46 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



as far as the forking of the trunci arteriosi. If desirable the heart may be removed 

 from the body by cutting through the aortic arches and the sinus venosus; then 

 mount the heart under water in a small wax-bottomed dish. Note the thick 

 spongy walls of the ventricle and its small central cavity. Find the openings 

 of the two auricles into the ventricle. The walls of the auricles are quite thin 

 and the right one is considerably larger than the left. The large opening of 

 the sinus venosus into the right auricle is readily found. The conus arteriosus 

 contains a conspicuous structure, the spiral valve, which almost fills its cavity. 

 It is a fold, bent slightly into an S-shape, extending lengthwise in the conus, its 

 dorsal edge attached along the entire extent of the dorsal wall of the conus, its 

 ventral margin free. In the cross-section of the truncus arteriosus observe the 

 two partitions which divide it into three channels, one for each aortic arch. 

 The ventral channel leads to the carotid arch, and the middle one to the systemic 

 arch; the beginnings of these two channels are in the conus arteriosus in front 

 of the termination of the spiral valve, so that the blood to reach them must flow 

 over the cuplike widened anterior end of the valve. The dorsal channel of the 

 truncus arteriosus passes into the pulmo-cutaneous arch; it starts farther down 

 in the conus below the anterior end of the spiral valve, so that blood reaches it 

 by flowing over the free ventral edge of the valve. Read Holmes, pp. 277-79, 

 and understand the function of the spiral valve and the partitions in the truncus 

 arteriosus in directing the venous blood into the pulmo-cutaneous arch, mixed 

 blood into the systemic arches, and the arterial blood into the carotid arch. 

 Draw the dissection from the ventral side. 



G. THE NERVOUS SYSTEM 



Remove the skin and clean away all muscles from the median dorsal region 

 of the animal. With a fine scissors and forceps remove the roof of the skull 

 bit by bit, being careful to keep the point of the scissors well up against the 

 bone so as not to jab into the soft brain tissue. First cut into the bone, then 

 pull away the pieces with the forceps. Remove the roof as far forward as the 

 nares. Next work posteriorly in the same way. Cut through the neural arches 

 of the vertebrae by lateral cuts, and pull out the central piece with the forceps. 

 The brain and cord should bt well exposed before their study is undertaken 

 (Holmes, chap, xvi, pp. 283-95). 



i. Dorsal aspect of the brain (Holmes, pp. 291-95). The brain is covered 

 with a pigmented membrane, the pia mater, which is particularly abundant in 

 the posterior part of the brain, where it is very vascular and fills a triangular 

 cavity, the fourth ventricle. The pia mater should be removed from the fourth 

 ventricle. 



The most anterior part of the brain comprises a pair of rounded olfactory 

 lobes, which are separated from each other by a faint median groove. Each 



