1903.] MONSTROSITIES IN PISHES. 9 



Thei^e is one pair of trigeminal nerves representing the right and 

 left 5th nerves of the right and left twin heads. The succeeding 

 cranial nerves are also normal, i. e. there is only a single pair of 

 each. A rudiment of inner or adjacent pairs of trigeminal 

 ganglia may be recognised in the form of a thin elongated 

 band of tissue containing smaU nerve-cells and lying in the middle 

 line underneath the region of the pons. This band of tissue has 

 no central or peripheral nerve-fibres. The pons and cerebellum 

 are single, but their internal structure shows traces of duplicity, 

 especially in the case of the pons. The medulla oblongata 

 is slightly expanded transversely, but otherwise is normal. 



In the anterior part of the spinal cord there is a curious and 

 interesting reappearance of duplicity, coextensive with the 

 duplicity of the notochords, and with the presence, ventral to 

 them, of a median composite muscular mass representing united 

 adjacent lateral muscles. In this region, the spinal cord is greatly 

 expanded in a transverse direction, its cavity is spindle-shaped, 

 and, in addition to the usual nerve-roots, it gives ofi', on the 

 ventral aspect in each segment, a pair of small additional motor 

 roots which are distributed to the median muscular mass just 

 mentioned. 



There are two pairs of olfactory organs and nerves. The outer 

 eyes (right eye of right twin head and left of left head) are 

 normal, but lie further back than usual, so that their optic nerves 

 pass backwards as well as outwards from optic commissure to eye- 

 ball. The inner or adjacent eyes may or may not be fused with 

 one another. In the former case, there is usually a single lens, 

 which is sometimes larger and sometimes smaller than in the 

 normal case ; the sclei-otic and choroid coats are single ; the retinae 

 never unite, each showing its own choroidal fissure, optic nerve, 

 and choroidal gland. In all cases the external rectus muscles are 

 absent ; the superior obliques are absent or rudimentary, but the 

 remaining ocular muscles are present in two sets. 



The heart and the ventral aorta are normal, but the dorsal 

 aorta and its roots, and the choroidal and carotid arteries, require 

 description. The union of the main collecting-trunks on either 

 side to form the dorsal aorta is carried backwards for a very con- 

 siderable distance, and takes place only at the level of union of 

 the notochords. The pseudobranch on either side receives a 

 branch from the hyoid artery, and its efierent vessel passes to the 

 choroidal gland of the corresponding (i. e. outer) eye. The 

 choroidal glands of the inner or adjacent eyes are supplied by 

 blood which has not passed through the pseudobranch. In the 

 specimen from which PI. I. figs. 3, 4, & 8 were taken, the arrange- 

 ment of vessels is quite symmetrical and is indicated in PL II. 

 fig, 12. A transverse arch vessel connects the upper aoi-tic roots, 

 and gives ofi" a common choroidal artery which soon bifurcates. 

 The two resulting vessels pass through the separate pituitary 

 spaces, and are distributed to the choroidal glands of the adjacent 

 eyes. 



