1 Comparative Quantitative Analysis of Osseous Anatomy of the Craniovertebral Junction of the Tiger, Horse, Deer, Bird, and Human

10.1055/b-0034-81378

1 Comparative Quantitative Analysis of Osseous Anatomy of the Craniovertebral Junction of the Tiger, Horse, Deer, Bird, and Human

Goel, Atul, Shah, Abhidha, Gaikwad, Santosh, Dhande, Prakash, Kothari, Manu

The anatomy of craniovertebral junction bones of the tiger, horse, deer, and bird was analyzed and compared with human bones ( Figs. 1.1, 1.2, 1.3, and 1.4 ). The evolutionary changes and structural alterations that have occurred due to the functional variations are clearly seen in the comparison. Understanding the anatomy of these animals clarifies the function of the various components of the bones of the craniovertebral junction. The evolutionary changes in the shapes and architectural design of the craniovertebral junction bones in each of these animals have been perfected to suit the job at hand. Despite the wide variations in the size of the bones, the basic patterns of structure, vascular and neural relationship, and joint alignments have remarkable similarities and a definite pattern of differences.

Apart from the sizes and weights, there are several structural variations in the bones of these animals that depend on their functional needs. The more remarkable difference in joint morphology is noticed in the occipitoatlantal joint. The occipitoatlantal articulation is remarkably large and deep, resembling a hinge joint in all the four animals studied. The odontoid process is C shaped in the deer, horse, and bird and is denslike in the tiger and humans. The transverse processes of the atlas are in the form of large wings in animals having heavy heads and thicker neck muscles like the horse, deer, and tiger. The arches of the atlas are large and flat, but the traverse of the vertebral artery resembles, to an extent, that of the human vertebral artery. The rotatory movements of the head at the craniovertebral junction are wider ranged in the horse and deer when compared with that of the tiger and humans. In the bird, the craniovertebral joints are designed to provide a circumferential 360° degree movement of the head. The bones of the craniovertebral junction of all the three mammals are adapted to the remarkable thickness and strength of the extensor muscles of the nape of the neck.

Fig. 1.1 View of the posterior surfaces of the C1 and C2 verte-brae of the (A) tiger, (C) deer, and (D) horse. Note the differences in the sizes and shapes of the bones of each animal in relation to human bones (B).

**Fig. 1.2a–k Images of the craniovertebral junction bones of a horse.** a Inferior view of the posterior aspect of the skull showing the large occipital condyles (1). b Superior (anterior) surface of the atlas, as seen from the superior and anterior perspective, showing the deep cup-shaped articular surface (1) for the articulation with occipital condyles. c Inferior surface showing the atlas articulated with the occipital bone. Note the acute flexed position of the head in relation to the atlas. d Posterior view showing the occipitoatlantal articulation. e Ventral view of the C1 vertebra showing the (1) ventral arch, (2) ventral tubercle, (3) transverse process, (4) transverse foramen, (5) atlantal fossa, and (6) superior articular facets. f Dorsal view of the C1 vertebra showing the (1) dorsal arch, (2) inferior articular facets, (3) transverse foramen, (4) alar foramen, and (5) lateral vertebral foramen. g Superior view of the axis vertebra of the horse showing the C-shaped configuration of the (1) odontoid process and the deep impressions for the (2) longitudinal ligament. The superior articular facets of the axis are seen in relation to the odontoid process. h Anterior view of the C1-C2 articulation. The atlantoaxial joints are relatively flat when compared with the deep concavity of the superior facets of the atlas in relation to the occipital bone. i Posterior view of the C1-C2 articulation. j Lateral view of the C1-C2 bones. The notch for the C2 spinal nerve is converted into the lateral vertebral foramen following ossification of the ligament. k Inferior view of the head of a horse.

**Fig. 1.3a–h Images of craniovertebral junction bones of a deer.** a Ventral view of the C1 vertebra showing the (1) ventral arch, the (2) ventral tubercle, (3) the transverse process, and the (4) superior articular facets. b Dorsal view of the C1 vertebra showing the (1) dorsal arch and the (2) alar foramen. c Anterior view of the axis vertebra. The articular surface of the C-shaped (1) odontoid process can be seen in continuity with the superior articular facets forming a saddle-shaped joint. d Superior view of the axis vertebra of the deer showing the C-shaped (1) odontoid process and the confluent (2) superior articular surfaces. The saddle-shaped articular surface can be vividly seen. e Anterior view of the C1-C2 articulation. f Posterior view of the C1-C2 articulation. g Posterior view of the craniovertebral junction. h Lateral view of the head of the deer. Note the location of the occipital condyles and the prominence of the occipital crest.

