Paper - Vertebrate cephalogenesis 4 (1919)

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Ayers H. Vertebrate cephalogenesis. IV. Transformation of the anterior end of the head, resulting in the formation of the 'nose'. (1919) J Comp. Neurol. 30: 323-.

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Mark Hill.jpg This historic 1919 paper by Ayers describes development of the head and nose region in a number of different species, including man.



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Vertebrate Cephalogenesis

IV. Transformation of the Anterior End of the Head, Resulting in the Formation of the 'Nose'

Howard Ayers

Twenty-Six Figures

Introductory

From the investigations of. many anatomists we have come to recognize, first, that the jaw apparatus (including the tongue and mouth) is a mechanism built up out of old-time head cartilages and newer elements derived from the gills, which has been added to the primitive vertebrate head — in fact, a mechanism of the trigeminus. Second, that the whole auditory apparatus is a mechanism built up out of surface sense organs sunk below the surface, together with structures derived from the gills. Third, that the eyes and adnexa are the end result of the outpushing from the brain of two hollow globes, whose walls are made up of the pigmented light sensitive cells of that part of the central nervous axis behind the lamina terminalis, with numerous associated parts. These three prominent organ systems have been quite thoroughly worked out and their component parts traced back to their origins. In speaking thus of the evolution of these three prominent additions to the head, we include those other necessary structures, as blood, nerve and lymph supply, and the muscles, connective- tissue parts and skin, which go to correlate and cover these organ systems. As regards the nose, it is generally agreed that the olfactive organs are derived from a pair of sensory organs, formed near the anterior end of the head which very early sink below the surface of the skin of the snout, later to become housed in a cavity called the nasal chamber. The details of this process and the structures involved have not been definitely worked out and described.

The following condensed account of some of the results of my study of the vertebrate nose has for its object, making clear the manner in which the human nose has come to be and also to homologize the several structures entering into its formation. The literature is extensive and regarding such structures as, e.g., Jacobson's organ, the N. terminalis, and the vomeronasal nerve, by no means harmonious either in statement of fact or interpretation. No attempt will be made here to review the literature, it being considered more important to lay the foundation for a rationals tudy of the nose and nasal region of the vertebrate head.

Beginning with the stage of head development presented by Amphioxus, we pass to Ammocoetes, Petromyzon, Bdellostoma, and Man. Already in the Marsipobranchs, the main features of human nasal anatomy are l^tid down, since the nasal chamber of Bdellostoma contains the terminal organs supplied by three pairs of cranial nerves with the endings of the invading branches of the trigeminus.


1. Amphioxus

The anterior end of the head region of Amphioxus (figs. 1 and 12) is compressed from side to side and has the shape of a spearhead, viewed either from the side or above. Included within it we note the anterior end of the central nervous system with the terminal paired but unseparated eye rudiments and the primitive olfactive organs. This part of the nervous axis contains the ventricular cavity and is the earliest stage known to us of the vertebrate brain. Connected with the brain from before, backward, are the following paired nerves (figs. 1, 2, 3, 4, 5, 6, 7, and 12) : 1. N. terminalis. 2. The N. opticus which does not extend beyond the brain contour. 3. N. olfactorius. 4. N. septalis.


The N. terminalis^ is connected to the right and left side of the anterior end of the ventral plate of the brain, and owing to the relatively large size of this pair of nerves they appear, when viewed from above, like a bifurcation of the brain. They run forward to the tip of the head. The N. opticus is entirely imbedded in the front wall of the brain anterior to the ventricle. In Amphioxus we have a stage of the evolution of the eye antedating the formation of optic cups which, from Bdellostoma on, presents such a prominent feature of vertebrate anatomy. The N. olfactorius (figs. 2, 3, 4, and 7), being relatively small, is a short nerve which arises from the anterodorsal wall of the brain near to the median line and, while the right and left olfactorius arise from the right and left halves of the brain respectively, they are usually drawn close together into one trunk as they approach the olfactive organ, although they occasionally remain separate and distinct their entire length. They run dorsad and cephalad and innervate the right and left halves of the olfactive cup just as their homologues do in Ammocoetes and all other vertebrates. The N. septalis arises from the dorsal wall of the brain on either side of the median line above the posterior limit of the ventricular cavity. The two nerves curve upward, forward, and outward, and run to the sides of the anterior end of the head, innervating the territory mainly caudad of the N. terminalis and as far back as the hypophysis.

