VENOUS SYSTEM 225 



FiTid the pericarddal opening in the middle line, imme- 

 diately dorsal to the sinus venosus. Insert a seeher, and pass 

 it down the canal into the abdominal cavity. 

 B. The Venous System. 



The venous system of the dog-fish consists of three great 

 divisions ; (1) the veins opening into the sinus venosus, which 

 return blood from all parts of the body to the heart ; (2) the 

 hepatic portal system, which conveys to the liver the blood 

 from the alimentary canal, pancreas, and spleen ; (3) the renal 

 portal system, which conveys to the kidneys the blood from 

 the part of the body behind the cloaca. 

 1. The veins opening into the sinus venosus. 



These veins are most readily traced by following 

 them back from the sinus venosus, and they will be 

 described in this way, though it must be remembered 

 that the flow of blood in them during life is towards 

 the sinus venosus. They are chiefly remarkable for 

 their enormous size in places, the dilated portions 

 being spoken of as sinuses. 

 To dissect the veins, slit open the ventral wall of the sinus 

 venosus transversely, and wash out the contained blood : then 

 follow the veins with a seeker, slitting them open along their 

 ventral walls, and washing out the blood. If preferred, they 

 may be injected with plaster of Paris, or with a colowed solu- 

 tion such as Prussian blue suspended in water ; or they may 

 be inflated with air. 



Fig. 49. — ScylliTim 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.Ih.) 



A, roof of the cranium. B, internasal septam. C, mucous canals. D, fold 

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

 leading from the cerebral vesicle to the olfactory vesicle. P, third ventricle. 

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

 vertebral neural plate. K, pituitary body. L, cerebellum. M, restiform body. 

 MN, lower jaw. MX, upper jaw. U, iuterorbital sinus. UP, vertebral 

 neural plate. ITS, neural spine. O, basi-hyal, supporting the tongue. 

 P, pericardial cavity. PC, pericardio-peritoneal canal. Q, sinus venosus. 

 B, auricle. S, ventricle. T, conus arteriosus. TE, teeth. XT, cardiac aorta. 

 V, aperture of afferent branchial vessel. VO, vertebral centrum. VV, first 

 branchial cleft. X, spiracle, or hyo-mandibular cleft. Y, notoohord. Z, 

 spinal cord. 



Q 