**Fig. 1.4a–f Images of the bones of tiger.** a Ventral view of the C1 vertebra showing the (1) ventral arch, (2) transverse process, (3) superior articular facets, and (4) alar foramen. Note the wings of the transverse process and the depth of the superior facets. b Dorsal view of the C1 vertebra showing the (1) dorsal arch and (2) transverse process. c Lateral view of the axis vertebra showing the denslike odontoid process (1) and the characteristic spinous process (2). Note the shape of the odontoid process and the articular surface of the facet that resemble the human C2 vertebra. d Anterior view of the C1-C2 articulation. e Posterior view of the C1-C2 articulation. f Lateral view of the skull of the tiger showing the large occipital crest.

Comparative Anatomy of the Bones of the Horse, Deer, Tiger, Bird, and Human

Tables 1.1 and 1.2 compare the C1 and C2 vertebrae, respectively, of humans to the horse, deer, tiger, and bird.

In quadrupeds, the cervical spine is a vertical part of the entire vertebral column, and the thoracic spine is more or less horizontally oriented. The head of the tiger, deer, and horse protrudes anteriorly in such a fashion that it is in the maximally flexed position at the occipito atlantal joint articulation ( Fig. 1.2 ). In contrast, the cervicothoracic junction is aligned in the maximally extended position. This asymmetric placement of the vertebrae in quadrupeds ensures an energy-saving balance of the head when the animal is in the resting position.1 While in the resting position, the movements permitted at the occipitoatlantal articulation are primarily of extension (flexion being a gravity-assisted passive movement); accordingly, the posterior cervical neck musculature is markedly strong in these animals. The occipital crest is remarkably thick, providing a site for muscular attachment. The occipital crest is most remarkably thick in the tiger, as seen in Fig. 1.4 . In humans, the entire spine assumes a general vertical orientation and its curvatures are much less pronounced when compared with the quadrupeds studied. The vertical stance of the human being places the head directly over the neck in line of the weight bearing of the rest of the spine.1 The muscles of the nape of the neck and the occipital crest in humans are significantly smaller by comparison. The atlantoaxial bone and joint complex of the tiger have much more remarkable resemblance to humans than the bones of herbivorous animals like the horse and deer.

**Table 1.1** Analysis of C1 vertebrae
Definition of Parameters	Humans (cm)	Horse (cm)	Deer (cm)	Tiger (cm)	Bird (cm)
Anteroposterior diameter of superior facet of C1	1.7	4.0	1.0	3.0	0.3
Transverse diameter of superior facet of C1	1.3	3.0	1.5	2.0	0.5
Anteroposterior diameter of inferior facet of C1	1.3	4.0	1.2	2.6	0.3
Transverse diameter of inferior facet of C1	1.2	4.0	1.0	3.0	0.25
Vertical height of anterior arch of C1*	1.0	9.0	1.2	2.0	0.4*
Vertical height of posterior arch of C1	0.7	16.0	1.7	3.8	0.45
Height of superior facet of C1	1.6	5.0	1.5	2.5	0.3
Anteroposterior diameter of spinal canal at C1	2.5	3.5	2.0	2.4	0.7
Transverse diameter of spinal canal at C1	2.3	5.0	1.9	3.0	0.8
Distance of vertebral artery foramen from midline	2.7	3.0	1.4	2.7	—
Horizontal length of C1 anterior arch*	3.5	8.3	2.2	6.5	0.65*
Horizontal length of C1 posterior arch	4.5	11.0	3.1	7.5	1.2
Length of transverse process of C1	0.4	9.7	3.8	5.0	—
Width of transverse process of C1	1.8	4.7	1.5	3.5	—
*In birds, the anterior arch is replaced by a vertebral body that articulates superiorly with the occipital condyle and inferiorly with the body of C2.

Platybasia

There is a platybasia in all three animals studied. The clivus and the anterior skull base are in the same horizontal plane. The maxilla and the upper jaw protrude anteriorly from the cranial base. The brain size is relatively small, and the olfactory nerves are well developed and long, reaching to a length of ∼1 foot (∼30 cm) in the horse. The cerebellum is proportionately large in animals as compared with the cerebral hemispheres. In humans, the angulation of the anterior skull base in relationship to the clivus is probably related to the relatively large size of the cerebral hemispheres.

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