Book - Vertebrate Zoology (1928) 28
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXVIII The Head And Neck
The structure of the head in an adult vertebrate animal is somewhat complicated, and bears little resemblance to the simpler segmented nature of the trunk-region. The segmenta- tion is obscured, added to which there is the complication introduced by the presence of the special paired sense-organs (nose, eye, and ear) and of the gill-slits. The somites do not all form straightforward myotomes as in the trunk, but give rise to the eye-muscles ; and lastly, it is difficult to recognise the segmental nerves because the dorsal and ventral nerve-roots remain separate. Nevertheless, the head is built strictly on a segmental plan, and it is easy to unravel its structure by considering early stages of development.
The embryo of the dogfish, for example, passes through a stage in which the mesoderm on each side of the body is segmented into a complete row of somites, from the front to the hind end of the body. There is no difference between the somites of the future head-region and those of the trunk, and they grade insensibly into one another. The 1st somite is, however, peculiar in that it is connected with its fellow of the opposite side by a strand of mesoderm-cells which passes in front of the tip of the notochord. Such a connexion would be impossible between somites situated farther posteriorly, because the notochord separates those of one side from those of the other. The 1st somite is called the premandibular somite, and it is innervated by a ventral nerve-root : the oculomotor. The 2nd somite is rather larger than the others, it is called the mandibular somite and is innervated by the trochlear nerve. The 3rd somite is the hyoid somite, and it is innervated by the abducens. These first three somites will become differentiated into the extrinsic eye-muscles, and they all lie in front of the auditory vesicle, for which reason they are called the prootic somites.
The 4th somite is the 1st of the metotic somites, and it is similar to the ones following it. It and the 5th somite in the dogfish eventually disappear, being squashed underneath the large developing auditory sac, and they either do not have, or do not retain, any ventral nerve-roots. In Petromyzon, however, no somites are lost, and the 4th becomes the most
Fig. 167. Reconstruction of the head of a dogfish embryo, showing the segmentation. 77/, oculomotor ; IV, trochlear ; V, trigeminal ; VI, abducens ; VII, facial ; IX, glossopharyngeal ; X, vagus ; nerves ; as, auditory sac ; gi to #3, first to third gill-slits ; hm, hypoglossal muscles ; hn, hypoglossal nerve ; op, ophthalmicus profundus nerve ; s, spiracle ; si to s8, first to eighth somite ; sn, spinal nerve ; the arrows show the position of the posterior limit of the neurocranium : P, in Petromyzon ; Sc, in Scyllium ; Sq, in Squalus.
anterior of the myotomes of the body. In the dogfish, it is the 6th somite which gives rise to the most anterior myotome of the body.
It is now necessary to turn to the relations which the dorsal nerve-roots bear to the somites. Above the premandibular somite, the cells of the neural crest group together to form the ganglion of the ophthalmicus profundus nerve. This nerve is lost in adult Scyllium, but it is present in Squalus, and it is the dorsal root of the 1st segment, corresponding to the oculomotor.
Above the mandibular somite is the ganglion of the tri- geminal nerve which is the dorsal root of the 2nd segment, corresponding to the trochlear. The hyoid somite lies under the ganglion of the facial nerve, which is accordingly the dorsal root of the 3rd segment, corresponding to the abducens. It is possible, therefore, to recognise three prootic segments.
The glossopharyngeal nerve is the dorsal root of the 4th segment, overlying the 4th somite. The vagus represents parts of four dorsal roots joined together, and it corresponds to the 5th to 8th segments. The ventral roots of the 4th and 5th segments which disappear in the dogfish, are present in Petromyzon. The ventral roots of the 6th and following segments are present in the dogfish, innervating the anterior myotomes, and contributing to the hypoglossal nerve which accompanies the growth downwards and forwards of portions of the myotomes to form the hypoglossal muscles.
So far, then, the only difference between the head and trunk- regions is that in the former, the dorsal and ventral nerve-roots remain distinct from one another, and that in the three prootic somites the ganglia of the dorsal roots lie outside the little somites, instead of median to them as in the trunk.
It is now time to turn to the gill-slits, which arise as out- growths from the pharynx on each side, and connect with the ectoderm. The gill-slits are formed at a level below that of the somites, in the region of the lateral plate, or unsegmented mesoderm. The connexion of the endoderm of the pharynx with the ectoderm in the formation of the gill-slits necessarily obliterates to the mesoderm in places and confines it to the bars between the gill-slits. These are the gill-bars (gill-arches, or visceral arches). The remnants of the splanchnocoelic cavity in this region are restricted to the cavities in the gill-bars (as in the primary gill-bars of Amphioxus).
