6 The Structure of the Spinal Cord of the Ostrich 



Caudal to the lumbo-sacral enlargement, the pia returns to the more 

 simple sheath-like form as seen in the thoracic and cervical regions. 



A work on the comparative and embryological anatomy of the spinal 

 cord meninges has recently been published by Sterzi.^ 



In the mammalian embryo (Ovis aries), 15 mm. long, the author de- 

 scribes a mesenchyma perimeningeale which first produces a definite 

 spinal cord membrane in the embryo of 20 mm. This he calls meninx 

 primitiva. In the 80 mm. long embryo this membrane is differentiated 

 into an outer layer or dura mater, and an inner layer or meninx secon- 

 daria. The two are separated by an intradural space. The dural layer 

 is separated externally by the epidural space from an endorhachide 

 which Sterzi finds always distinct from the dura. In the 157 mm. 

 embryo the meninx secondaria is further differentiated into an outer or 

 arachnoideal layer and an inner or pial layer. In his comparative series 

 the author finds the Petromyzon as representing the 20 mm. embryonal 

 stage. The Eana esculenta and Lacerta viridis represent the 80 mm. 

 stage. The development shown by the 157 mm. embryo with a differen- 

 tiated arachnoid he finds only in the mammals. Our findings in the 

 ostrich do not correspond with this. In Sterzi's series the birds are re- 

 presented by Gallus domestica in which he describes a meninx secondaria 

 not yet differentiated into pia and arachnoid. In the ostrich we find, as is 

 above described, an arachnoidal layer which presents all the distinguish- 

 ing features of that of the mammalian cord. 



GENERAL MACROSCOPIC DESCRIPTION. 



The abrupt change from the slender cervical spinal cord of the ostrich 

 to the thick medulla oblongata gives a rather definite level at which the 

 cephalic end of the cord may be said to be located. From this point 

 extending caudal ly it stretches throughout the entire length of the spinal 

 canal, its slender tapering end extending to the last coccygeal vertebra. 

 It measures 81 cm. long in a small ostrich, the middle of whose back 

 stands about 60 cm. above ground, and whose head in the ordinary up- 

 right position is 45 cm. higher, or 105 cm. above ground.^ 



From each side of the cord throughout its length is given off a series of 

 fine rootlets which unite, within the dural sheath in segmental bundles, to 

 form the dorsal and ventral nerve roots. These, together, pierce the 



'Sterzi, Anatomia comparata ed all'ontogenesi delle Meningi midollari: 

 Atti del Reale Institute Venento di Scienze, Lettere ed Arti, Tomo LX, 1900- 

 1901. 



* All the measurements hereafter stated are taken from this same specimen. 



