Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

Section 10. The pectoral limb and girdle (Figure 4, [Sheet 16]) have only a very vague resemblance to the corresponding structures in the rabbit. The girdle (g.) is a transverse bar lying ventral to the pericardial wall, and sending up a portion (sc.), dorsal to the attachment of the limb, which answers to the scapula and supra-scapula of the forms above the fish. Three main cartilages, named respectively the pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the limb. With these, smaller cartilaginous plates, rods, and nodules articulate, and form a flattened skeletal support for the fin.

Section 11. The pelvic girdle and limb (Figure 2, [Sheet 15]) are similar in structure, but the pro-pterygium and meso-pterygium are absent, and the cartilage answering to the meta-pterygium goes by the name of the basi-pterygium. In the male, but not in the female, the pelvic fins are united behind the cloaca, and there are two stiff grooved copulatory organs, the claspers (cl. in [Figure 1]), which have a cartilaginous support (cl.c.). These claspers form the readiest means of determining the sex of a specimen before dissection.

Section 12. The skull consists of a cartilaginous cranium, and of jaw and visceral arches. The cranium persists throughout life, in what closely resembles a transitory embryonic condition of the higher types. There is a nasal capsule (na.c.), a brain case proper, and lateral otic (auditory) capsules (ot.c.) containing the internal ear. (This should be compared with the frog's embryonic skull.) The upper jaw has a great bar of cartilage, the palato-pterygoid, as its sole support; the arch of premaxilla, maxilla, jugal, and squamosal-- all membrane bones-- is, of course, not represented. In the frog this bar of cartilage is joined directly to the otic capsule by a quadrate portion, but this is only doubtfully represented in the dog-fish by a nodule of cartilage in the pre-spiracular ligament (p.s.). The lower jaw is supported, by Meckel's cartilage (M.C.). The hyoid arch consists of two main masses of cartilage, the hyomandibular (h.m.), and the ceratohyal (c.h.); the former of these is tilted slightly forward, so that the gill slit between it and the jaw arch is obliterated below, and the cartilage comes to serve as the intermediary in the suspension of the jaw from the otic mass. There are five branchia[l] arches, made up pharyngo-, epi- and cerato-branchials, and the ventral elements fuse in the middle line to form a common plate of cartilage. Outside these arches are certain small cartilages, the extra branchials (ex.b.) which, together with certain small labials by the nostrils and at the sides of the gape, probably represent structures of considerably greater importance in that still more primitive fish, the lamprey. The deep groove figured lateral to the otic capsule is the connecting line of the orbital and anterior cardinal sinuses; the outline of the anterior cardinal sinus in this figure and in [Figure 1] is roughly indicated by a dotted line.

Section 13. [Figure 3a] is a rough diagram of the internal ear-- the only auditory structure of our type (compare Rabbit, [Sheet 7]). To dissect out the auditory labyrinth without injury is a difficult performance, but its structure may be made out very satisfactorily by paring away successive slices of the otic mass. Such a section is shown by [Figure 3b]; through the translucent hyaline cartilage the utriculus and horizontal canal can be darkly seen. The ductus endolymphaticus (vide Rabbit) is indicated by a dotted line in our figure. It is situated internal to the right-angle between the two vertical canals, and reaches to the surface of the otic capsule.

Section 14. The brain shows the three primary vesicles much more distinctly than do our higher types. The fore-brain has large laterally separated olfactory lobes (rh.), there are relatively small "hemispheres" (pr.c.), the stalk of the pineal gland tilts forward, and the gland itself is much nearer the surface, being embedded in the cartilage of the brain case, and the pituitary body is relatively very large, and has lateral vascular lobes on either side. Following the usual interpretation of the parts, we find optic lobes (op.l.) as the roof of the mid-brain, and behind a very large, median, hollow, tongue-shaped cerebellum (c.b.). The medulla is large, and certain lateral restiform tracts (r.t.) therein, which also occur in the higher types, are here exceptionally conspicuous.

Section 15. The dog-fish has ten pairs of cranial nerves, corresponding to the anterior ten of the rabbit very closely, when we allow for the modification the latter has suffered through the conversion of some part of the spiracular cleft to an eardrum, and the obliteration of the post-hyoid branchial slits.

The first and second nerves are really brain lobes, and nerves of the special senses of smell and sight respectively.

The third (oculomotor), the fourth (patheticus), and the sixth (abducens) are distributed to exactly the same muscles of the eyeball as they are in the rabbit.

The fifth nerve, has, in the dog-fish, as in the rabbit, three chief branches. V.2 and V.3 fork over the mouth just as they do in the mammal; V.1 passes out of the cranium by a separate and more dorsal opening, and runs along a groove along the dorsal internal wall of the orbit, immediately beneath a similar branch of VII., which is not distinct in the rabbit. The grooves are shown in the figure of the cranium, [Sheet 18]; the joint nerve thus compounded of V. and VII. is called the ophthalmic (oph.). It is distributed to the skin above the nose and orbit. When the student commences to dissect the head of a dog-fish he notices over the dorsal surface of the snout an exudation of a yellowish jelly-like substance, and on removing the tough skin over this region and over the centre of the skull he finds, lying beneath it, a quantity of coiling simple tubuli full of such yellowish matter. These tubuli open on the surface by small pores, and the nerves terminate in hair-like extremities in their lining. These sense tubes are peculiar to aquatic forms; allied structures are found over the head and along a lateral line (see below) in the tadpole, but when the frog emerges from the water they are lost. They, doubtless, indicate some unknown sense entirely beyond our experience, and either only possible or only necessary when the animal is submerged.

In addition to the ophthalmic moiety mentioned above, the seventh nerve has a vidian branch (vid.) running over the roof of the mouth, and besides this its main branches fork over the spiracle, just as V. forks over the mouth, and as IX. and X. fork over gill clefts. This nerve in the rabbit is evidently considerably modified from this more primitive condition.