Now, down each of these gill-bars or visceral arches there passes a large branch of a dorsal nerve-root. The most anterior visceral slit is the spiracle, and separating it from the mouth is the mandibular arch (or 1st visceral arch) down which the trigeminal nerve passes. Between the spiracle and the 2nd visceral slit (1st gill-slit) is the hyoid arch (or 2nd visceral arch), and down this there passes the facial nerve. In a similar way, the glossopharyngeal nerve passes down the 3rd visceral arch, behind the 1st gill-slit ; and a branch of the vagus runs down each of the 4th, 5th, 6th, and 7th visceral arches.
Since the dorsal nerve-roots are segmental in arrange- ment, the visceral arches are segmental also, for they correspond. This means that the spiracle and gill-slits are intersegmental in arrangement. It must be remembered, however, that this segmental arrangement of the visceral arches is not the same thing as the primary and fundamental segmentation of the somites, because the visceral arches lie in the lateral-plate mesoderm (not in the segmented vertebral plate). The cavities enclosed in the mesoderm of the visceral arches are really part of the originally continuous splanchnocoel, and not myoccelic cavities. This is important, for it explains why the muscles to which the mesoderm of the visceral arches gives rise are innervated by dorsal and not ventral nerve-roots, although they are striped and voluntary. Ventral nerve-roots only innervate somatic striped muscles derived from the segmented myotomes of the vertebral plate. Muscles formed from the visceral mesoderm (inner wall of the splanchocoelic cavity) in the region behind the gill-slits are of course the smooth muscles of the gut, innervated by the autonomic system. That the muscles of the visceral arches should differ from these latter in being striped and voluntary is due to the fact that, unlike them, they are attached to skeletal structures. These skeletal structures support the jaws and the branchial arches, and their movements are involved in the processes of biting and breathing, which are related to the outside world. Smooth muscles are only related to the events which go on inside the animal.
Since the visceral arches correspond to the segmentation of the body, the structures in them correspond also. These consist of the skeletal elements just mentioned, and of the blood-vessels which run up round the gut from the ventral to the dorsal aorta. So Meckel's cartilage and the quadrate correspond to the trigeminal nerve and the 2nd segment, and the hyomandibula and ceratohyal correspond to the facial nerve and the 3rd segment. The blood-vessels in these two arches disappear in the higher vertebrates, but that in the 3rd visceral arch corresponding to the glossopharyngeal nerve and the 4th segment of the body becomes the carotid. Similarly, the systemic blood-vessel corresponds to the 4th visceral arch (5th segment of the body) down which the first branch of the vagus nerve runs. The pulmonary artery corresponds to the 6th visceral arch (7th segment of the body) down which the 3rd branch of the vagus runs.
The segmentation of the head is now clear, and it may be asked how many segments of the body does the head occupy ? Before this can be answered it is necessary to be clear as to whether " the head " is to be regarded as everything in front of the hindmost part of the skull (occipital arch), or whether it extends as far back as the gill-slits. In point of fact, it is necessary to distinguish between the dorsal or " neural head," and the ventral or " visceral head," for they differ in extent. The hindmost region of the neural head is indicated by the position of the occipital arch of the skull, that of the visceral head by the position of the last visceral arch. It is interesting to find that the number of segments in either kind of " head " varies in different animals. The neural head of Petromyzon occupies 4 segments, that of Scyllium 7, that of Squalus 9, that of Amphibia 6, that of Amniotes probably 8. Similarly, the number of segments in the visceral head varies from 10 in Petromyzon to 8 in Scyllium, 9 in Hexanchus, and 10 in Heptanchus, while the number is reduced in land- vertebrates which no longer breathe by gills. In the formation of the neural head, more and more segments of the trunk are incor- porated during evolution. The occipital arch is therefore not formed by the same segment in different groups of vertebrates, but this fact does not affect the homology of the occipital arches. This structure has a representative in the common ancestor of Craniates, whatever segment of the body it may be in. In the more primitive forms the neural head is short, and the occipital arch becomes displaced backwards. The primitive extent of the visceral head is probably about 10 segments, for not only is this the number in Petromyzon and in the primitive selachian Heptanchus (both of which have 8 visceral slits, although Petromyzon loses one), but Amphioxus in its development passes through a stage (the so-called " critical stage ") when it has 8 pairs of symmetrically arranged gill-slits.
