1276 



SPLANCHNOLOGY 



and a pars intermedia, separated from each other by a narrow cleft, the remnant 

 of the pouch or diverticulum. The pars anterior is extremely vascular and consists 

 of epithelial cells of varying size and shape, arranged in cord-like trabeculse or 

 alveoli and separated by large, thin-walled bloodvessels. The pars intermedia is u 

 thin lamina closely applied to the body and neck of the posterior lobe and extend- 

 ing onto the neighboring parts of the brain; it contains few bloodvessels and 

 consists of finely granular cells between which are small masses of colloid material . 

 The pars intermedia in spite of the fact that it arises in common with the pars 

 anterior from the ectoderm of the primitive buccal cavity is often considered as 

 A B a part of the posterior lobe; 



which arises from the floor of 

 the third ventricle of the 

 brain. Although of nervous 

 origin the posterior lobe con- 

 tains no nerve cells or fibers. 

 It consists of neuroglia cells 

 and fibers and is invaded by 

 columns which grow into it 

 from the pars intermedia; 

 imbedded in it are large 

 quantities of a colloid sub- 

 stance histologically similar 

 to that found in the thyroid 

 gland. In certain of the 

 lower vertebrates, e.g., fishes, 

 nervous structures are pres- 

 ent, and the lobe is of large 

 size. 



From the pars intermedia 

 a substance, no doubt an in- 

 ternal secretion, causes con- 

 striction of the bloodvessels 

 with rise of arterial blood- 

 pressure. This substance 

 seems to have a stimulating 

 effect on most of the smooth 

 muscles, acting directly upon 

 the muscle causing contrac- 

 tion. It also increases the se- 

 cretion of the urine; of the 

 mammary glands when in 

 functional activity; and of 

 the cerebrospinal fluid. Ex- 

 tracts of this lobe also influ- 

 ence the general metabolism of the carbohydrates by accelerating the process of 

 glycogenolysis in the liver. 



The pars anterior exercises a stimulating effect on the growth of the skeleton and 

 probably on connective tissues in general. 



Enlargement of the hypophysis and of the cavity of the sella turcica are found in the rare 

 disease acromegaly, which is characterized by gradual enlargement of the face, hands, and feet, 

 with headache and often a peculiar type of blindness. This blindness is due to the pressure of 

 the enlarging hypophysis on the optic chiasma (Fig. 1180). 



Development of the Hypophysis Cerebri. This in the adult consists of a large 

 anterior, consisting of the pars anterior and the pars intermedia, and a small pos- 





FIG. 1182. Vertical sections of the heads of early embryos of the rab- 

 bit. Magnified. (From Mihalkovics.) A. From an embryo 5 mm. 

 long. B. From an embryo 6 mm. long. C. Vertical section of the 

 anterior end of the notochord and hypophysis, etc., from an embryo 16 

 mm. long. In A the buccopharyngeal membrane is still present. In B 

 it is in the process 9f disappearing, and the stqmodeum now communi- 

 cates with the primitive pharynx, am. Amnion. c. Fore-brain, ch. 

 Notochord. /. Anterior extremity of fore-gut, i. h. Heart, if. Infun- 

 dibuluni. m. Wall of brain cavity, me. Mid-brain. mo. Hind-brain. 

 p. Original position of hypophyseal diverticulum, _ py. ph. Pharynx. 

 sp.e. Sphenoethmoidal. be. Central, sp.o. Sphenooccipital parts of basis 

 cranii. tha. Thalamus. 