In Amphioxus the body surfaces innervated by the terminal and septal nerves (figs. 1 and 12) are fully exposed, except the hypophysial region which lies within the buccal cavity; with this exception they form part of the body contour. The olfactory organs are sunk below the body surface as a conical pit opening directly on the surface and more or less pushed to the left of the median line by the dorsal head fin fold.^

The distribution of the septalis nerve in Amphioxus is as follows: Arising from the dorsolateral territory of the brain above the posterior border of the ventricle the nerve trunk soon separates (figs. 1, 12 and 26) into two parts. The larger part curves forward and outward, dividing as it passes to its terminal territory behind the tip of the snout which is supplied by the N. terminalis. The smaller branch curves forward and downward over the surface of the notochord and innervates the surface territory about the anterior end of the buccal cavity as well as the walls of the terminal pocket of the mouth and the dorsal hypophysial organ which formerly occupied a surface position as the preoral pit of the larva.

The terminal territory of the Amphioxus body is thus a terminal sensory organ. As far as known, the sensory elements are isolated sensory cells distributed with some regularity of spacing throughout the epithelial covering of the body in the territory supplied by the terminal and septal nerves. These sensory cells have not been found in other localities. They are innervated by terminal twigs given off from the short fibrils which issue from the groups of subepithelial ganglion cells belonging to these two nerves (fig. 11). The close association of the terminal and septal nerves in their peripheral distribution is reflected in the central exchange of fibers (figs. 8, 9, and 10) and doubtless in their physiological functions, to the extent of their being grouped to gether as a single nerve physiologically. The remarkable persistence of these two nerves as anatomically distinct structures throughout the entire vertebrate series from Amphioxus to man is noteworthy and needs further attention from anatomists. The concentration of the chemical sense organs, olfactive sense organs, and light perceptive organs at the anterior end of the neural axis is strikingly shown in Amphioxus and the anatomy of the nasal chamber of man as herein described shows that the morphological relations of these structures have not been much disturbed throughout the evolution of the vertebrates. In Amphioxus the neural lips of the anterior neuropore maintain their original relations in the adult. This is indicated by the position of the terminal and septal nerves which supply the tip of the head and the organs connected with the ventral end of the neuropore, the hypophysis, while the olfactory nerves supply the organs connected with the dorsal end of the neuropore, the nose. The whole neuroporic territory is concerned with testing alimentary and respiratory supplies, i.e., food in the broad sense.


ABBREVIATIONS

A, place of origin of the hypophysial organ

B, last surface position of hypophysial organ

Br, brain.

C, position of hypophysial organ in adult

G, nasal gland (Jacobson's) organ

n, hypophysis

HC, hypophysial canal

h, hypophysial extension

B..S., hypophysial sac

h.O, lobus olfactorius

l.t, lamina terminalis

M, mouth

M' , anterior buccal pouch

AT, nose

n, external nasal opening

n./, nasal fold

n.n, nasal nerve

liC, nasal canal


'NF, nasal fold

A^.p, nasopalatine nerve

0, olfactory nerve

OL, L.O, olfactory lobe

ON, olfactory nerve

op, optic nerve

P, anterior buccal pouch

PN, prenasal sac

SN, septalis

Sk, cranial wall

Sj), nasal septum

S, hypophysial branch of septalis

T, n. terminalis

T.&S, n. terminalis and n. septalis

TO, terminal organ

TH, hypophysial branch of terminal

nerve tt, terminal nerve bundle V, velum V, valve Vt, ventricle



Fig. 1 View of left side of the head of Amphioxus, showing brain and nerves The terminalis, opticus, olfactorius, and septalis are the first four cranial nerves. The apical territory supplied by the terminal and septal nerves is distinctly marked off from the rest of the head and its close association with the optic and olfactory sensory structures is evident. The septal territory has already undergone partial migration into the buccal cavity. The path of migration of the hypophysial organ is indicated by the letters A and B and the dotted line B-C. Over the outline of the apical buccal pouch is shown the last surface position (in the larva) of the pre-oral pit (hypophysial organ) before its entrance into the mouth chamber.

