The spinal nucleus of the accessory nerve (SNA) comprises the group of somata (perikarya) of motor neurons that supply the sternocleidomastoid and trapezius muscles. There are many conflicting views regarding the longitudinal extent and topography of the SNA, even in the same species, and these disagreements prompted the present investigation. Thirty Sprague-Dawley rats (15 males, 15 females) were used. The SNA was localized by retrograde axonal transport of horseradish peroxidase. Longitudinally, the SNA was found to be located in the caudal part (caudal 0.9 -1.2 mm) of the medulla oblongata, the whole lengths of cervical spinal cord segments C1, C2, C3, C4, C5 and rostral fourth of C6. In the caudal part of the medulla oblongata, the SNA was represented by a group of perikarya of motor neurons lying immediately ventrolateral to the pyramidal fibres that were passing dorsolaterally after their decussation. In the spinal cord, the motor neuronal somata of the SNA were located in the dorsomedial and central columns at C1, in the dorsomedial, central and ventrolateral columns at C2 and in the ventrolateral column only at C3, C4, C5 and rostral quarter of C6. The perikarya of motor neurons supplying the sternocleidomastoid were located in the caudal part (caudal 0.9 -1.2 mm) of the medulla oblongata ventrolateral to the pyramidal fibres that were passing dorsolaterally after their decussation. They were also located in the dorsomedial and central columns at C1, in the dorsomedial, central and ventrolateral columns at C2 and only in the ventrolateral column at the rostral three-quarters of C3. The perikarya of motor neurons supplying the trapezius muscle were located in the ventrolateral column only in the caudal three-quarters of C2, the whole lengths of C3, C4 and C5, and in the rostral quarter of C6.

The aim of the study was to investigate the location of motor neuron somata of geniohyoid muscle in rat. Nine Sprague-Dawley rats were used in this study. Operations were performed under general anaesthesia. Nembutal sodium, 40 mg per kg intraperitoneally was used for anaesthesia. 0.02 to 0.05 ml of 30% horseradish peroxidase (Sigma Type VI) solution in normal saline was injected into the exposed right geniohyoid muscle. After 48 hr, the animals were fixed by perfusion through left ventricle of heart, first by 100 ml normal saline and then with 500 ml of 1.25% glutaraldehyde and 1% paraformaldehyde in 0.1 M phosphate buffer, pH 7.4, at room temperature, and finally with 500 ml of 10% sucrose in the same buffer at 4°C. The medulla oblongata and first cervical segment of spinal cord were removed, kept in 10% sucrose in above phosphate buffer at 4°C for 24 hr. Thereafter, their serial transverse sections were cut in a cryostat at a thickness of 60 μm. The sections were treated according to tetramethyl benzidine (TMB)-horseradish peroxidase (HRP) method. HRP-labelled neuron somata were observed at the following sites: (a) In ventral part of right main hypoglossal nucleus in upper two-thirds of the closed part of medulla oblongata. (b) In ventrolateral subnucleus of hypoglossal nucleus in lower third of closed part of medulla oblongata. (c) At spinomedullary junction, they were located in dorsomedial part of right ventral grey column; a few were also seen here scattered on right side of central canal and among corticospinal fibres.

The upper interparietal segment of the squamous part of the occipital bone develops in membrane and the lower supraoccipital part develops in cartilage. According to the available literature, the interparietal segment is ossified from 2 to 3 pairs of ossification centres and each of these centres has 2 nuclei. Interparietal bone is formed due to failure of fusion of these centres and/or their nuclei with each other. Many variations of interparietal bone have been reported by many investigators. In the present study, out of 25 human skulls studied, six skulls had some interesting variations of interparietal bones. Four interparietal bones were found in one skull, 2 interparietal bones were observed in another skull and the remaining four skulls had a single interparietal (Os inca) bone at the lambda. In addition to interparietal bones, some sutural bones were also observed in three skulls. These variations were in accordance with the ossification centres of the membranous part of the occipital bone. By their location and shape it was concluded that they were formed due to failure of fusion of nuclei of the third pair centres of ossification with each other, with opposite fellow and with the second pair centers. It was further concluded that these cases of interparietal bones were different from what had been reported earlier by other researchers and this prompted the present report that may be found useful for anatomists, anthropologists, radiologists and neurosurgeons.

The attachments of the lateral and medial ends of the extensor retinaculum of the human wrist were observed in 52 human upper limbs (from 26 cadavers) by dissection. In all the specimens used in this study, the lateral end of the retinaculum was found to be attached to the distal part of the anterior border of the radius and its medial end was attached to the styloid process of the ulna, the pisiform and the triquetrum.

Structures passing through the triangular space of the human upper limb have been studied in 50 upper limbs from 25 cadavers. In all the specimens used in this study, a branch of the circumflex scapular artery with an accompanying vein was found to pass through this space.