230 THE CAT. [chap. viii. 



muscles — posterior and hicral crko-arytenolds. — pass from tlio cricoid 

 to different parts of the arytenoids, which the former rotate out- 

 wards, and the latter inwards, thus either widening or contracting 

 the glottis. The thyro-arytowkh and the arti/cno-oinylottidcau muscles 

 have the same connexions as the true vocal cords and the aryteno- 

 cpiglottic ligaments respectively. The fjhsso-cpiglottldean muscles 

 pass from the hack of the tongue downwards to the base of the 

 front of the epiglottis, and the hyo-epiglottidean muscles are very 

 small ones, extending from the hinder surface of the hasi-hyal, down 

 to nearly the same part of the epiglottis as that into which the last- 

 mentioned muscles are inserted. 



Lastly, the arytenoid muscles connect and tend to approximate the 

 two arytenoid cartilages. 



§ 8. In treating of respiration and the respiratory organs, we have 

 mainly been occupied ^dth that which ministers to the nutrition and 

 warmth of the body by enabling it to obtain its due supply of oxygen. 

 "We have, however, also noted that the process of respiration is in part 

 a process of elimination and removal from the body of a portion of the 

 waste products of its vital activities. This now requires more careful 

 consideration. Life is a series of compositions and decompositions, 

 and in order that assimilation may go on, a process of disassimilation 

 must necessarily accompany it. With the addition of new and unused 

 material, there must go on a subtraction of old and effete material, and 

 this (as wo have lately seen) is mainly brought about by a process of 

 oxydation in the inmost parenchyma of the body. \Vc have seen, 

 in studying alimentation, that colloids have to be changed into 

 crystalloids, that they maybe conveyed to the colloidal parenchyma, 

 into which they have to be transformed. Similarly in the process of 

 disassimilation, the effete colloidal parenchyma has to be reconverted 

 into crystalloids that it may be convoyed away and excreted, thougli 

 the crystalloids of excretion are generally different from those of 

 nutrition. 



It has also already been pointed out that the digestion of the 

 food is aided by the juices of the salivary glands and pancreas, and 

 other similar structures. Now these juices do not exist Yas such in 

 the blood, but are formed from it by a mysterious power which 

 certain cells possess thus to form new products. The exercise 

 of this power is called " .secrctioji," and it is a power analogous 

 to that by which the various tissues arc enabled to add to their 

 own substance from the life-stream which batlics them, though 

 their substance does not exist, as such, in that stream. Thus 

 " assimilation" is a sort of "secretion." Nevertheless it cannot be 

 said that " secretion " is a sort of " as.similation." *' Assimilation " is 

 a process of forming products and adding them to the body ; but 

 "secretion" is a process of forming products wliich arc to be got rid 

 of, or are destined to aid in other life processes. Tlius secretion is a 

 special function, and as such has a special organ — a gland. Gland.^, 

 as wo have already seen, arc cither simple or complex involutions 

 of an epithelial surface. We have seen simple ones in the sweat 



