CIRCULATORY SYSTEM. 247 



girdle, and its dorsal wall by the wide basi-bran- 

 chial plate and the fifth cerato-branchials. 



2. The heart is a single tube bent on itself in an S -shape, 

 and dilated to form a series of chambers separated 

 by transverse constrictions. 



Determine the position and general characters of the 

 divisions of the heart in order from behind forwards, pressing 

 them aside when necessary, but making no further dissection. 



a. The sinus venosus is a thin-walled transversely 



placed tube attached along its whole length to 

 the posterior and dorsal wall of the pericardial 

 cavity. It receives laterally the veins returning 

 blood from the body, and opens in front into the 

 auricle by a median aperture. 



To expose the sinus venosus, press the ventricle slightly 

 forwards. 



b. The auricle is a large thin-walled sac occupying 



the dorsal half of the pericardial cavity. It is 

 triangular in shape, with its posterior , angles 

 produced into processes which, in the natural 

 position of the parts, project laterally at the 

 sides of the ventricle. 



To expose the auricle, push the globular ventricle to one 

 side. 



c. The ventricle is a thick-walled globular sac lying 



Fig. 51. Scyllium canicula. A longitudinal and vertical section 

 through the head and anterior part of the body. The section of the 

 brain is taken a little to one side of the median plane, and the spinal 

 cord has been cut across a short way behind the brain. The cut 

 surfaces of the cartilaginous skeleton are dotted, (a. m. m.) 



A, roof of the cranium. B,internasal septum, c, amp ullary canals. D, fold 

 of the wall of the hemisphere in which lies the choroid plexus. E, aperture 

 leading from the cerebral ventricle to the olfactory ventricle. F, third ventricle. 

 G, stalk of the paraphysis. H, optic lobe. I, optic chiasma. IP, inter- 

 vertebral neural plate. K, pituitary body. |_, cerebellum. M, restiform body. 

 MN, lower jaw. MX, upper jaw. N, interorbital sinus. NP, vertebral neural 

 plate. NS, neural spine, o, basi-hyal, supporting the tongue. P, pericardial 

 cavity. PC, pericardio-peritoneal canal. Q, sinus venosus. R, auricle. S, 

 ventricle. T, conus arteriosus. TE, teeth, u, cardiac aorta. V, aperture of 

 afferent branchial vessel. VC, vertebral centrum. W, first branchial cleft. 

 X, spiracle or hyo-mandibular cleft. Y, notochord. Z, spinal cord. 



