892 



DEVELOPMENT OF THE SENSE ORGANS. 



In the hindbrain are found the cerebellar hemispheres, which grow backwards to meet m 

 the middle line. The vermis is formed at the 7th month. The cerebellum covers in the part 

 of the medullary tube lying below it, which is not closed, as far as the calamus. The pons 

 arises in the floor of the hind-brain at the 3rd month. 



The spindle-shaped narrow after-brain forms the medulla oblongata, with the opening of 

 the medullary tube in its upper part 



[The following table, from Quain, shows the destiny of each cerebral vesicle : 



( Cerebral hemispheres, corpora striata, 

 J 1. Prosencephalon, . . . J corDua callosum, fornix, lateral 

 (fore-brain) 



II. 



in. 



Anterior 

 Vesicle, 



Middle 

 Vesicle, 



Posterior 

 Vesicle, 



Primary | 



- * I 2. Thalamcnccphalon, . 

 (inter or 'tween brain) 



Primary ( 3. Mesencephalon, . . 

 . . . [ (mid-brain) 



I 4. Epencephalon, . . . 

 Primary | (hind-brain) 



. . . "i 5. Mctcncephalon, . . 

 (after-brain) 



corpus callosum, fornix, 

 ventricles, olfactory bulb. 



( Thalarai optici, pineal gland, pitui- 



-! tary body, crura cerebri, aqueduct 



( of Sylvius, optic nerve. 



( Corpora quadrigemina, crnri cerebri, 

 ' aqueduct of Sylvius, optic nerve 



( (secondarily). 

 . I Cerebellum, pons, anterior part of 



\ the fourth ventricle. 

 . / Medulla oblongata, fourth ventricle, 



\ auditory nerve. 



Spinal Cord.- The spinal cord is developed from the medullary tube behind the medulla 

 oblongata, fust the grey matter around the canal, while the white matter is added afterwards 



outside this. The ganglionic cells increase 

 by division in amphibians (Lominsky). At 

 first the spinal cord reaches to the coccyx. 

 In the adult, the spinal cord reaches only 

 to the 1st or 2nd lumbar vertebra, so that 

 it does not elongate so much as the verte- 

 bra can. It is a question how far this want 

 of harmony in the development of these 

 two structures may lead to disturbances of 

 sensibility or paralysis of the lower limbs in 

 children. The first muscles are formed in 

 the back at the 2nd month ; at the 4th 

 month they are red. The spinal ganglia 

 are formed from a special strip of epiblastic 

 cells. They are seen at the 4th week, and 

 so are the anterior spinal roots and some of 

 the trunks of the spinal nerves, while the 

 posterior roots are still absent. At this 

 period the ganglia of the 5th, 7th, 8th, 9th, 

 and 10th nerves and part of their origins 

 are present, while the 1st, 2nd, 3rd, and 

 12th nerves and the sympathetic are not 

 yet far differentiated (His). The peripheral 

 nerves grow out from the ganglia of the 

 spinal cord (first the motor and afterwards 

 the sensory nerves), and penetrate into the 

 other parts of the body (His). At first 

 they are devoid of myelin. 



ac, an- 

 medullary 



Fig. 690. 

 Diagram of an embryonic fowl's brain 

 terior commissure; amv, anterior 

 velum, and below it the aqueduct of Sylvius 

 and the cerebral peduncles ; ba, basilar artery ; 

 bg, corpora bigemina ; cat, internal carotid 

 artery ; cbl, cerebellum ; cW, ch 4 , choroid plexuses 

 of the third and fourth ventricles ; h, cerebral 

 hemispheres ; inf, infundibulum ; It, lamina 

 terminali8 ; li, lateral ventricle ; obi, medulla 

 oblongata ; off, olfactory lobe and nerve ; ope, 

 optic commissure ; pin, pineal gland ; pit, pitui- 

 tary body ; ps, pons Varolii ; r, floor of fourth ven- 

 tricle ; st, corpus striatum ; v\ third ventricle ; 

 c*, fourth ventricle (Quain, after Mihalkovics). 



452. THE SENSE ORGANS. Eye. The primary optic vesicle grows out from the fore- 

 brain towards the outer covering of the head or epiblast, and soon becomes folded in on itself 

 (4th week), so that the stalked optic vesicle is shaped like an egg-cup (fig. 691, I). The cavity 

 in the interior of this cup is called the secondary optic vesicle. The inflected part becomes 

 th- retina (IV, r), while the posterior part becomes the choroidal epithelium (IV, p). The 

 stalk becomes the optic nerve. At the under surface of the depression there is a slit the 

 choroidal fissure which permits some of the mesoblast to gain access to the interior of the 

 eye. This slit forms the coloboma (II); it is prolonged backward on the stalk, and contains 

 the central artery of the retina. The margins of the coloboma afterwards unite completely 

 with each other, but in some rare conditions this does not take place, in which case we have to 

 deal with a coloboma of the choroid or retina, as the case may be. In the bird the embryonic 

 coloboma slit does not close up, but a vascular process of the mesoblast dips into it, and passes 

 into the eye to form the pecten (p. 796). The same is the case in fishes, where there is a large 

 vascular process of the meso- and epiblast forming the processus falciformis (p. 796). 



The depression or inflection of the optic vesicle is due to the downgrowth into it of a 