Fig. 2 Anterolateral view of the brain of Amphioxus (short type), showing the terminal, septal, and olfactory nerves and the region of the lamina terminalis.

Fig. 3 Anterior view of the same brain of Amphioxus.

Fig. 4 Ventrolateral view of the same brain of Amphioxus, showing the first three nerves and the infundibular territory.

Fig. 5 Anterodorsolateral view of a brain of Amphioxus (.transparent) , to show the ventral territory of the neuropore in the adult.

Fig. 6 Lateral view of the brain of Amphioxus, to show the roots of the septal nerve, the ventricle with its olfactory, infundibular, and spinal prolongations.

Fig. 7 Dorsolateral view of brain of Amphioxus, to show paired olfactory nerves.

Fig. 8 Ventral view of brain of Amphioxus to show the course of fibers of terminal nerve.

Fig. 9 Posterior ventrolateral view of Amphioxus brain, to show course of fibers of terminal and septal nerves.

Fig. 10 Anterolateral view of another brain, to show course of fibers of terminal and septal nerves.



2. Ammocoetes and Petromyzon

The head region of Ammocoetes has the shape of a truncated cone with the snout truncated from above, downward, and backward. In the larval Petromyzon the superficial territory innervated by the terminalis and septalis which in Amphioxus forms part of the body surface is withdrawn into the protection of a nasal canal along with the olfactory organ and opens secondarily to the outside through the nasal canal. Confining our attention for the present to the morphological equivalents of the parts just described in Amphioxus, we find the spear-shaped primitive tip of the head withdrawn bodily into the nasal chamber to form the nasal septum of Ammocoetes, each half receiving a rich nerve supply from its N. terminalis (fig. 13). The characteristic condition of the septum in Ammocoetes furnished the key for the solution of the problem of the homologies of the nasal organ of vertebrates. On either side of this septum (fig. 14) we find the olfactive organs, each with its N. olfactorius. The N. septalis passes to the septum along with the median bundles of the olfactory nerves. The optic nerves are well developed and leave the base of the brain below and behind the N. terminalis. Owing to the formation of heavy lips surrounding the mouth — which in the adult became the 'sucking disk' — the nasal tube opens on the top of the head in front of the brain region. The olfactory organs and nerves are completely separated from each other in Ammocoetes by this nasal septum, which from its origin we recognize as a fundamental as well as an original landmark in head anatomy. At the base of the septum is a glandular structure (figs. 14, 15, and 16) which is innervated by the terminal nerve. This paired gland is a more or less constant feature of the vertebrate nose from Ammocoetes up to man. It has been described in many forms as the organ of Jacobson. Between the Amphioxus and Ammocoetes condition of the terminal and nasal region of the head there has occurred a translation of this region ventrad and caudad in the sagittal plane with a concomitant enlargement of the nasal organ and the separation of its right and left halves by the interposition of a septum formed by the spear-shaped tip of the head. In other words, the olfactive organs have migrated ventrad along the sides of the septum. Thus the olfactive organs come to lie laterad of the terminal and septal territory, instead of dorsal, as in Amphioxus.



Fig. 11 End twig of terminal nerve with ganglion cells and peripheral fibrils

Fig. 12 Ventral view of snout and mouth of Amphioxus, to illustrate relations of brain to anterior buccal pouch, and the distribution of the terminal and septal nerves as seen from below. The hypophysial organ with dorsal buccal groove running caudad and the band of thickened epithelium running cephalad are shown. In the drawing the head is expanded laterally to make room to show the parts clearly.

Fig. 13 Sagittal section of brain and nasal organs of Ammocoetes, to show relations of olfactory and terminal nerves to the brain and the olfactory and terminal organs.

Fig. 14 Horizontal section of brain and nasal organs of Ammocoetes, to show relations of olfactory and terminal nerves and the nasal septum.

Fig. 15 Three cells from the 'nasal' epithelium of Ammocoetes, one olfactive sense cell and two ciliated epithelium cells from the ciliated 'gland' of the terminal organ.

