BIRDS. 357 



and presses off that membrane and a part of the yolk-sac from the 

 abdomen, and the chick leaves the egg^. 



There now remain for consideration the organs of animal life in 

 the class of birds. The mass of the spinal marrow is smaller 

 than that of the brain, whilst the reverse was observed in fishes 

 and reptiles. A narrow canal runs through its middle. Where 

 the nerves of the wings (plexus hrachialis) arise, the spinal marrow, 

 which in the upper part of the neck is thin, becomes broader; a 

 still more remarkable swelling appears in the lumbar region, where 

 the nerves of the hind limbs arise. Here the posterior strands 

 separate laterally from each other, and the central canal expands 

 into a sac which is filled with a watery fluid. From this part 

 downwards the spinal marrow becomes constantly thinner, runs 

 through the tail, and finally terminates in a fine thread ; a so-named 

 Cauda equina is not present^. 



The brain is more fully developed than in the two preceding 

 classes. In the first place, its mass is larger; in general, at least, 

 it surpasses in relative magnitude that of most reptiles, and of all 

 fishes^. It entirely fills the cranial cavity. Again, the greater 



1 Amongst the nuraeroua writings on the development of the chick we name only 

 M. Malpighii Diss. epistoMca de Formatione puUi in ovo in his Oi^ra omnia, Londini, 

 1687, fol. II.; A. Haller Deux Mtjmoires sur la formation du coeur dans le }midet, 

 Lausanne, 1758; G. F. Wolff Tkeoria generationis, Halse, 1759, Svo (ed. sec. ibid. 

 1774); ejusd. Ueher die Bildung des Darmlcanals im hehriiteten Hiihncliens, iiherseizt von 

 J. F. Meckkl, Halle, 18 12, Svo; Chr. Pander Dissertatio inaug. sistens historiam 

 metamorphoseos quamovum incubatum prioribus quinque diebus subit. Wirceburgi, 1817, 

 Svo (and his Beitrdge, with 14 beautiful plates by D'Alton, already cited above) ; 

 K. E. Von Baer Ueber Entioickelungsgeschichte der Thiere, Konigsberg, 1828, 4to, s. 

 I — 140, and in Bdedach's Physiologie, Bd. II. 2te Aufl. s. 335 — 446; lastly, the 

 excellent plates of M. P. Erdl, in the first part of his work, interrupted by his death, 

 Die Entwickelung des Menschcn und des HiUmchens, Leipzig, 1845, 4to. We could not 

 dwell upon the great difference of opinion of the latest writers with regard to the 

 layers of the germ and other questions not yet sufficiently cleared up, but we have 

 deviated only slightly from the description given in our first edition, for which the 

 investigations of VoN Baer supplied the ground-work. 



2 Compare Nicolai in Reil's Arcliiv, XI. s. 156—219, with a figure of the entire 

 spinal marrow of a goose. 



^ The proportion is very various ; thus the ratio between the weight of the brain 

 and that of the whole body has been found in the sparrow as i : 25, in the chaffinch as 

 1 : 22, in the goose as i : 300, Halleb Elem. Physiol, iv. pp. 9, 10 ; Cuv. Lee. d'Anat. 

 camp. II. pp. 151, 152, 26 ^d. in. pp. 79, So. Still smaller than in the goose is the 

 ratio in struthious birds; in the Lidian Casuary as i :67o or even i: looo, Meckel 

 Archil' fiir Anatomie und Physiol, vi. 1832, s. 352. 



