Branches are sent upward and forward over the 

 surface of the brain. Anastomoses between the vessels of the op- 

 posite sides are formed anterior to the optic chiasma. The main 

 branches of the carotids pass backward along the sides of the hypo- 