Fig. 16 Part of a section of the 'gland' of the terminal organ.



In Petromyzon marinus the tip of the primitive head has been sunk still further below the surface of the body and surrounding it the nasohypophysial canal has been complicated by the formation of sacs and pockets with valves and folds for the reception and control of the water to be tested. The olfactive organs have been expanded and pushed forward as well as downward, while the terminal and septal structures occupy the ventroposterior portion of the nasal capsule. This portion of the complicated nasal organ has been described as a gland. It is made up of a series of pockets or tubes which open out on the face of the wedge-shaped terminal organ to become continuous with the folds of the septal region of the nasal chamber (figs. 17 and 17A).


3. Bdellostoma

In Bdellostoma the terminal region of the Amphioxus head has been withdrawn still deeper into the head of the adult hagfish by the formation of a long and capacious nasal canal (nasohypophysial canal) (figs. 21 and 22). The N. terminalis supplies a well-developed sense organ having to do with the testing of the respiratory water (fig. 20). About it are developed valvular folds from the lining of the nasal canal which control the admittance of the water to the terminal organ, the septal epithelium, and the nose and of course to the hypophysial canal. The nasal respiratory mechanism can quickly close off the sense-organ chamber and expel the water in the nasal canal forward as well as backward. The optic nerves are long and extend outward from the base of the brain to the optic cups, no lens being present. The N. terminalis runs dorsad within the brain (figs. 18 and 19) before breaking through to the surface and then runs ventrad within the median fissure between the olfactory lobes to the horizontal level of the terminal organ when it runs cephalad to its terminations in the tip and sides of this structure. The N. septalis leaves the dorsal region of the olfactory lobe of the brain inside the median fissure and near its anterior end, but separate from the median olfactory nerve bundle which leaves the tip of the median part of the olfactory lobes. The septal nerve also runs ventrad before coursing forward to supply its peripheral territory. The olfactory nerves are enormously developed and the nerve of each side supplies three complete and two half folds (plates) of the compound nasal organ, all of which, heretofore, has been called the olfactory organ. The N. terminalis in Bdellostoma presents a stage intermediate between Amphioxus and the Selachians.^ In the former, the olfactory nerves are small and in their primitive terminal position on either side of the dorsal end of the neuroporic raphe, while the N. terminalis is relatively large, leaving the brain nearly midway between the olfactory organ and hypophysis, and strictly terminal in its positions as regards the adult brain.



Fig. 17 Lateral view of the 'nasal' organ of Petromyzon marinus. The reference letters show position of sections A to G, figured below. 17A Is a dorsal view of the nasal organ of the same Petromyzon. A. Section through tip of prenasal sac. B. Section through expanded portion of prenasal sac. C. Section through the edge of nasal funnel, the nasal canal, the anterior ends of the anterior olfactory pockets, and the naso-hypophysial canal. D. section through the nasal funnel, showing the funnel valve and the anterior part of the nasal sac with the free folds. E. section through the posterior part of the nasal funnel, the body of the nasal sac with free nasal folds, the ventral half of the nasal septum and the nasohypophysial canal. F. section through the nasal sac with completed septum and free nasal folds in right and left nasal chambers, the anterior end of the nasal gland (terminal gland) and the nasohypophysial canal. G. section through the posterior part of nasal body showing posterior nasal pockets, the enlarged posterior end of nasal gland, the ventral part of septum and the nasohypophysial canal.


Fig. 18 Lateral view of the right olfactory, septal, and terminal nerves of Bdellostoma dombeyi, inside the median fissure separating the olfactory lobes. The dissection exposes the intercerebral course of the terminal nerve for a short distance.

Fig. 19 Same view as figure 18 of another Bdellostoma brain.


