First Dissection.-- Pin out the frog in a dissecting dish, ventral surface uppermost, and cover with water. Open up the skin along the mid-ventral line. Note the large sub-cutaneous lymph spaces, the pelvic and pectoral girdles, and the anterior abdominal vein. Cut into the body cavity on one side of this latter, cut across in front of where the vein dips down to liver, and peel the body wall away from it. The xiphisternum will probably be cut in this operation. In early spring the females are greatly distended with ova, and the greater portion of the ovary may, with advantage, be removed. The oviduct is dead white then, and larger and much more in evidence than the (pinkish) intestine even. Turn over the viscera, and compare with [Sheet 11]; one lung is often found greatly inflated, and then projects back into the body cavity; the stomach is, in some cases, pushed forward and hidden behind the shoulder girdle. Observe the allantoic bladder, the spleen, gall bladder, portal vein, and pancreas. By squeezing the gall bladder gently, the bile duct will be injected with bile, and will be apparent if the stomach is turned over. The oesophagus, just in front of the stomach, should be cut through, and the rectum, and the mesentery and alimentary canal supported by it, removed. This will expose the urogenital organs. (Vide Figures given.) These vary greatly, especially in the females, at different seasons. The condition figured would be seen in late autumn, or winter. In spring females are often found copulating with males, and then the ovary itself is inconspicuous, while the lower part of the oviduct is enormously distended with ova, so as to be mistaken sometimes for the ovary by those who fail to note that the ova are enclosed by a thin semi-transparent skin (wall of oviduct). The vena cava inferior is seen between the kidneys and the renal portal vein beside the ureter. Cutting through the mesentery supporting the kidney laterally, the dorsal aorta is exposed, and on either side of it the sympathetic chain and rami communicantes, often tinged with black pigment. This black pigment is a frequent but variable feature of the frog's anatomy, and usually dapples or blackens the testes, and also sometimes darkens the otherwise pale pink arteries. Behind the kidneys the sciatic plexus also becomes visible. Careful drawings should be made. Cut off the head of the frog, shave off top of brain case, and put the head in strong spirit.
Second Dissection.-- A fresh frog is required. Pin out under water as before, and open up body cavity. Now carefully remove the muscle from the ventral portion of the shoulder girdle, to expose the clavicles and coracoids. Cut away xiphisternum, and then cut through clavicles and coracoids on either side, and remove ventral part of shoulder girdle, to expose the heart. Open out the cut portions of body wall and pin. The veins going towards the heart should now, with a little examination, be evident. Make out the external jugular, the innominate, and its two branches, and the pulmo-cutaneous and vena cava superior. Clear by carefully picking away any shreds of semi-transparent tissue. Make out, by feeling, the position of the hyoid body, and of its anterior cornua. Note the hypoglossal nerve (first spinal) running ventral to this, and the ninth cranial nerve, running parallel to it but dorsal to the hyoid-- hidden therefore by the hyoid, and reappearing in front. The vagus may also be made out less distinctly, running "postero-ventrally" towards the heart. By clearing the muscle by the ramus of the jaw, VII. may be seen, and the third branch of V., running across the jaw at about the middle of its length. Pick off the thin transparent pericardium from the heart very carefully, and proceed to cut away all the veins made out. The truncus arteriosus may then be followed up as it branches. Note all the branches shown in the figures in this book. The precise position of the vessels will vary to a certain extent with the attitude in which the frog is pinned. The cutaneous artery will prevent the student following up the aortic arch until it is cut; then the arch may be followed round until it meets its fellow to form the dorsal aorta. Note the sympathetic again. Make careful drawings of all this. Cut off lower jaw, and note posterior nares and Eustachian openings. If time allows, remove the heart, and examine by cutting open and washing. (Compare, [Section 44]) Remove eyeball, to see the first and second branches of the fifth nerve, and the Vidian (i.e. palatial) branch of the seventh.
Third Dissection.-- Read the account of the frog's skull carefully. Take the head of a recently killed frog and drop into boiling water for a minute. Then pick off, very carefully, muscle, connective tissue, nerves, and etc., to clear the cranio-facial apparatus; examine the bones, compare with figures given in this book, and draw. Take the head, which has been in spirit a fortnight or so, pick away cranium, and compare brain with figures given. Examine ventricles, by taking sections, after drawings have been made.
-The Dog-Fish-
First Dissection.-- Examine external characters, nasal grooves-- no internal nares-- fins, spiracle, scales passing over lips, and cloaca. Cut off tail below the cloacal opening. The males are distinguished by the large claspers along the inner edge of the pelvic fin. Open up body cavity. Usually this is in a terrible mess in the fish supplied by dealers, through the post-mortem digestion of the stomach. Wash out all this under a stream of water from a tap or water-bottle. Frequently the testes are washed out of the male in this operation and ova from the loose ovaries in the female. Now compare with figure given in this book, allowing for the collapse of the stomach, if it has occurred. Cut through the oesophagus and rectum, and remove alimentary canal from body; cut open and wash out the intestine, and examine spiral valve. Now make a careful examination of the cloaca and its apertures, and dissect away the peritoneum hiding the kidney. In the female find the opening of the oviducts in front of the liver. Remove liver, and cut off body now behind pectoral fin. Before throwing tail and hinder part of body away, note the myotomes of body wall, the notochord and vertebral body, neural canal, and, in the tail, the haemal canal. [(See {[Section 9] the Dog-fish})]
{Lines from First Edition only.}
-The relation of the vertebral bodies to the notochord may be very well seen by taking successive slices, about one-tenth of an inch thick, through the vertebral body. The cartilage is hard and semi-transparent, the notochord jelly-like, least at the centres of the centra, and at a maximum intervertebrally.-
[The notochord is a soft jelly.] Cut away the ventral part of the pectoral girdle, to open pericardium. With a seeker, make out the pericardio peritoneal opening. Cut into the sinus venous, and run seekers into the Cuvierian and hepatic sinuses. [Cut open the Cuvierian and posterior cardinal sinuses, and run seekers into their affluents.] Dissect along the truncus arteriosus to afferent branchials. [Cut away the heart and oesophagus; run a seeker up the dorsal aorta and cut along it from the ventral side to subclavian and efferent branchial arteries.] Skin the top of the head. Note, while doing this, the yellow, jelly-like sense-tubuli beneath the skin. Shave off top of brain-case, and leave the head in spirit for a week or so.
Second Dissection.-- Place the head with the ventral side downward, skin all the dorsal surface as yet unskinned. Refer to book for precise position of the anterior cardinal sinus, and then cut down through body wall into this just over gill slits. The tenth nerve will become visible, with its "slit" branches athwart the floor of the sinus. Clear to make this more evident, and make out its lateral line and visceral branches, and the ninth nerve.
{Lines from Second Edition only.}
[The pharyngo-branchials may be felt beneath the sinus. Run a seeker from the dorsal aorta to the efferent branchials.]
Proceed now to orbit, and, without any dissection beyond the removal of skin, make out recti and oblique muscles of eyeball, and the optic, third and fourth nerves. Cut through these structures carefully and remove, exposing nerves seven, and five, as described and figured in the text. Examine the otic capsule by taking successive slices through it to show the labyrinth of the ear. -Remove the dorsal wall of the skull to obtain a dorsal view of brain. If this is sufficiently hard, examine it; if not, return it to spirit for a more convenient occasion.- [Examine brain.]