Text Book of Biology Part 17

-The Rabbit_

May be killed by chloroform, or pota.s.sium cyanide, or drowned. It may also be readily suffocated with house-hold gas. It should be killed immediately before use, as otherwise the gastric juice attacks the wall of the stomach, and the dissection is, in consequence, rendered extremely disagreeable. A very young rabbit is unsatisfactory as regards the genitalia, but otherwise there is no objection to a little one, and it has this advantage-- that it may be immersed more conveniently under water, in a large pie dish, for purposes of fine dissection. The external features of the animal should be examined: eyelids, whiskers and teeth, toes, a.n.u.s, perineal s.p.a.ce on either side of the same, urogenital opening, and position of the ribs, vertebral column, and limb girdles beneath the skin should be made out. Then the animal should be pinned out through the legs, the ventral surface uppermost, the skin opened up along the middle line from pelvic girdle to symphyses of jaw; separated from the body wall below by means of the handle of a scalpel, and turned back; and then the abdominal wall should be cut into and two flaps pinned back to expose its contents. Note the xiphisternum. The caec.u.m and colon will be recognised (Section 16); the stomach, the right and left central, and left lateral lobes of the liver will probably be apparent; and the urinary bladder (especially if distended) in the middle line behind. Without any further dissection, but simply by turning the parts over, all the structures of the abdomen in Figure 1, Sheet 1, will be identified.

Seek especially for and note particularly, the gall bladder, bile duct, and portal vein, pancreatic duct, sacculus rotundus, vermiform appendix, ureters (by pulling urinary bladder forward), genital ducts (looping over ureters), spleen, kidneys, and adrenals. The vena cava inferior is seen dorsally. The genital duct guides the student to the genital gland; if the subject is a male, the testes may be exposed by dissection, or by pulling the vas deferens gently the scrotal sac will be turned inside out, and the testes brought into view. The ovary lies exposed without dissection posterior to the kidney. Examine all this carefully, and make small sketches of points of interest-- the duodenal loop and the pyloric end of the stomach, for instance; the meeting of colon, caec.u.m, and sacculus rotundus again; or the urinary bladder and adjacent parts. Note the dorsal aorta and vena cava and their connexions behind. (Compare figure of circulation.) Cut through pelvic girdle, and remove one hind leg, to see bladder and genital ducts better (compare Sheet 10). Wash away any blood that may flow. Turn all the intestines over to the animal's right, and see the dorsal aorta and vena cava inferior of the abdomen, the inferior mesenteric artery, and the spermatic (or ovarian) artery (compare, of course, with figure in book). In front, immediately dorsal to the spleen, is a variable quant.i.ty of lymphoidal tissue, which must be very carefully cleared to see the superior mesenteric and coeliac arteries. Separate Spigelian lobe from stomach, and look for vagus nerve descending by oesophagus, solar plexus around the superior mesenteric artery, and thrown up very distinctly by the purple vena cava inferior beneath, and the splanchnic nerve. To see the abdominal sympathetic behind, gently remove the peritoneum that lies on either side of the aorta; blood-vessels will be seen running in between the vertebral bodies, and the sympathetic chain, with its ganglia, made out very distinctly, as it runs across them longitudinally. Now cut oesophagus just in front of stomach, and cut the r.e.c.t.u.m, cut through the mesentery supporting the intestine, and remove and unravel alimentary ca.n.a.l; cut open, wash out, and examine caec.u.m and stomach. Bleeding to a considerable extent is inevitable, chiefly from the portal vein. The liver had better remain if the same rabbit is to serve for the second dissection.

Second Dissection.-- Skin front of thorax and neck. Note subclavian veins running out to fore limbs-- avoid cutting these. Cut through ribs and remove front of thorax, to expose its contents; cut up middle line of neck, and clear off small muscle bands, to expose bloodvessels; pick away carefully whatever is left of thymus gland; make out structure of heart and blood-vessels, as described, in Chapter 3; note larynx and trachea. Now proceed to the examination of the nerves of this region. See phrenic nerve, by vena cava inferior, and between heart and lungs, and sympathetic, running over the heads of the ribs.