In Bdellostoma the olfactory nerves have become enormously increased and overshadow the N. terminalis, which, while it remains terminal in its peripheral distribution leaves the brain near the anterior end of the olfactory lobes within the median fissure. The development of the olfactory lobes is so great that the primitive anterior end of the brain is covered over and its relation obscured. The great increase in size of the olfactory nerves causes them to enfold the forebrain in an inclosing overgrowth of the olfactory lobes, which apparently forces the roots of the terminalis upward, but they retain their relative position with reference to the exit of the olfactory nerve, viz., mesad and ventrad of it. In Amphioxus we found branches of the septal nerve innervating the hypophysial organ. In Bdellostoma the hypophysial branches are given off from the terminal nerve. In mammals it has been shown by Huber and Guild^ and LarselP that both terminal and septal branches run to the organ of Jacobson, i.e., the terminal organ. In Bdellostoma, the ganglion cells of the terminal nerve lie in the terminal organ (fig. 23).



Fig. 20 Ventral view of Bdellostoma 'nasal' organ, to show the distribution of the olfactory, terminal, and septal nerves, the median terminal organ and other 'nasal' folds.

Fig. 21 Same view of nasal organ and nasohypophysial canal in Bdellostoma.

Fig. 22 Lateral view of nasal organ, brain, and nasohypophysial canal of Bdellostoma.

Fig. 23 A. Section through the mucosa covering terminal organ of Bdellostoma. Five layers of epithelium cells are shown above the basement membrane beneath which lies a plexus of nerve fibrils given off from the ganglion cells lying below. B. Four ganglion cells from same section. C. Surface view of epithelium from same terminal organ to show relation of sensory cells to other surface cells.


4. Chimpanzee and Man

In man as described by Brookover^ and in the chimpanzee, as my dissections disclose (fig. 24) the N. terminalis leaves the brain ventrad and mesad of the olfactorius and passes forward to the lamina cribrosa. In man they pass through this plate along with the septal and olfactory nerves and run ventrad to terminate in and about Jacobson's organ. In the chimpanzee the nerve on leaving the brain enters the pia and takes its course forward to near the lamina cribrosa where it passes into the dura and leaves the cranial chamber along with the bundles of olfactory fibers. Its course outside the cranial chamber was not traced. The recently published researches of Larsell^ on several mammals show conclusively that both the terminal and septal nerves are, in this class, preserved in their original relations to the olfactory nerve and brain. The nasal chamber in man, therefore, contains the surface distribution of these three most ancient cranial nerves as well as the surface terminations of invading branches of a fourth and more recent cranial nerve, the trigeminus (fig. 25).

5. The Anterior Cranial Nerves

We thus find that the nasal septum and related parts form one of the most ancient and least changed morphological complexes in vertebrate anatomy. Reacting to physiological necessity, the advanced outposts of the central nervous apparatus of vertebrates were withdrawn early in the life of the phylum more or less deeply into the protective hood furnished by the overgrowth of the muscles supported by added skeletal structures, and body covering from territory lying behind the region of the primitive brain, in fact, from the territory of the trigeminus nerve; we therefore find that the advanced outposts of the brain, the most anterior sense organs of the body surface, in all forms above Amphioxus are tactile sense organs of simple or complex structure belonging to the trigeminus as distinguished from the earlier group of chemical sense organs, belonging to the terminal, olfactory, and septal nerves which form the anterior sensory outposts in Amphioxus. This change in the character of the sensory outposts is of course a result of forward overgrowth of the trigeminus mechanisms. We also find that the trigeminus structure has not only surrounded and housed in this group of chemical sense organs, but has also invaded the original olfactory and terminal sensory territories supplied by the NN. terminalis, olfactorius, and septalis, and here performs some function of a tactile sort. Along with the housing of the chemical sense organs there has been built up in the long series of vertebrate forms a large variety of control systems of valves, doors, guide folds for the control of water and air currents, their admission, guidance, and expulsion varying in different animals as the case may be. The emergence of vertebrates from water breathing to air breathing has not affected the physical conditions of the functioning of the chemical sense organs, for they still are kept wet and pick up their stimuli out of a liquid medium, thanks to the moisture supplied by the 'mucosa.' With the exception of the jaw apparatus and related parts, no single change of, or addition to head structures has caused greater changes in contour or anatomical detail than the housing of the chemical sense organs. Owing to the failure to recognize the presence of both the terminal and septal nerves, the first arising ventrad of the olfactorius, while the second arises dorsad of the olfactorius, much confusion exists in accounts of the so-called nervus terminalis. In all vertebrates yet examined, one of these nerves is found to be present, which may arise near the dorsal surface of the brain or near the ventral surface. In many forms both pairs of nerves have been found. Further investigation will doubtless show that both nerves are present in all vertebrates. In order to make final decision, it will be necessary to trace the nerves to their central as well as peripheral terminations. The presence of coordinating sympathetic fibers in the terminal and septal nerves seems to be definitely proved in a number of the species studied. The diagrams shown in figure 26 cover four stages in the transformation of the apical head region of Amphioxus into the nasal chamber and septum of the higher forms.




