Table 1.
Comparisons of the central nervous system between sheep and humans.
| Features/Gross | Sheep | Human | References |
|---|---|---|---|
| Brain shape | Smaller and elongated | Larger and rounded | (18) |
| Skull thickness (mm) | 5.0–6.0 | 6.5–7.5 | (19) |
| Brain mass (g) | 130–140 | 1,300–1,400 | (14) |
| Four lobes defined by external landmarks | Present | Present | (12) |
| Sulci and gyri | Present | Present | (12–14) |
| Cerebral cortex | Primarily neocortex | Primarily neocortex | (13, 20) |
| Motor cortex | Located in frontal lobe (superior frontal gyrus) | Located in frontal lobe | (21) |
| Somatosensory cortex | Located in frontal lobe (middle frontal gyrus) | Located in parietal lobe | (21) |
| Cortical layers | Distinct cellular layers I-VI | Distinct cellular layers I-VI | (22) |
| Cortical interneuron | Significant role | Significant role | (23) |
| Frontal lobe | Small | Very large | (18) |
| Olfactory bulb | Large and well-developed | Small | (24, 25) |
| Optic chiasm | More pronounced | Less pronounced | |
| Orbit indentation | Side | Front | (26) |
| Visual cortex | More lateral | More midline | (26) |
| White matter | Abundant | Very abundant | (27) |
| Cerebrum | More elongated | Less elongated | (18) |
| Rigid tentorium cerebelli | Present | Present | (28) |
| Cerebellum | Smaller, located posteriorly (behind the cerebrum) | Larger, located caudally | (29) |
| Meninges | Thick, well-developed | Thick, well-developed | (18) |
| Subventricular zone | Laminar structure | Laminar structure | (30) |
| Subgranular Zone | Laminar structure | Laminar structure | (30) |
| Hippocampus | Ventral aspects of cerebrum | Ventral aspects of cerebrum | (24) |
| Basal ganglia | Separate caudate and putamen | Separate caudate and putamen | (14, 16) |
| Substantia nigra pars compacta and pars reticulata cell diameter (μm) | 9–26 and 10–23 | 14–50 and 20–30 | (22, 31) |
| Substantia nigra pars compacta and pars reticulata average volume (mm3) | 13 and 152 | 68 | (22, 31) |
| Substantia nigra pars compacta and pars reticulata average cell number | 39,481 and 51,800 | 436,000 | (22, 31) |
| Gross spinal cord | Located posteriorly | Located caudally | (18) |
| Vertebral bodies (Cervical spine) | Taller than wide | Wider than tall | (32) |
| Lumbar spine curvature | Kyphotic | Lordotic | (32) |
| Spinal canal width | Wider (Identical to human) | Wider | (32) |
| Spinous process (cervical and thoracic regions) | Longer | Smaller | (32) |
| Spinous process (Lumbar regions) | Longer | Longer | (32) |
| Sciatic nerve origin | L6-S2 | L4-S3 | (33) |
| Pineal gland | Large and round; located at the interface between the cerebral hemispheres and cerebellum, not lobulated | Small, pine cone shaped, located within the posterior wall of the third ventricle near the center of the brain, lobulated | (34) |
Where no journal reference is given, the human features/gross are adopted for comparison with sheep data from the Nervous System by Snyder et al. (18) with permission from Elsevier.