The relations and destinies of the three prootic somites are constant in all vertebrates above the Cyclostomes (in which the eyes are degenerate), and they may now be considered.
In the first place, it is interesting to note that the ist or premandibular somites correspond to the anterior head- cavities or anterior gut-diverticula of Amphioxus, and that the front ends of all chordates correspond. Just as in Amphioxus the anterior head- cavity (of the left side) opens into an ectodermal pit (the preoral pit), so in Selachians (Torpedo) the premandibular somites open into an ectodermal inpushing (the hypophysis), and this connexion between premandibular somites and hypophysis also occurs in some reptiles and birds. The hypophysis is therefore probably homologous with the preoral pit of Amphioxus. This connexion between a meso- dermal pouch and the ectoderm is similar to that which occurs in Balanoglossus and the larvas of Echinoderms, forming the co-called " water-pores " and " proboscis-pores." (It may be mentioned that the so-called " anterior head-cavities " of some Selachians are merely parts of the premandibular somites, and have no segmental value.)
The morphological anterior end of the body in Craniates is a point near the middle of the mesodermal strand connecting the premandibular somites with one another. Just behind this point is the front end of the notochord, and the preoral gut ; just in front of it the hypophysis grows in from the superficial ectoderm, and just above it is the floor of the fore- brain near the optic chiasma and the point of closure of the neuropore. This morphologically anterior point of the animal is represented in many skulls near the dorsum sellae, which lies immediately behind the pituitary body. That part of the head which lies in front of this is the result of secondary forward growth.
In the conversion of the prootic somites into the eye-muscles in the dogfish, for example, the walls of the somites become thickened by the formation of muscle-fibres, and the contained ccelomic cavity is obliterated. The premandibular somite wraps round the optic nerve from behind, and becomes divided
Fig. 168. Reconstructions showing stages in the conversions of the first three somites into the extrinsic eye-muscles in a dogfish.
A to E, successive stages, ab, abducens ; exr, external rectus muscle ; /, facial nerve ; hy, hyoid or 3rd somite ; infr, inferior rectus muscle ; inob, inferior oblique muscle ; inr, internal rectus muscle ; m, mandibular or 2nd somite ; oc, oculomotor nerve ;" opn, optic nerve ; p, profundus ophthalmicus nerve ; pa, trochlear nerve ; pm, premandibular or 1st somite ; ros V and VII, superficial ophthalmic branches of trigeminal and facial nerve ; suob, superior oblique muscle ; sur, superior rectus muscle ; tr, trigeminal nerve.
into four pieces. The two dorsal portions are the internal and superior recti muscles, the two ventral portions are the inferior oblique and the inferior rectus muscles. The mandibular somite grows forwards above the premandibular and gives rise to the superior oblique ; and the hyoid somite, also growing forwards, becomes attached to the posterior part of the eyeball, forming the external rectus muscle.
In higher vertebrates, the prootic somites are not always separately recognisable as such, and in these cases the eye- muscles appear to arise from masses of mesenchymatous mesoderm-cells. In others, the manner of development is the same as that described for the dogfish.
In the lower vertebrates, the head has no greater mobility than any other part of the body ; indeed, in several bony fish it has none, for the back of the skull is connected with the pectoral girdle by a chain of bones. The neck has not yet evolved in these animals. The neck is a region of flexibility which enables the head to be moved without moving the body. This is made possible by the specialisation of the most anterior vertebrae. In reptiles, the differentiation of the first two vertebrae into the atlas and axis appears, and the head is then able to hinge on the transverse axis (as in signing " yes"), and on the longitudinal axis (as in signing " no "). Movement of the head to the side is effected by the flexibility of the next posterior vertebrae, the ribs of which do not get attached to the sternum. In this way, the cervical vertebrae differ from the thoracic. When the neck is very long and capable of extensive twisting, it is common to find vertebrarterial canals, formed between the centra and the ribs fused on to them. These canals protect the artery from being kinked when the neck is twisted. In mammals, the neck contains seven vertebrae, except in three species only.
Literature
Goodrich, E. S. " Proboscis Pores " in Craniate Vertebrates. Quarterly Journal of Microscopical Science, vol. 62, 1917.
On the development of the Segments of the Head in Scyllium.
Quarterly Journal of Microscopical Science, vol. 63, 1918.
van Wijhe^J. W. Ueber die Mesodermsegmente und die Entwicklung der Nerven des Selachierkopfes. de Waal, Groningen, 1915.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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