Fig. 24 Ventral view of right cerebrum of Troglodytes (chimpanzee) to show intracranial course of N. terminalis.

Fig. 25 Sagittal section of nasal chamber of man from Toldt, to indicate extracranial course of N. terminalis drawn in from Brookover's description. The septal nerve (Vomeronasal) practically parallels the course of the terminal branches. The invading branches of the trigeminus are also shown.

Fig. 26 Six diagrams illustrative of the translation of the chemical sense organs of the neuroporic territory of Amphioxus, from the exposed position on the surface of the head to the inclosed condition found in Bdellostoma where they are housed in the 'nasal' chamber. A. Pre-Amphioxus stage with the hypophysial organ still outside of and in front of the buccal cavity. B. Amphioxus stage with the hypophysial organ inside the buccal cavity, said organ being the first of the apical territory to find protection. C. Post-Amphioxus stage in which the overgrowth of the trigeminal structures has housed in the remainder of the apical territory of the Amphioxus head. This apical region furnishes the nasal septum together with the sensory structures associated with it in the nose of the higher vertebrates. D. Bdellostoma stage in which a nasopharyngeal partition has appeared partly separating nasal and buccal chambers, fl. Ventral view of stage C. F. Ventral view of stage D.


Stage A is pre-Amphioxus condition. The hypophysial organ (pre-oral pit) is shown on the outside of the head in the position it reaches during larval life just before it enters the buccal chamber. Here it is in the surface territory innervated by the septal nerve.

Stage B represents the relation of the parts in the adult Amphioxus. The hypophysial organ is housed in the buccal cavity.

Stage C is a pre-Bdellostoma condition antedating the formation of a nasobuccal partition. The entire head territory of the Amphioxus stage is now housed in the buccal chamber and projects into it along the sagittal plane, forming a partial septum which partly separates the nasal portion of the common nasobuccal chamber into right and left nasal chambers.

Stage D represents the Bdellostoma condition. The primitive apical territory is now enclosed in a 'nasal capsule/ open below, which is divided into right and left halves by the terminal organ, which forms a complete septum for structures of the capsule. The nasal chamber is further separated from the buccal cavity by a horizontal partition. The nasohypophysial canal is an extension forward and backward of the nasal chamber.

The cranial nerves in man are not, therefore, twelve in number, but fourteen. In the order of their connection with the brain we may tabulate them as follows:

Nerve 1, terminalis Nerve 8, abducens

Nerve 2, opticus Nerve 9, facialis

Nerve 3, olfactorius Nerve 10, acusticus

Nerve 4, septalis Nerve 11, glossopharyngeus

Nerve 5, oculomotorius Nerve 12, vagus

Nerve 6, trochlearis Nerve 13, accessorius

Nerve 7, trigeminus Nerve 14, hypoglossus


6. Comments on Function

We know almost nothing of the function of the terminal organ, of Jacobson's organ, of the hypophysis. We are perhaps warranted in assuming that the terminal, olfactory, and septal nerves have to do with special chemical senses, of which above Amphioxus the olfactory sense plays the predominant role. Next in importance stands the terminal sense organ, which we find from the fishes on, as the organ of Jacobson, a paired sense organ of the nasal chamber which appears reduced in importance as compared with the olfactory organ, although in the Ophidia the reverse appears to be the case. The functions of the nerves in the nasal chamber may be divided as follows:

