The last group of organs to be dealt with for our present question is that of the Head-Cavities.
The walls of the spaces formed by the cephalic prolongations of the body-cavity develop into muscles and resemble the muscle-plates of the trunk, and with these they must be identified, as has been already stated. As equivalent to the muscle-plates, they clearly are capable of serving as very valuable guides for determining the segmentation of the head. There are then a pair of these in front of the mandibular arch, a pair in the mandibular arch, and a pair in each succeeding arch. In all there are eight pairs of these cavities representing eight segments, the first of them præoral. As was mentioned above, each of the sections of the head-cavity (except perhaps the first) stands in a definite relation to the nerve and artery of the arch in which it is situated.
The comparative results of these three independent methods of determining the segmentation of the head are in the subjoined table represented in a form in which they can be compared:—
Table of the Cephalic Segments as determined by the Nerves, Visceral Arches, and Head-Cavities.
| Segments | Nerves | Visceral Arches | Head-Cavities or Cranial Muscle-Plates |
|---|---|---|---|
| Præoral 1 | 3rd and 4th and ? 6th nerves (perhaps representing more than one segment) | (?) | 1st head cavity (in my figures 1pp.) |
| Postoral 2 | 5th nerve | Mandibular | 2nd head-cavity (in my figures 2pp.) |
| ——3 | 7th nerve | Hyoid | 3rd head-cavity |
| ——4 | Glossopharyngeal nerve | 1st branchial arch | 4th head-cavity |
| ——5 | 1st branch of vagus | 2nd branchial arch | 5th head-cavity |
| ——6 | 2nd branch of vagus | 3rd branchial arch | 6th head-cavity |
| ——7 | 3rd branch of vagus | 4th branchial arch | 7th head-cavity |
| ——8 | 4th branch of vagus | 5th branchial arch | 8th head-cavity |
In the above table the first column denotes the segments of the head as indicated by a comparison of the three sets of organs employed. The second column denotes the segments as obtained by an examination of the nerves; the third column is for the visceral arches (which lead to the same results as, but are more convenient for our table than, the visceral clefts), and the fourth column is for the head-cavities. It may be noticed that from the second segment backwards the three sets of organs lead to the same results. The head-cavities indicate one segment in front of the mouth, and now that the ophthalmic branch of the fifth has been dethroned from its position as a separate nerve, the eye-nerves, or one of them, may probably be regarded as belonging to this segment. If the suggestion made above (p. [431]), that the walls of the first cavity become the eye-muscles, be correct, the eye-nerves would perhaps after all be the most suitable nerves to regard as belonging to the segment of the first head-cavity.
EXPLANATION OF PLATES 15, 16, 17.
Plate 15. (The Head during stages G—K.)
Complete List of Reference Letters.
1aa, 2aa, etc. 1st, 2d, etc. aortic arch. acv. Anterior cardinal vein. al. Alimentary canal. ao. Aorta. au. Thickening of epiblast to form the auditory pit. aun. Auditory nerve. aup. Auditory pit. auv. Auditory vesicle. b. Wall of brain. bb. Base of brain. cb. Cerebellum. cer. Cerebrum. Ch. Choroid slit. ch. Notochord. com. Commissure connecting roots of vagus nerve. 1, 2, 3 etc. eg. External gills. ep. External epiblast. fb. Fore-brain. gl. Glossopharyngeal nerve. hb. Hind-brain. ht. Heart. hy. Hyaloid membrane. In. Infundibulum. l. Lens. M. Mouth involution. m. Mesoblast at the base of the brain. mb. Mid-brain. mn. v. Mandibular branch of fifth. ol. Olfactory pit. op. Eye. opn. Optic nerve. opv. Optic vesicle. opth V. Ophthalmic branch of fifth. p. Posterior root of spinal nerve. pn. Pineal gland. 1, 2 etc. pp. First, second, etc. section of body-cavity in the head. pt. Pituitary body. so. Somatopleure. sp. Splanchnopleure. spc. Spinal cord. Th. Thyroid body. v. Blood-vessel. iv. v. Fourth ventricle. v. Fifth nerve. Vc. Visceral cleft. Vg. Vagus. vii. Seventh or facial nerve.