Page images
PDF
EPUB

the specimen hardened in formalin. Measurements from the sections show its thickness to vary from 36 , to 65 y, with an average thickness of 53 l. Completely fused with and continuous with the pia mater are its three ligaments. These were described by STREETER as occurring in the pia of the ostrich cord and they are remarkably well developed in that of the emu. The ligamenta longitudinalia lateralia course along the lateral surfaces of the cord proper in a line ventral to that along which the dorsal and ventral nerve roots approach each other (LII, Figs. 2. 3 and 4). The ligamentum longitudinale ventrale (Llv, Figs. 3 and 4) is triangular or dentate in section, its apex projecting into the relatively wide fissura mediana ventralis. Neither of these ligaments disturbs the outer contour of the pia and can only be detected in the fresh by a slight difference in the color of the lines it occupies. The lateral ligaments appear grooved into the white substance of the cord and increase in thickness with the increase in the diameter of the cord, i. e., from the neck caudad. They appear fusiform in transverse section, thinning at the edges to become continuous with the pia. Measurements at their thickest part give an average thickness of 198 y. All three are true ligaments, being composed almost entirely of large, well defined elastic tissue fibers with the characteristic stellate cells in transverse section, while the pia proper is composed mostly of white fibrous tissue.

In addition to the usual filaments bearing blood vessels and connecting the dura with the periosteum of the vertebral canal, there occurs along each side of the specimen and a short distance caudad to the points at which the spinal nerves penetrate the dura, a series of special supporting filaments. These ligamenta epiduralia occur in groups of 4 to 6 each, approximately in line, and one of them, that nearest the nerve, is always larger than the others (Lep, Figs. 1 and 2). In the formalin material this larger one appears whiter than the surrounding tissue, cylindri. cal and well defined, and microscopic examination reveals it to, be a bundle of elastic fibers. Upon opening the dura these. bundles along each side are found to be opposite and connected, with the strongest of the ligamenta denticulata.

Each of the specimens possessed 48 pairs of spinal nerves, or 48 segments in its spinal cord.

In the cervical divison and for about one-third the length of the spinal cord of the emu, the spinal nerves are attached at angles inclined toward the head, or in other words, the respective intervertebral foramena through which the nerves pass are slightly more cephalad than the levels at which their dorsal and ventral roots come together on the periphery of the cord. Along the middle third of its length the nerves are attached more approximately at right angles to the axis of the cord. As in the Ostrich, the spinal cord occupies the entire

Se vertebral canal and there is no

LIT cauda equina.

The sacral and coccygeal nerves are arranged so that their respective foramena are but little caudad to the points of attachment of their dorsal and ventral roots.

In consequence of the angles at which the nerves are attached to the cord, the fila radicularia of the

M dorsal and ventral roots of all the spinal nerves fray along the sur. face of the cord in both directions. The fila of each root form cephalad and caudad divisions of the root, which divisions, increasing as they course along their respective lines of insertion, approach each other Fig. 1. The 23rd and 24th seg. and fuse to form the root (see

ments of the spinal cord of the

emu, dorsal aspect. Dura mater Fig. 2, ventral root).

intact. Nr.-nerve roots; Lep.-As to the arrangement of their Ligamenta epiduralia. Natural fila radicularia and their angles of size after formalin. attachment, the spinal nerves of the emu differ from those

[graphic]

of the mammals and especially man. In the human, with the exception of the first two or three cervical nerves, all the spinal nerves of the adult pass through their intervertebral foramena caudad to the levels of their attachment to the cord and all the fila of their dorsal and ventral roots are distributed cephalad. In the human fætus the cord occupies the entire vertebral canal and the foramen of each nerve is then opposite the segment or level of the cord at which the nerve is attached, but, owing to the fact that the vertebral column grows more rapidly than the spinal cord and continues to grow after the cord has attained its adult length, the foramena become displaced caudad.' The cord being attached to the encephalon, the effects of the increase in the length of the vertebræ are superimposed so that the nerves become pulled downward and the nerve roots are necessarily drawn caudad into progressively increasing lengths till the final effect is that the conus medullaris seldom reaches lower than the upper end of the first lumbar vertebra and the whole lumbar and sacral portion of the vertebral canal contains only the resulting cauda equina. Like the ostrich, the emu may be said to have two brains, the encephalon and the lumbar brain", or the remarkably large intunescentia lumbalis. The latter, necessary for the control of the chief mass of the body musculature, lies in an especially enlarged portion of the vertebral canal. Thus the two ends of the central nervous system must become mechanically fixed long before the growth of the animal is complete, and as the vertebræ increase in length the intervening portion of the spinal cord must also increase in length. This results in remarkably long segments in the cervical and thoracic regions of the spinal cord of the einu, and as compensation, explains the fact that the nerves and nerve roots are not drawn caudad instead, as they are in man. Further, between fætal lise and maturity the lower thoracic and the lumbar vertebræ of man undergo a proportionately greater gain in length than do the cervical vertebræ,' thus accen

.' Measurements of the length of the first five cervical vertebræ and of the lumbar vertebræ of several human skeletons at birth and several adults, show

tuating the drawing downward of the more caudal pairs of nerves. 'The young ostriches, as I have observed them on the ostrich farms of this State, appear to have necks very much shorter in proportion to the length of the body than do the adult birds, and in all probability the same is true for the emu. If this is true, then during growth the cervical vertebræ must undergo a proportionately greater gain in length than any below them, which would explain the fact that the nerves of the upper third of the cord are attached at angles slightly cephalad instead of caudad.

Another feature to be noted in the spinal nerves of the emu is that the roots and beginning portions of the lower cer. vical and the thoracic nerves pass from the cord dorsalward to their respective intervertebral foramena (see Fig. 1). This differs from the course of most of these spinal nerves of the mammals and means that this portion of the cord lies deeper in the bony framework about it. The small neck nerves proper are similar in this respect to the neck nerves of the mammals. As the natural result of the dorsal direction of the nerves, the nerve roots approach each other along the dorso-lateral as pect of the cord, instead of the lateral or ventro-lateral aspect (see Figs. 3 and 4).

Fig. 2. Lateral longitudinal or horizontal section of 20th segment passing at level of central canal. D-dura mater; P-pia mater; Lep-Ligamentum epidurale; Ill – ligamentum longitudinale laterale; Lla-ligamentum longitudinale ventrale; l'r-ventral root; Fz-fila radicularia of ventral root; Sg--- substantia grisea; Co-canalis centralis; Nmm -- nucleus marginalis minor; Se-segmental enlargement; M-level midway between nerves. x 4. The parts were outlined by camera. Fig. 3.

Sc and M. Transverse sections of 22nd segment. Se- taken through segmental enlargement corresponding to level similarly indicated in Fig. 2; 11 - taken midway between the enlargements of the adjacent segments at level similarly indicated in Fig. 2. Lli—ligamentum longitudinale ventrale; Fr-fila radicularia of both roots; other reference letters same as in Fig. 2. Out. lined by camera. X 4.

Fig. 4. Si and N/-Transverse sections of 17th segment. Taken at lev. els corresponding to those of Fig. 3. Reference letters, drawing and magnification same as in Fig. 3.

that between birth and the adult stages the cervical vertebræ undergo an average gain in length of about 2;0%e, while the lumbar vertebræ gain about 450 % in length

[graphic][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small]
« PreviousContinue »