f Olfactory nerve Testing alimentary foods

Chemical sense < Terminal nerve \ m i- • ^ *■ j

[Septal nerve / ^^'^'""^ respiratory foods

[Testing for solid bodies in

m .-1 rr. • • 1 ] the respiratory currents and

Tactile sense Trigeminal nerve < . ^, "^ , ^,

I sensing the pressure and the

[ current flow

Although the chemical sense organs have been housed in the nasal chamber, they have, so far as the structure of the sensory surfaces are concerned, remained in a primitive state. These sense organs have not developed accessory parts in such degree as have the eye and ear. In the case of the eye, the vitreous body, lens, cornea, lids, etc., have been added to increase the precision or enlarge the range of its functions, there has been added to its light-perceiving function the optical reactions. In the case of the ear, there has been a parallel evolution of the primitive function of wave-motion perception by the addition of tone perception, with the cochlea as its anatomical expression. In both cases there has been a progressive increase in the number of protective structures as well as of parts serving the increase of precision and enlarged range of function. In the case of the chemical sense organs of the nasal chamber there has not been such considerable increase of subsidiary parts or perfection of the sensory structures, and they therefore remain organs whose stimuli belong more to the subconscious domain of reaction to the environment than is the case with either the eye or ear. Even when conscious attention is directed to the reactions of the nasal organs, they can only partly be brought into the realm of definiteness. This is proved by the fact that from the days of Aristotle down to the discovery of the terminal nerve there was never a hint of anything more than an olfactive function. Even in man to-day the olfactive function is a vague and uncertain sense in itself and needs, in order to make certain the interpretation of the stimulus, the assistance of other sense organs, e.g., the eye or the ear. To illustrate, most persons are sure they can recognize the odor of the rose, specifically, a given variety of rose, with whose characteristics they are familiar, but blindfolded and lacking tactile stimuli they cannot identify with certainty the source of the odor, often indeed it may call up the memory of violet odor or some other odor. It is different with the eye. By sight alone and within a considerable range of distances we can recognize any object of definite form which we have seen before. The ear stands between the nose and eye with regard to definiteness and certainty of the results of the stimuli. While we can 'smell' an odor and not be able to identify it, and can hear a tone and not be able to place it in the scale, we can always recognize objects by sight, by either their form, color, size, motion, or all combined. All three senses are quite equally and similarly limited by the upper and lower limits of intensity of stimuli. Although the nasal senses lack conscious definiteness, when compared with the eye or ear, they are not on that account less determinative of physiological (and psychological) reactions. They are primitive and fundamental senses. When stimulated, the nerve reactions, even though subconscious, may be propagated far and wide throughout the nervous apparatus, much like 'sympathetic' reactions. Rarely is there an instantaneous response such as so frequently results from stimuli of the auditory nerve. The cranial nerves of the nasal chamber have not the intimate associations with the 'voluntary' muscular apparatus that the auditory apparatus has. Our knowledge of both the peripheral and central relations of the four cranial nerves having endings in the nasal chamber is very imperfect. Even more fragmentary and obscure is our knowledge of their functions.

Winding Way and Valley Road Cincinnati, January 20, 1919

Literature Cited

1 Van Wlihe, J. W. 1918. On the nervus terminalis from man to Amphioxus. Proceedings Koninkljke Akad. van Wetenschappen, vol. 21, No. 1 and 2.

2 Ayers 1907 Vertebrate cephalogenesis — Amphioxus and Bdellostoma.

3 LocY, William A. 1905 On a newly recognized nerve connected with the forebrain of selachians, Anat. Anz., Bd. 26 pp. 33-63, 111-123. Also other papers by the same investigator.

4 HxjBER, G. Carl AND Guild, Stacy R. 1913 Observations on the peripheral distribution of the nervus terminalis in Mammalia. Anat. Record, vol. 7, pp. 253-272.

5 Larsell, Olof. 1918 Studies on the nervus terminalis: Mammals. Jour. Comp. Neur., vol. 30, pp. 3-68.

6 Brookover, C. 1914 The nervus terminalis in adult man. Jour. Comp. Neur., vol. 24, pp. 131-135.

1917 The peripheral distribution of the nervus terminalis in an infant. Jour. Comp. Neur., vol. 28, pp. 349-360.


Cite this page: Hill, M.A. (2020, December 4) Embryology Paper - Vertebrate cephalogenesis 4 (1919). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Vertebrate_cephalogenesis_4_(1919)

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