By the common carotids will be found the large white vagus nerve, the greyish sympathetic, and a small branch of X., the depressor. Make out branches of X. named in text. The big white cervical spinal nerves will be evident dorsally. Clear forward into the angle between the jaw and the bulla tympani, to see XII. and XI.; IX. will be found, lying deeper, dorsal to the carotid artery and body of the hyoid. Compare with figure given of this. Skin the cheek, and see VII. running over it.

Cut through malar and remove it; cut through lower jaw-bone and turn it back, to see the third branch of the fifth nerve on its inner side; examine the muscles of eyeball, and remove it, to expose the first and second branches of V.-- the latter is especially deep within orbit.

Remove, open, wash out, and examine the heart. Shave off the dorsal wall of cranium, to expose hemispheres of brain, and then put the head in strong spirit for a week or so. With a second rabbit, this dissection may advantageously be varied by removing the lower jaw, cutting -up- [through] soft palate, and observing openings of the Eustachian tubes. [The tonsils (on the ventral side of the soft palate) must not confused with these.] The heart should also be cut out, washed out and examined (Compare Sections 38, 44.)

Third Dissection.-- (Before this is performed the mammalian skull should have been studied and examined.) Take the head of a rabbit, the brain of which has been hardened by spirit, and carefully remove cranium; be particularly careful in picking away the periotic bone, on account of the flocculi of cerebellum. It is difficult to avoid injury to the pituitary body embedded in the basisphenoid bone. Examine with the help of Sheet 8. Make the sections there indicated.

-The Frog_

May be killed by drowning in dilute methylated spirit, or by chloroform.

Take a recently-killed frog, and examine a drop of its blood, spread out on a gla.s.s slip, under the microscope; compare it with your own.

Before using the high power, put a cover gla.s.s over the object, of course. Sc.r.a.pe the roof of the mouth of the frog gently, to obtain ciliated epithelium; and mount in very weak salt solution-- the cilia will still be active. Squamous epithelium may be seen by the student similarly sc.r.a.ping the interior of his own cheek. Take a piece of muscle from one of the frog's limbs, tease out with needles upon a gla.s.s slip, and examine. To see the striations clearly, the high power will be needed. Compare a piece of muscle from the wall of the alimentary ca.n.a.l. Similarly examine nerve and connective tissue.

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 s.p.a.ces, 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 r.e.c.t.u.m, and the mesentery and alimentary ca.n.a.l 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 rumus 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 att.i.tude 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 pa.s.sing 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 r.e.c.t.u.m, and remove alimentary ca.n.a.l 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 ca.n.a.l, and, in the tail, the haemal ca.n.a.l. [(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.]

-Amphioxus_

Two specimens of this type should be obtained. It should be examined entire by the naked eye and with the low power of the microscope. Immersion, in glycerine will render it more transparent; or it may be cleared with oil of cloves, put up temporarily in that, or permanently in Canada balsam. One specimen should then be pinned out in the dissecting dish, ventral side uppermost, and the atrium opened to expose liver and pharynx. A part of the pharynx may be examined with the low power to see the form of the gill slits. The second specimen should be soaked in turpentine for some time, and then dropped into melted paraffin wax. Transverse sections may then be cut with a razor, the paraffin wax removed from these by solution in turpentine, the turpentine in its turn dissolved out by alcohol, and the sections, after immersion in oil of cloves, may be transferred to Canada balsam for examination and preservation. This work should not be attempted until some practical histological work has been done in botany, and it may be altogether avoided by the purchase of stained and mounted sections.

-Development_

Laboratory work in this portion of the science is not usually undertaken by elementary students of biology, but the reader will probably find it helpful, in the realization of the facts given in this book, to look out for frog sp.a.w.n, in February and March, and to catch and examine tadpoles of various sizes. A small dissecting dish may be made by pouring melted paraffin wax into one of those shallow china pots chemists use for cold-cream, and tadpoles may be pinned out with entemologists' pins and dissected with needles. But this is a work of supererogation. Partially incubated hen's eggs may be obtained at a small cost almost anywhere, and the later stages profitably examined and dissected under warm water. For a clear understanding of the allantois and amnion, this last is almost indispensable. A few microscopic slides of sections of embryonic chicks should also be compared with our rough diagrams.